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Is Swap Necessary?
johnnyb writes "Kernel Trap has a great conversation on swap, whether it's necessary, why swapless systems might seem faster, and an overall discussion of swap issues in modern computing. This is often an issue for system administrators, and this is a great set of posts about the issue."
One can have 1GB of RAM for a fairly cheap price.
I really doubt that majority of newest desktop PCs need to swap on the HD at all.
The unused/used portions argument from the article isn't quite true. You don't have to swap every unused bit,
if you have enough RAM, leave everything there. It's R-A-M. don't access parts you don't need.
If you don't have them in the RAM, read them from the drive,
don't waste time putting them where they mostly are in the first place.
I'm willing to bet that people who need performance, don't often run 10 applications at the same time. If they do, they
surely know what are they doing.
IMHO the average user should get enough RAM and no swap, let the OS optimize things a bit.
I ran linux without a swap file on 128 MB of memory a couple of years go. It was an accident, I didn't create a swap partition. I never had a problem (forutnately). Of course, I wasn't doing the heavy duty stuff I am now (scientific computation).
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Sometimes, when a process goes haywire, it will start munching RAM. If important programs like, say, sshd or X, can't malloc when they need to, they'll die ignominiously. Swap gives you the chance to kill the rogue process before your OS goes kaput. Its slowness can actually help for this.
but today's production, heavily loaded system will still need the ability to swap to/from disk.
Already, there are systems that minimize that need, set-top boxes, embedded systems in general. But each of those is seriously modified (kernel-wise, mostly) to achieve the responsiveness, the frugality of resource treatment that a general purpose desktop computer can't expect to enjoy.
That doesn't mean that developers should stay in the same rut, assuming that hardware that confined system design in the '60s, '70s... '00s will perpetually assign similar constraints.
IMO, desktops still need to swap... for now. but let's not paint ourselves into a performance corner.
Notice how sluggish the system is after doing something disk-intensive like watching a movie. That's because the kernel is caching as much of the movie as possible to memory and swapping your running apps out. And kernel developers think this is a good thing, so it isn't going to change any time soon. IMHO for a desktop system this makes no sense, that's why I run my 1GB RAM machines with zero swap.
As I RTFA & previous comments here, I was rather suprised at how argumentive people were getting over this. Some people are saying swap is an absolute necessity & a swapless system was a broken system, while other's said swap was an obsolete solution to a problem that no longer exists (expensive RAM). This seems odd to me, because as far as I can tell, the decision of whether & how much swap to use is based mostly on two things: specific situations (and thus there is no general answer to 'Is Swap Necessary?'), and opinion. And either way, with the Linux kernel today (and for quite a while now), I can choose for myself whether or not, and how much, swap I want to use. So if I am in a situation that I think requires swap, I can use it, and in a situation that I think would be hurt by having swap, I don't have to use it. So I don't see why there's so much hoolabaloo about this: nobody is forcing anyone to do it one way or the other. And if someone else thinks it should be done different from how I would do it, that's their decision, not mine.
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In the 90's, I ran a 10 line BBS on an Amiga 4000 with 16 megs of Fast ram, 2 megs of Chip ram, and 0k for the swap file. :)
I know, I know, the Amiga didn't HAVE virtual memory. Well actually it did if you had an 040 and installed a memory management program such as GigaMem, but so few people had a use for such a thing that it was practically unheard of.
Oh, and before someone jumps in saying that I wasn't able to do anything else, that is totally NOT the case.
Very often I was doing lots of stuff. The difference is developers were used to working within memory constraints, and now days developers are used to systems growing into the applications.
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The "swap=2x RAM" thing is obsolete admin trivia that simply refuses the die. It comes from the days when physical RAM was mapped into swap to simplify the swapping algorithm. If you didn't have at least a 1:1 correspondence between RAM and swap performance would suffer immensly. Starting with Linux 2.4 and up this is simply no longer true, there is no benefit from using excessively large swap partitions. Same goes for Sun OS and the BSDs these days.
Instead, the swap needed depends on the sort of usage pattern your machine has. If it's a desktop with 1-3GB of RAM, a swap partition of 1GB is completely adequate. Want the machine to swap as little as possible and utilize all the RAM, so turn down swappiness a bit to avoid Mozilla/Firefox from being paged out when you leave for 15 minutes.
On a server you need a whole lot more swap, the more the better. Not because it's necessarily any faster, it might be slower in fact with a high swappiness setting the system decides you don't really need that 2GB DB in memory if it's been unused for a month. But when you do run out of memory in legitimate use, the shit will really hit the fan if there isn't enough swap to pick up the slack.
I forgot to explain swappiness. This is a entry in proc, /proc/sys/vm/swappiness, that you can plug a numerical value between 0 and 100 into. The higher the number, the more eager Linux will be to swap out applications from RAM to disk. There's a lot of conflicting opinions on what values you should use. Kerneltrap had a good article on it recently.
Personally I use a value of around 20 or less for desktop machines. This keeps Mozilla being paged out after a short while, that really shouldn't be happening on modern hardware. Too bad you can't achieve the same effect in Windows 2000. Some people swear that a swappiness of 0 is ideal for their desktops, your mileage may vary. It's fun to play with in any case, any changes you make take effect instantaneously.
there's a definite pattern with regard to swