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How Much Virtual Memory is Enough?

Cliff posted more than 7 years ago | from the how-large-is-your-swap-partition dept.


whitroth asks: "Ten years ago, Received Wisdom said that virtual memory should be, on the average, two to two-and-a-half times real memory. In these days, where 2G RAM is not unusual, and many times is not that uncommon, is this unreasonable? What's the sense of the community as to what is a reasonable size for swap these days?"

cancel ×


lots (5, Funny)

emphatic (671123) | more than 7 years ago | (#16004033)


Re:lots (0, Offtopic)

Xocet_00 (635069) | more than 7 years ago | (#16004057)

Haha. Redundant.

I'm not really sure it's possible for the first thing said in a discussion to be "redundant".

Weee! Now I'm going to get an "offtopic". Sweet.

Re:lots (0, Redundant)

Millenniumman (924859) | more than 7 years ago | (#16004083)

This would be a redundant first post:

"Ten years ago, Received Wisdom said that virtual memory should be, on the average, two to two-and-a-half times real memory. In these days, where 2G RAM is not unusual, and many times that not that uncommon, is this unreasonable? What's the sense of the community as to what is a reasonable size for swap these days?"

Re:lots (0, Offtopic)

Xocet_00 (635069) | more than 7 years ago | (#16004258)

He gets offtopic too? Not all slashdot discussions confine themselves to only the immediate topic discussed in the article. I'd say by the third post, we've got a legitimate discussion going. Pfft.

Not much, anymore... (5, Insightful)

FishWithAHammer (957772) | more than 7 years ago | (#16004034)

Under Windows it seems it'll swap out whether the free RAM is needed or not, no matter what (there's a registry setting to change this though). Under Linux, you won't swap much anyway unless you need it.

I run a Core Duo laptop with 1GB of RAM and have never swapped out in Linux, no matter what I was doing.

Re:Not much, anymore... (5, Informative)

Propaganda13 (312548) | more than 7 years ago | (#16004056)

Simple. Monitor your own resource usage and figure out what YOU require. Everyone has different hardware, programs, and habits.

Re:Not much, anymore... (0)

Anonymous Coward | more than 7 years ago | (#16004065)

What needs to be edited in the registry to stop the swaping out of RAM?

Re:Not much, anymore... (5, Informative)

larry bagina (561269) | more than 7 years ago | (#16004090)

system control panel -> advanced -> performance options -> advanced - > virtual memory.

Set to no paging.

Re:Not much, anymore... (3, Informative)

LiquidCoooled (634315) | more than 7 years ago | (#16004199)

This works really well until the one day you leave everything running, startup halflife and flick around the levels (uses memory faster...)

bleeding thing cannot smoothly say "You are running out of memory, Setting up an emergency page file now...." without something crashing.

Fix this problem and you are cooking on gas. A modern computer should be able to accomodate every malloc upto memory+free disk space and it can't easily.

Re:Not much, anymore... (1)

Tatsh (893946) | more than 7 years ago | (#16004433)

100% agreed. I have never gotten a problem after setting this on my previous desktop and new laptop, both which have 1GB of RAM.

Re:Not much, anymore... (1)

Xocet_00 (635069) | more than 7 years ago | (#16004077)

What registry entry is that? I'd love to be able to spin down my disk (laptop w/ 1.5GB) occasionally while in Windows. Something always seems to be swapping.

doesn't everyone know this? (-1, Offtopic)

missing000 (602285) | more than 7 years ago | (#16004092)

vi. definitely vi.

Re:Not much, anymore... (3, Insightful)

toller8 (705418) | more than 7 years ago | (#16004097)

Different jobs, different needs....

Two of my (Linux) servers have lots of memory and lots of small processes so anything that does swap out swaps out quick. These don't use a lot of swap (512Mb?) and don't have gig sized processes to write into swap... so they don't really need the 2+ gig of allocated swap.

One other (Linux) server has big processes (1Gig or more) and when they have to swap out, watch the machine fall apart while the process is swapped out - it takes a while to write 1 gig of ram into swap! Since the process is large, swap needs to be large.... Just hope that server needs to have 3 or 4 multi gig processes swapped out....

So, YMMV! Know your machines and what *may* need to swap out and you can live on the edge and figure your minimum swap.... or you could be safe and boring and have x.5 times RAM... After all... who needs that critical app to run after memory gets tight and the kernel kills it cause it was the memory hog?


Re:Not much, anymore... (1)

KingJoshi (615691) | more than 7 years ago | (#16004100)

Exactly. And being on a laptop, the HD speed is slower and a complete waste of battery and time. So I have ZERO virtual memory. On Linux it'd be a different story because it's smart enough not to needlessly swap. I barely go over 500 MB, unless I'm playing Civ4, in which case it might go to a gig. But with 2GB RAM, why does Windows swap so much? Well, for me, not any more.

Re:Not much, anymore... (1)

Ctrl-Z (28806) | more than 7 years ago | (#16004356)

I suppose that depends on what you're doing with it. First of all, not all laptops have slow hard drives. Secondly, if you're using a laptop as a developer, it's pretty easy to eat up 2GB of RAM. Especially if you're running Oracle, SQL Server, and several instances of various pieces of server software. I was surprised when I recently discovered that when SQL Server is doing a lot of stuff, it will continue to allocate memory until you have 4-10 *MB* left. That is, unless you find the advanced option to cap the memory usage.

Re:Not much, anymore... (0)

Anonymous Coward | more than 7 years ago | (#16004394)

So I have ZERO virtual memory.

How did you manage that? Windows seems to force a minimum of a 2 megabyte pagefile.

Re:Not much, anymore... (1)

lnjasdpppun (625899) | more than 7 years ago | (#16004490)

Disable the page file completely. It's an option in the same screen where you set the page file size.

I've had Windows ignore my first request to disable the page file before, but as is the way with Windows telling it twice actually got the message through.

Re:Not much, anymore... (5, Informative)

megaditto (982598) | more than 7 years ago | (#16004103)

To control how much 'it will swap' on Linux:
#echo [0-100] > /proc/sys/vm/swappiness

A better question is how much memory you can address. Could your 32 bit Windows system address over 2^36 bits of memory (64GB), for example? And could you allocate over 2GB to windows kernel?
Could your 64-bit linux system address over 2^48 bits of memory?

Re:Not much, anymore... (3, Informative)

SanityInAnarchy (655584) | more than 7 years ago | (#16004157)

Swapping out makes sense sometimes, though. For instance, there are tiny chunks of the system -- daemons and such -- that are pretty much never accessed. I'd rather reclaim that, if only to cache something worthwhile.

Also, remember that suspend2 requires swap, so figure how much of an image you'll need (and how much is cache that can be freed) and get a bit more than that. My own rule of thumb is, swap is roughly 1x to 1.5x RAM, so that I can be sure I have room for the suspend. But I have the space, and Windows doesn't use swap for this anyway, it uses hiberfil.sys

Pre-emptive swapping... (5, Informative)

Junta (36770) | more than 7 years ago | (#16004189)

Linux has futzed with this a lot (and lets the user tweak VM behavior a lot, but /proc/sys/vm/swappiness goes a long way...), both linux and Windows will swap well ahead of not having free memory (for good reason). Just wanted to go into detail because I keep seeing people complain that they see swap used in linux or windows when they still have free memory, not realizing this isn't a bad thing generally.

There are generally two strategies:
-The common-sense one where you swap when you run out of memory. This makes a lot of practical sense on systems with limited write cycles (flash based swap, though you really never ever should do that anyway), and systems that want to spin down drives to conserve power for battery conservation. Performance wise (this may surprise people who haven't spent time thinking about it), this can often be bad. Avoiding swapping is generally only good on systems where resource utilization is carefully managed and you know it won't swap ever (the IO operations of unneeded can interfere with the productive activity of a constantly busy system). This is actually a vast minority of systems in the world (no matter how l33t one may think themselves, they most certainly don't have a usage pattern that would be impacted by the extraneous IO operations of occasional write to swap.

-Pre-emptive swapping. When the IO subsystem is idle and the system can afford to copy memory to swap area, it does so (depending on criteria). Generally speaking it will select memory not accessed much and write it to disk, but leave the memory copy in place if the physical memory is not immediately needed. A fair amount of swap used in an apparently underutilized system is duplicated in physical memory and swap space. The benefit here is that if the process reads back that memory, it doesn't incur any penalty in reading it back despite it being also in swap (the system may make certain decisions on what is the best swap candidate and write to disk different data). The benefit of writing this stuff to swap even when not needed is clear when an application comes along that allocs more memory than the system has free in physical space. In the first strategy, this means the malloc blocks while data is written to disk, and the new application starting or needing a lot of data is severely impacted. In the pre-emptive swap case, system notices the condition, knows what memory it has a backup in swap of that hasn't been used lately, and can free that memory and satisfy the malloc pretty much instantly.

To those who have 1GB of RAM or so it becomes less likely that the system will have to flush memory from physical RAM, but there is a balance to be struck between memory used directly invoked by applications, what the application memory access pattern is, and what ram you can use to buffer filesystem access. If your total application memory allocation is 75%, it still may make sense performance wise to only keep 50% of your physical memory dedicated to the applications, (the other bit relegated to swap), and 50% of the memory to buffer disk I/O.

Re:Pre-emptive swapping... (1)

afidel (530433) | more than 7 years ago | (#16004357)

My response is that preemptive swapping on Windows can often be too agressive. If I launch a game like WoW then all of my other processes get swapped out, even though I have 1.5GB of ram and the WoW process only takes ~500MB max. This means that while I have ~700MB of free ram whenever I alt-tab my browser and other programs have to get swapped back in from disk, a slow and annoying process. Is it good to user some ram as a disk cache, yes, but at what point does it become counterproductive? Personally I wish there was some way to tune this.

Re:Pre-emptive swapping... (2, Informative)

cookd (72933) | more than 7 years ago | (#16004461)

That's not due to pre-emptive swapping. Pre-emptive swapping makes your hard disk work more when the system is idle, but it doesn't force anything out of memory.

Your issue is due to an incorrect decision somewhere (not sure where) about how much memory to make available to WoW's direct (memory allocation) and indirect (disk cache) needs. WoW IS taking advantage (directly or indirectly) of that extra memory, but it probably only makes a 0.1% performance difference and you would rather it left your other programs in RAM. That is a hard situation to tune for.

Note that there are (at least) two different ways for memory to be used even when it shows up as "free". One is via disk cache. The other is via large temporary allocations that are made, used, and then freed before they really register on the performance monitor.

Re:Pre-emptive swapping... (1)

L0stm4n (322418) | more than 7 years ago | (#16004358)

Where are my mod points when I need them?

Get this guy ro +5

But more on topic... (4, Insightful)

Junta (36770) | more than 7 years ago | (#16004363)

My strategy generally is to use a file for swap rather than a partition, even in linux. I figure that if memory has to be swapped in from disk, it's already crappy going to disk so the extra overhead doesn't matter much, and I have freedom to adjust it up or down depending on my needs. (This is a desktop/laptop circumnstance). I generally start at 512MB or so, increasing maybe if IO is faster on the drive. I view swap like a rumble strip on a road before a stop sign. With no swap, you don't realize a process leaked memory until it's too late, with swap, while it eats through your swap the performance will degrade and you'll see the end coming ahead of time, and may be able to head it off with a kill. It may be well an good your 4GB of ram is technically capable of handling the same load your 1GB RAM+1GB swap handled in the past, but having some noticable impact when things start going wroing is nice. I realize theoretically there are better approaches, but nothing gets in your face like poor performance and tons of disk accesses.

On a production server or a problematic system where I want support and the OS likes to dump a core to swap, I'll ensure a generous swap partition is available (generally observed active swapx1.5+physical memory size). In this case a file-backed swap may depend on layers of the kernel that are in an invalid state, and a swap partition is more likely to be reliably writable. The only system I would even theoretically hibernate on is my laptop, and I only ever suspend to ram or shutdown completely, so I don't consider my laptop as needing a swap partition of any significant size.

Re:Not much, anymore... (1)

mnmn (145599) | more than 7 years ago | (#16004365)

Core Duo?

You havent used the laptop long enough.

Re:Not much, anymore... (1)

Transcendent (204992) | more than 7 years ago | (#16004474)

You've never run Matlab for a big project, I take it...

Depends (4, Interesting)

beavis88 (25983) | more than 7 years ago | (#16004042)

My rule of thumb these days is 1.5x RAM, unless you're at 2GB, in which case I go with 2GB swap as well. This is for *gasp* Windows, though.

Rules of thumb are dumb (1)

temojen (678985) | more than 7 years ago | (#16004208)

It really does depend.

It depends on what you're doing with the computer, and what hardware resources are available. Out of memory is bad. Very bad. On systems which have oodles of RAM, I tend to give low or no swap; on systems tight on RAM I may give 10x or more the amount of RAM.

Here, "oodles of RAM" and "tight on RAM" are very dependant on what the system's being used for. For a home NAT gateway 64MB may be oodles; for an image processing station, 1GB may be tight (especially when dealing with medium or large format photographs).

My desktop currently has 1GB RAM and 8GB swap, since I work with 100MPixel 16 bit images when I'm not working on hundreds of 14MPixel 16 bit images at the same time.

Re:Rules of thumb are dumb (1)

statusbar (314703) | more than 7 years ago | (#16004324)

On Linux sometimes there are surprises. On a system with 2 gigs of ram, 2 gigs of swap, I found locatedb causing huge flushes of runnable binaries into swap or whatever causing a massive system slowdown - Even though the system was normally not under any real load and no big memory allocations ever. So I shut off swap completely, and the system was consistently fast and never crashed! of course, 2 gigs was enough for what I needed. if it wasn't ... kaboom!


Re:Depends (1)

ColaMan (37550) | more than 7 years ago | (#16004277)

From my MythTV box, which has a few other misc processes running:

top - 11:50:40 up 4 days, 15:14, 2 users, load average: 0.05, 0.01, 0.00
Tasks: 76 total, 1 running, 75 sleeping, 0 stopped, 0 zombie
Cpu(s): 5.9% user, 0.6% system, 4.0% nice, 88.5% idle, 1.0% IO-wait
Mem: 483476k total, 463240k used, 20236k free, 1804k buffers
Swap: 899632k total, 232k used, 899400k free, 275920k cached

Hmmm. Guess I don't really need swap after all.

Re:Depends (1)

Malc (1751) | more than 7 years ago | (#16004476)

I have 2GB of memory. I have my swap set at 768-1536 MB. I thought I'd set it lower than that... could Windows have increased the min. value permanently when it needed to increase? Whatever the case, I do not want 2-3GB of swap space. That means for my usage that I've got a problem. I find that when you're using as much swap as real memory that Windows starts to grind. Even after freeing memory, just bringing up the start menu can cause it to thrash the hard drive. So I'm realistic and keep my swap space to a size that makes sense. Why waste disk space on something I'm not going to use? Until recently, as far as I'm concerned, swap space is only there for emergencies. These days though I often hit 2.5GB usage (VMWare, some big JVMs, Photoshop, etc).

If you have enough, none (4, Insightful)

bob whoops (808543) | more than 7 years ago | (#16004044)

Back when I had 512MB of memory, I had a 512MB swap partition, but I noticed that I never came close to using all of it.

When I got my new machine with 1G, I never bothered to make one at all, and I've never had a problem with it. If I do ever find myself in a situation where I need some swap space, I could always just create a swap file. It's a lot more convinient because it wouldn't have to be a fixed size, doesn't take up space when I don't need it, and I have one less partition

Especially if you have 2G or more, I don't see a real reason to use swap

LVM (2, Informative)

XanC (644172) | more than 7 years ago | (#16004074)

If you use LVM (which you should, it's great!), you can expand and contract your swap partition as needed.

Re:If you have enough, none (5, Interesting)

MyDixieWrecked (548719) | more than 7 years ago | (#16004111)

not creating a swap partition at all is a bad idea, imo...

you never know when some runaway process is going to eat all yer RAM and need to use swap... no matter how much RAM you've got.

I typically just make a 1 or 2 GB swap partition since I've got more than enough space to spare. I mean, back in the days when 128MB of RAM was considered a lot, and a 5GB drive was considered huge, no one would consider using 20% of their storage space for swap. Now, it's not unusual to have 300GB of storage, so what's 1% of that being used for swap?

I've also got a serious collection of 2-6GB harddrives kicking around, now, so I've been using them for swap. It's really pointless to have a 4GB partition for data, so I just use the entire 6GB drive for swap on some machines.

my primary server right now has a 4GB swap partition and 1.25GB of RAM... a piece of bad AJAX code that ran overnight wound up using all the RAM and had some seriously detrimental effects on the performance of the server. it took 25 minutes to ssh in in the morning and when I finally got in, I found that the load averages were at over 100 (I've NEVER see that before).

my point is that even if you have a LOT of RAM, it's still handy to have some spillover available.

Re:If you have enough, none (5, Interesting)

edmudama (155475) | more than 7 years ago | (#16004169)

If you've got a 300GB primary drive, it's foolish to use a 5GB drive for your swap. While you gain the benefit of having that drive separate from the primary (and potentially not contending for the bus), those drives are so far apart technology wise that you'd probably be better off with a swap partition on your most modern disk.

That 2/5/6GB drive may have a 20MB/s sequential rate at OD and half that at ID. Modern drives more than double that sequential performance (or triple), which is what's critical when swapping in/out a large job. Many drives in that generation don't support UDMA either, and talk with PIO, meaning you get no data checksum on your transfers.

You can span generations when you're using a cost reduced modern drive (fewer heads, same formats) but the drive that was stretching to make 5GB across 6/8 heads will be a real POS compared to modern drives performance wise.

Thrashing is bad, but thrashing to a slow disk I'd think would be worse. It is even compounded since that 5GB drive is probably PATA, meaning you're going to have your swap drive and primary drive sharing a cable, which will basically nuke most of the savings of 2 disks since they'll be reselecting master/slave at almost every command.

Re:If you have enough, none (4, Insightful)

EvanED (569694) | more than 7 years ago | (#16004289)

Well, again, that depends; if your usage patterns don't cause enough memory use to justify swapping, and you're just creating a swap partition for the emergency where some program decides to break, then it hardly matters if your swap drives are slow, because they are never accessed.

Re:If you have enough, none (1)

MyDixieWrecked (548719) | more than 7 years ago | (#16004540)

well, machines of mine that have a 300GB drive don't have the smaller drives (10GB) in them. my point was really that when you've got 300GB of storage, does the difference between 500MB and 5GB of swap really matter? why not just go with the latter?

however, in response to your mention of the fact that older drives will be slower... you're right. I just use those drives in machines because I really dont' know what else to do with them. I threw out all of my 1GB drives a couple years ago and now I've got piles of 4-8GB high performance (as of like 1999) scsi harddrives kicking around that I don't know what else to do with.

Re:If you have enough, none (4, Insightful)

gfxguy (98788) | more than 7 years ago | (#16004202)

you never know when some runaway process is going to eat all yer RAM and need to use swap... no matter how much RAM you've got.

Frankly, while I do use swap, in this case I'd rather have the process crash sooner rather than later.

Re:If you have enough, none (0)

Anonymous Coward | more than 7 years ago | (#16004260)

you never know when some runaway process is going to eat all yer RAM and need to use swap... no matter how much RAM you've got.

That's why I like per-process ram limits - even if it goes bezerk it won't bring the system to a crawl.

Re:If you have enough, none (2, Insightful)

hackstraw (262471) | more than 7 years ago | (#16004427)

you never know when some runaway process is going to eat all yer RAM and need to use swap... no matter how much RAM you've got.

Personally, I prefer a runaway process to run out of resources and stop vs take over my whole system. It takes a long time to page out 1+ Gigs of RAM. It takes a long time to unpage all of that at shutdown or even when an app is closed.

Swap completely depends on the computer's real RAM available and the purpose of the computer and the OS on said computer.

To adequately answer the question, "How much Virtual Memory is Enough?" The correct answer is "It depends".

Having too much swap on a HPC type of machine is a nightmare and will kill performance. Having too little swap on a general purpose server (moreso real RAM) is going to hurt performance. Paging out too much on a laptop with a slow disk can be very painful and slow down the shutdown process.

There is no right answer.

Re:If you have enough, none (0)

Anonymous Coward | more than 7 years ago | (#16004509)

If you have a runaway process, then all you have bought is a bit of time. It will still hit a wall and your system will be incredibly slow (i.e. difficult to recover from).
The real answer is if you need swap for NORMAL running, then have it. If you are not swapping under NORMAL condition, then turn it off.

Re:If you have enough, none (0)

Anonymous Coward | more than 7 years ago | (#16004539)

that doesn't make any sense.

if you have a runaway program eating memory, it will also eat all your swap

and if it doesn't you have just wasted a ton of disk space for no reason.

so, in short what you say makes no sense.

Enough... (3, Funny)

talkingpaperclip (952112) | more than 7 years ago | (#16004046)

640k should be enough for anybody.

Chicken Cross Road (0)

Anonymous Coward | more than 7 years ago | (#16004162)

To get to the other side!

Mod me up, Baby! You know that's at least as funny as the parent post!!

Equal to. (1)

digitalsushi (137809) | more than 7 years ago | (#16004047)

I just make mine equal to my ram these days.

I use this (5, Insightful)

Anonymous Coward | more than 7 years ago | (#16004062)

2X physical memory for under 2G RAM
2G swap for up to 8G RAM
+1G swap for every 4G RAM beyond that

Re:I use this (1)

Asm-Coder (929671) | more than 7 years ago | (#16004148)

You can load more than 4gig on a computer. I know Windows combined limit used to be 4gig ram + virtual, but I didn't ever encounter a limit on any other OS. But I've never seen a motherboard capable of handling more than 4 gig of ram.

Re:I use this (1)

afidel (530433) | more than 7 years ago | (#16004412)

Windows 2003 Standard x86 is limited to 4GB of virtual address space. 2003 Enterprise x86 is limited to 64GB. Full details available here [] . Personally I work with server with more than 4GB daily, though so far none of ours have more than 4GB per processor board =)

Re:I use this (1)

Malc (1751) | more than 7 years ago | (#16004501)

1x-2x for 2GB

I use a range so that I get a warning from Windows when it is increasing the swap space. I really don't want to be using the maximum value because everything is slow and the hard drive is being thrashed. With more memory, I don't need so much virtual memory - it's just there as an emergency buffer if I have a run-away process or run a new app that I'm not used to.

Every watched SQL Server? It does not swap. Instead it reduces its memory demands when the system needs more RAM. Interesting solution. Makes sense too... all it's data is already on disk. Sorry, tangential...

Re:I use this (1)

Malc (1751) | more than 7 years ago | (#16004513)

Ooops: should have proof read. I would have spotted /. treating my text as HTML...

This is what I tried to say:
1x-2x for <= 1GB
0.5x-1.5x for <= 2GB
512MB-1024MB > 2GB

1GB ram using XP (3, Informative)

Karloskar (980435) | more than 7 years ago | (#16004066)

I disable virtual memory on computers with more than 1GB of ram unless the user is going to be manipulating large images. Never had a problem yet.

Don't forget disk cache (5, Insightful)

uler (583670) | more than 7 years ago | (#16004121)

One of the real advantages of using swap isn't to avoid memory exhaustion at all; by moving infrequently accessed pages from memory you make more room for the disk cache, thereby possibly improving overall system performance by reducing hard drive reads.

You've got virtual memory?!? (1)

EmperorPenguin (983135) | more than 7 years ago | (#16004165)

Let's create a ramdisk!

Re:1GB ram using XP (1)

goarilla (908067) | more than 7 years ago | (#16004371)

You don't always know if your process is gonna handle large images
A short example click on this link 9 []
My brother posted this link to me once because
When i clicked the link and watched all of the pictures ... my ram was gone and i only had 80 mb swap left :S
I couldn't even admire the next round without killing firefox first

Re:1GB ram using XP (1)

fitten (521191) | more than 7 years ago | (#16004373)

You disable virtual memory? That can't be good. I'm assuming you mean the swap file, which is not the same thing as virtual memory. In any case, conventional/modern recommendations is to leave it alone and leave it set at letting Windows manage the size. There are oodles of postings as to why (search for posts by DriverGuru, particularly on ArsTechnica) you should do this. It's a question that comes up frequently.

Depends... (1, Informative)

PianoComp81 (589011) | more than 7 years ago | (#16004069)

It depends on what you want to use. If you don't care about Hibernate mode, then you probably wouldn't need any swap (or much). However, if you want to use that mode, you need to have at least the same amount of swap space as memory. I've tried it with less, and it wouldn't even attempt to go to sleep (for obvious reasons - swap is used to store what's currently in RAM when going into hibernate mode).

Re:Depends... (0)

Anonymous Coward | more than 7 years ago | (#16004109)

No it's not.
There's a seperate hibernation file for this purpose, hiberfil.sys in c:\ (or whatever disk you have windows on)

Re:Depends... (1)

aminorex (141494) | more than 7 years ago | (#16004265)

So what happens to the data that is already in swap, if your swap is all required in order to hibernate? It's an endless recursion, unless you have a swap partition that you are not currently swapping on, to which your system is yet clever enough to hibernate.

Re:Depends... (4, Informative)

Limecron (206141) | more than 7 years ago | (#16004446)

This is completely wrong.

In Windows, your RAM is saved to a file called "hiberfil.sys" which is the exact size of your physical RAM. Your swap file stays exactly the way it is, otherwise you'd lose the data that was swapped to it.

In Linux, it depends on what program you are using to suspend, but typically, it's a file in /tmp.

Well, there's what I do and then there's reality (3, Informative)

TLouden (677335) | more than 7 years ago | (#16004079)

If you really want to know, I use 1-2 GB swap with 1GB ram and the same for 512MB ram.

However, you might just do what I do and try out different values to figure out what works. If you're talking about a linux system a real-time memory/swap usage graph can be added to most window managers so that you can see what's happening. You could also try to estimate usages based on what the machine is expected to do.

Think for the future (1)

solafide (845228) | more than 7 years ago | (#16004101)

I have 10GB swap on my desktop with 2.5GB RAM, because I have 700GB of hd space, and I don't have to be conservative with it. 10GB is enough for any application that I'm liable to install to need for the next, eh, 4 years, and in 10GB swap you can have some incredibly huge python range() arrays.

On the other hand, on my laptop with 256MB, I have (iirc) 1GB swap, and on my laptop with but 96MB ram, I have (again, iirc) 25% of my disk in swap. (But then, the disk is only 2GB anyway...)

So do as much as you can spare. Sure, you may never use it, but hd space is cheap these days.

Re:Think for the future (1)

Millenniumman (924859) | more than 7 years ago | (#16004128)

Hard drive space is cheap and plentiful for desktops, but it is scarce, no matter how much you are willing to spend, for laptops. The biggest for a SATA drive is 120GB, and the biggest laptop drive is 160GB. And with a laptop, an external drive is not a convenient option.

Re:Think for the future (1)

Barny (103770) | more than 7 years ago | (#16004149)

Yeah, the OP asked the wrong question, should be "what % of your very cheap hdd storage do you use for swap?".

Personally i use 5G of mine, thats enough should anything start gobbleing it up (have seen one buggy game use over 6G once, 1.8 in real mem rest was swapped).

But then, i beta a lot of games that have memory handleing like the titanic had water pumps ;)

Excuse me while I reminisce... (1)

ian_mackereth (889101) | more than 7 years ago | (#16004106)

The first time I used a friend's 128k RAM Macintosh, I noticed how busy the HD seemed to be.
After some poking around in the system, we found that we were in the topsy-turvy situation of having the OS running in RAM and all the applications running in the swap file on the HD!

As soon as he got rid of the silly voices and other frippery (cool, though!), it went back to behaving in a more sensible manner.

I think RAM prices have fallen faster than HD speeds have risen, so it has more impact than it used to to have applications requiring swap file space.

Re:Excuse me while I reminisce... (2, Informative)

Anonymous Coward | more than 7 years ago | (#16004206)

You're full of it.

1. The 128 KB Mac did not have a HD (though there were some companies that made disks that plugged into the floppy port).

But more importantly:

2. There was no "swap" (Virtual Memory) for the Mac OS until System 7, which wouldn't run anything less than a Mac Plus.

More is better! (5, Funny)

Millenniumman (924859) | more than 7 years ago | (#16004116)

I use 4x750 GB hard drives (RAID), purely for virtual memory. It increases the speed on the RAM preprocessing directive, but demodulates the core processing utility monitor. I find it to be a good setup, especially for running Naibed Linux.

Re:More is better! (1)

SilentUrbanFox (689585) | more than 7 years ago | (#16004201)

You forgot to mention the deflector dish.

Set it and forget it (4, Insightful)

StikyPad (445176) | more than 7 years ago | (#16004118)

According to MS, it's 1.5 times the total RAM [] . I assume you're asking because you're trying to avoid a fragmented page file. While the benefits of an unfragmented page file are dubious at best (since it will be randomly accessing different parts of the page file), it's better to err on the side of caution: If you have 2GB of memory, you likely have an equally compensating-for-something hard drive, so you probably won't miss 3GB of space, or even 4. It's better to waste a little space than have Windows run out of Virtual Memory. Otherwise, just let it do its dynamic page file adjustment thing.

If you're asking about creating a swap partition for Linux then 1.5X is also recommended. Just be generous, unless -- for some reason -- you've got 2GB of RAM and a 50 meg hard drive. Too much is always better than not enough.

Re:Set it and forget it (0)

Anonymous Coward | more than 7 years ago | (#16004239)

While the benefits of an unfragmented page file are dubious at best (since it will be randomly accessing different parts of the page file)
The benefit of an unfragmented page file is not for the page file itself, it's for the rest of the file system. Fewer fragments on the page file means larger continuous blocks for the rest of the disk...

Re:Set it and forget it (1)

Doctor Memory (6336) | more than 7 years ago | (#16004353)

This seems like a reasonable limit to me. The last time I installed Solaris it wanted to give me ~800M (on a machine with 1G of memory), I overrode it to take a gig. Of course, under Solaris /tmp is mapped to swap space, so it's usually a good idea to give yourself a little more room anyway. OTOH, even debugging stuff in NetBeans while running Sun's app server, I've never eaten more than half a meg of swap, so it looks like Solaris at least has a pretty good swapper. I do recall the only time I ever had NetBSD crash on me, I was rebuilding the kernal and passed "-j" as an argument to make(1) (which tells it to create as many subprocesses as it wants to process things in parallel). Ran out of swap and everything went dark....

Still 2-3x physical RAM (1)

georgewilliamherbert (211790) | more than 7 years ago | (#16004123)

I still use a small multiplier, typically 2-3x physical RAM, for swap partition sizes on Solaris, Linux, xBSD, etc.

Systems typically are paging less now that we have multiple gigs of RAM per server, but if something goes wrong, the disk is so cheap that having the overhead installed and ready to use is fine. Having a live, active safety margin is just good sytem planner sense.

If you skimp on OS hard disks so much that 2-3x physical RAM is an excessive chunk out of the hard disks, then you're doing something wrong.

Remember though... (1)

spysmily1 (962459) | more than 7 years ago | (#16004136)

I'm not sure about any nongaming applications, but some games require that you have a swap file regardless if you need it or not. This will usually be indicated by an error message stating this fact or some other oddball message. Of course this is an easy fix, just start using a swap file again

auto (3, Insightful)

Joe The Dragon (967727) | more than 7 years ago | (#16004137)

just let windows set it for you.

How Much Virtual Memory is Enough? (1)

fishtop (998697) | more than 7 years ago | (#16004139)

The conventional wisdom was (1) for Windows and (2) when memory was a lot more expensive. In general, you never want to use virtual memory. You absolutely never want to need it. This has been true since the days that swapping was done to fixed head drums. You do not want to rely upon it while you are doing anything important. Get enough real memory, and set the virtual memory to whatever makes sense for your usage pattern.

My rule (1)

Nezer (92629) | more than 7 years ago | (#16004144)

Here is my rule and is founded on years of managing Unix/Linux systems professionally.

Never, ever use less than 1x physical RAM. Some operating systems use swap space on disk to through core dumps on a crash (Digital Unix... err Tru64 is one). Without a single swap space big enough to hold a physical RAM dump you might be SOL when it comes time for the vendor to diagnose. (In the case of Tru64 you're already SOL because they can't diagnose anything.)

2x physical, which is what I was always told as the recommended amount, is just ludicrous. It was then and it is even more so now.

So my rule is, first, know how much memory your machine uses under heavy load but not when the system is ailing because of an app leaking memory or some other out-of-the-norm issue. Now add that, plus a reasonable margin of error (I like 50%), to your physical and you have a good number. Remember what I said above about always having at least physical? It's very likely (though I've never run into this issue) that a system that wants to dump core to swap space may very well want to dump what's in active swap along with it. As far as the OS is concerned this memory space can be important (though, in the case of a system panic it seems impossible that the offending code would be sitting in swap space).

It should go without saying but the amount of paging space actually used should always be kept to a negligant amount. If this gets out-of-hand you need to add more RAM.

Finally, it's better to err on the side of too much than too little.

Re:My rule (0)

Anonymous Coward | more than 7 years ago | (#16004383)

As memory serves (haha!), under certain Unices popular at the time Linux first started attracting attention, the 2x rule was a necessity. The reason was: memory writes went to both physical ram and the swap space. It sped up swapping out (you could just throw away the physical pages, since they were mirrored in swap), and if you had less than 1x physical you weren't getting any benefit at all. (And although it may have seemed ludicrous if your OS of choice didn't do this, it wasn't ludicrous if you had to take away your physical memory's size...)

Obviously, neither scenario is the case any longer, so pretty much it's useful as a buffer to hold little-used memory (for instance, initialization code on applications you start and leave running for a long time), or as an emergency buffer when memory usage spikes.

Still using the convention... to a point (1)

cyanics (168644) | more than 7 years ago | (#16004175)

I typically will use no less that 2g for swaps, however, as the more memory is added to the system, the less swap I use.

It is never less than 1 - 1.5x the ram size. You NEED to have enough space to dump the entire contents of memory out in the event of a panic (for tracing) and then anything else that is still resident in swap at the time. Start with 2GB and add 1GB per 512MB of memory. You should be sitting safe in most situations.

Let the OS decide (1)

hackwrench (573697) | more than 7 years ago | (#16004177)

I let the operating system increase and decrease as it sees fit. It has a better picture of the situation than I do. I use Windows XP, though. Is this possible in Linux? I find it strange that someone hasn't mentioned this already. Usually the latest beta of Firefox, Minefield, triggers the OS to grab more.

Re:Let the OS decide (1)

KillerBob (217953) | more than 7 years ago | (#16004426)

Linux doesn't work that way. Of course, Linux also has a significantly lower memory overhead, usually, and is also a lot more conservative about how it allocates memory. Usually, a system won't actually start swapping unless you have a huge number of programs open, or you have very little physical memory.

Case in point... my laptop has 1GB of RAM. Not really that much by modern standards, if you ask me, but not really important, either. On the laptop, I have had, at the same time, GAIM, gxine (listening to streaming audio from Club977), GIMP (like Photoshop), and running through Cedega, Guild Wars. On a system with an Athlon64 3500+ and 1GB of RAM, there was absolutely no swapping happening at all. Now, I've got 2GB of SWAP set up on that laptop, but with that much stuff open at the same time, it says something that there's no swapping happening.

By comparison, my desktop machine runs Windows XP Professional (yes, it's legal). Said machine has an Athlon64 3000+, so it's pretty much comparable to the laptop at that end. It also has 2GB of physical RAM. Also, where the laptop has a Radeon XPress 200M 128MB, the desktop has a Radeon X1300 256MB. Strangely, under XP Pro, I have had my system slow down to an absolute crawl while playing a single instance of Guild Wars (and having nothing else running), because I had set the swap to start at 32MB and grow to up to 1024MB. Why? Because even though Windows had twice the physical RAM of my laptop, and even though Windows didn't have nearly as many programs open, Windows still felt the need to increase the size of my swap file because it was doing swapping. Ultimately, I set the swap on that machine to be permanently 1024MB.

The Winbox has a *lot* more overhead, and the Windows OS is significantly more liberal in its memory use and memory distribution. It's not as good at garbage collection. And before somebody points it out... the lappy is running Linux, with X.Org 6.9 and XFCE 4.3.90 with compositing enabled (and full transparency effects), and ClamAV 0.88.2, on top of SSH server, cpu frequency scaling (the Conservative governor, usually underclocking to 1GHz from 2.2GHz peak). The Windows machine, at startup, is running AVG Antivirus, RKLauncher, Window Shade 1.2, and... Windows.

As to the actual question posed... it's been answered dozens of times, but I may as well chime in with my $0.02. As you can probably guess, how much virtual memory you need depends on what you're doing, and what OS you're using. Under Linux, with a respectable amount of memory and reasonable use, I would say you don't need virtual memory at all. By Respectable, I'd say 1GB+. If you're running less than 1GB of RAM, or if you're going to do some heavy use, I'd say 1:1 Physical:Virtual, with a caveat: Physical + Virtual should probably be at least 1GB. Under Windows? Let the system manage it, most of the time. Just make sure that you don't let the system give you so much virtual memory that Physical + Virtual > 4GB... All versions of Windows that are currently available only create memory addressing space for 4GB, and any more than that simply won't be used. Vista changes how memory is addressed, but it's not on the market yet. :)

Enough? (1)

AJWM (19027) | more than 7 years ago | (#16004200)

What is this "enough memory" of which you speak?

Oh, wait, virtual memory. I thought you were talking about real memory.

Never mind.

No swap at all (3, Interesting)

DrZaius (6588) | more than 7 years ago | (#16004207)

I think it was one of the Live Journal guys at OScon that said, "If your server starts to swap, you've lost the battle".

With all of our 64bit 4GB of ram minimum hosts floating around, there is no longer a point to having swap -- if you server really is swapping, it's under a huge load and the io is making the problem worse. Let the OS kill a few processes to get it back under control

Re:No swap at all (4, Interesting)

georgewilliamherbert (211790) | more than 7 years ago | (#16004245)

If the server starts to swap, you've lost the battle. But randomly killing things or locking up is losing the war.

It's fine to set off alerts and alarms if you're paging. You should set off alerts and alarms if your servers start paging. Randomly killing things instead? Insanity.

You can never build reliable services for users/customers unless you can handle random or accidental error conditions gracefully. Swap space is a cheap and easy key way to do that.

Re:No swap at all (1)

Rufus211 (221883) | more than 7 years ago | (#16004376)

If your server starts to swap, you've lost the battle

Not necesarally. A smart pre-emptive swapper can swap out things even if you have 50% free memory. If some amount of memory is only accessed extremely rarely (how often does '/sbin/getty 38400 tty2' really get used, or if a program starts leaking memory) the swapper can stick it on disk. Now this dead block of memory can be used for more cache, which is always a good thing.

You are thinking of "thrashing." (1)

Ivan Matveich (998090) | more than 7 years ago | (#16004377)

Swap devices are intended to hold long-inactive pages, like those of a stopped process. They are NOT "virtual memory."

Highly dependent (0)

Anonymous Coward | more than 7 years ago | (#16004230)

For me, my systems are highly dependent on what I do with them. Currently my swap space/file is at 0.75GB on a system with 1GB RAM. The only times that I would have encountered problems running out of memory is when dealing with Adobe's memory hogging programs. Since I'm not that big on graphics and video manipulations using those programs, I rarely run out of memory anyway.

However, on my removable hard disks where I do general testing, I set my swap size to none. 1GB physical RAM is more than adequate for my needs.

heavy windows usage = 0, anything else = default (2, Interesting)

Fry-kun (619632) | more than 7 years ago | (#16004311)

OP poses wrong question. Virtual Memory is built into the OS and cannot be turned off. What OP means is Paging or Swap File (i.e. simulating memory using HD space). The rest of this reply will ignore this difference.

Very simply, if you use windows and use it heavily (run some intensive tasks or need performance), turning off the page file will give you a nice performance boost.. or rather will not take away from performance.
I have 1GiB of physical memory on my laptop, and reaching the limit in Windows when my paging file was off, posed a challenge (in other words, it worked perfectly well without it)
This is because Windows attempts to use the paging file whenever it can (proactive), unlike Linux, which uses it only when there's no other way (reactive). Depending on the applications you're running, one of the approaches will be better than the other, though from what I've seen, I don't like what Windows does...
Caveat Lector: this might be because I wasn't seeing the slowdowns which might've been caused by reactive approach. I've still yet to formulate an opinion on it - but so far it looks very reasonable.

If using Linux, keep the swap partition and forget about it.
In Windows, the best way to figure out if you need your page file is to load up as many apps as you normally load, maybe a few more - and check the memory usage (don't trust "VM usage" in windows task manager, it doesn't show you what you think it shows you!). If the usage is lower than your physical ram by a [few] hundred MiBs, turn off the page file and don't look back. If it's closer, set the page file to a small size, usually no more than 512MiB. If you set the file, make its size static, so that Windows doesn't try to adjust it all the time (it's too stupid to understand that you want to keep it as small as possible)

Interesting to note that the paging file is not used for hibernation, even though you'd think it were almost tailor-made for that purpose. I've heard that early betas of Windows 2000 woke up from hibernation in a few seconds - I bet they were using the paging file for hibernation then... but I digress


None! (1)

vio (95817) | more than 7 years ago | (#16004347)

I gotta agree with what many have already said... monitor your usage for a while, and then adjust accordingly. I haven't had a swap file in over a year in any of my desktops/workstations (1-2GB of RAM) and with prices of RAM nowadays I don't forsee myself ever needing another swap file again. ... and killing that swap file is sooooo liberating :-)

Virtually none (1)

JBob-S (264875) | more than 7 years ago | (#16004369)

"Memory is like an orgasm. It's a lot better if you don't have to fake it." -- Seymour Cray, on virtual memory.

This is even more true today, with memory speeds growing faster than disk speeds. Don't use virtual memory for anything that is actually used more than once. I was recently asked to bump the swap on a 128G machine to 1X; imagine how long it would take to access all that memory only once, let alone multiple passes. That machine has a 1-2G swap. Any process that actively used more than 128G wouldn't finish in the lifetime of the project anyway.

It may make sense to enable a lot of swap if you're running Linux and turn off memory overcommit. This is "unused" memory that's required because many programs only run efficiently with overcommit turned on, and you don't lose performance because the memory is never addressed anyway.

Swap in any case should be based on memory size alone only for special purposes like suspend; otherwise, use the largest image you expect minus physical RAM.

Think About The Future + More (1)

alchemist68 (550641) | more than 7 years ago | (#16004379)

Memory usage depends on what YOU are using your computer for and the platform. I have an Apple PowerMac G5 with dual 2.7 GHz PowerPC G5 processors and 2.5 GB of RAM and IT ISN'T ENOUGH! Mac OS X pages out to virtual memory almost every day, because I'm constantly using Mozilla Firefox, Apple's Mail program, Micrsoft Office, OpenOffice (under X11), iTunes, MacTheRipper, HandBrake, Google Earth, VueScan, Preview, Adobe Reader, iCal, AddressBook, iWeb, QuickTime, AOL Instant Messenger, Skype, VLC, DVD2OneX2, Toast 6 Titanium, RBrowser, iPhoto, iMovie, iDVD, and iChat. If you have a Macintosh running under a recent Mac OS X, you'll want as much RAM as possible to avoid paging-out to virtual memory - makes your Mac faster overall.

Equal (0)

Anonymous Coward | more than 7 years ago | (#16004387)

If the computer has a lot of memory (1GB+) I usually set the swap partition to the same size as the memory. Works great for me.

Disk seek times haven't improved much (1)

pyite69 (463042) | more than 7 years ago | (#16004395)

A machine with 9ms seek times and 4G of swap in use is probably going to be unusable. Everyone seems to over-engineer swap space a little too much, 2x the amount of memory you have isn't a bad rule unless you have a very unusual situation.

old discussion (1, Informative)

Anonymous Coward | more than 7 years ago | (#16004411)

There's an old discussion [] amongst various kernel developers about this issue which, even though it's a couple of years old, is almost certainly going to be more insightful than anything you'll read on slashdot. You'll note that there doesn't seem to be a ready consensus even amongst the folks who know this stuff best, so if you see anyone posting the "correct" answer, call bullshit.

My advice is that you should just do the same thing as some random poster on slashdot who says "these days I set up my swap like blah blah blah" without any explanation or justification.

hibernation in windows (1)

sankekur (998708) | more than 7 years ago | (#16004423)

windows 2000, xp, and server 2003, do not use the swap file for hibernation, they use a file called "hiberfil.sys" in the root of the system volume. it is equal in size to the memory in the computer.

Eliminate Swap! (1)

SkyLord (8650) | more than 7 years ago | (#16004432)

On Larger systems, where the system and applications can be tuned to operate within a performance envelope, I think swap should be eliminated. It's a legacy concept, and will drag down any system if it has to operate in swap actively.

If the system is small and resources are scarce, then swap is needed, but I would still think the best solution would be to get a new system or expand resources to accomodate. Just piling on swap to handle an application that is overrunning RAM is like trying to put out a fire by throwing logs on it.

Personal Note: The fact that large database vendors can recommend 1x to 2x swap to RAM on large systems that have upwards of 256G of RAM is ludicrous.

Back in Windows 9x days there were tricks. (1)

The MAZZTer (911996) | more than 7 years ago | (#16004465)

Specifically, you could set your swap file to a FIXED SIZE to keep Windows from resizing the swap file, thus theoretically upping performance due to less disk activity.

Today in XP, I set a decent minimum size (1.5GB, to complement my 1.5GB memory... I had 0.5GB at one point and I haven't changed my virtual memory size since then, no real need to.) and then set the maximum as high as it can go, for emergencies (memory leak from hell). 32bit Windows XP allows 3gb maximum size (so it tells me), so that's what I have it set at.

Another old trick that works as well today is, if you have multiple hard drives, move your paging file to the non-Windows drive. So I have my C: drive set to no paging, and D: set to 1536-3072 (Windows recommends 2302 for it, fyi).

Unfortunately I have no clue if it actually produces measurable benefits, and I have no motivation to do benchmarking to see. For most people it's probably just best to leave it be at "System managed size" unless you know what you're doing.

4GB RAM, 4GB swap (3, Insightful)

Agelmar (205181) | more than 7 years ago | (#16004486)

I have 4GB of physical ram (ddr2-6400) and 4gb of swap. There are actually a few reasons for this, YMMV (obviously I think the answer to this question depends on what you do).

I have a lot of things running which, usually, are doing nothing. For instance, apache2, mysql, postfix, and courier-imapd-ssl are always running, but they're rarely actually *doing* anything. (If I get a hit or an email, it's relatively rare as I hardly have very little hosted off of my home box - nevertheless, I do want these running). So I'm happy to let these get swapped out. When I start up matlab, and start dealing with huge datasets, I know it's going to swap most of these out. That's good. It will also swap out some of my matlab data that's loaded but not currently being used (and yes, it's quite possible to have >4gb in your workspace). For me, I have the swap because I need it. Figure out what you need, and you will have the answer to your question.

Depends of if CPUS are Hot swappable (2, Informative)

markk (35828) | more than 7 years ago | (#16004512)

This depends - see other comments for most situations. However if you have a large Sun, HP, Fujitsu, IBM, etc. with 16+ CPUs and say 2 to 8 Gb per CPU (not uncommon in the big systems), then at minimum you need 3 times the --per CPU-- memory, becuse if one of the CPU's goes bad, the hot swap mechanism is going to use the swap space to keep the processes (at least on some of these systems) for moving them to the other CPU's as it marks the one as bad. You certainly don't need 2 times the total memory, or several hundred Gig. This is assuming the kind of NUMA architechure that I think all of these systems still have.
Generally we just used to use, say, 36 Gig local drives as (mirrored) swap for simplicity. In this environment you are probably on a SAN and people will say to move everything there, and that might be more true now than a year or two ago.

As always, it depends (1)

cookd (72933) | more than 7 years ago | (#16004519)

You need to add physical memory until your performance is adequate.

You need to add swap until you have enough. You have enough if you don't run out of (virtual) memory while doing whatever you need to do with the system.

On my main (fast) system, I have a lot of physical memory. I have about 1.5X that much swap set up, because occasionally I load a lot of programs at once.

On a test (slow) system, I have very little physical memory. I have about 10X that much swap set up, because I still need to run programs that allocate a lot of memory, but their working set tends to stay small, so they run ok even when 90% is in swap.

size of a core dump (0)

Anonymous Coward | more than 7 years ago | (#16004524)

The size of swap should be large enough to hold the contents of kernel memory when a panic+core dump occurs. Anything larger than that depends on workload.

The way I use swap (2, Interesting)

Thaidog (235587) | more than 7 years ago | (#16004536)

Many people think that you should save the fastest part of the HD to allocate as swap. But after thinking about it no matter where your swap is on a hard disk it's going to be noticeable when the system pages. what is not noticeable is the perceived difference in human terms of the VM at the outer most of the platter or the innermost. And if your allocating a gig of VM or more you're wasting space for system files or applications files where things like time to boot and application launch speed can be faster. (Along with prefetch data) In Linux you can use riser for your system root to further optimize this since most system application files are small in size. (Along with a small block allocation size.) Also, if you're using a secure VM file you may notice a difference if you move the paging file to a secondary drive with the file allocated at the outermost section. No matter what the system I make a page file. If it needs to be secure you should lock the applications pages to ram memory. I any event when using a disk you're going to notice paging and at that point you'll be waiting anyway... I doubt you'll care or notice at that point the page finsihed 2ms faster or not.
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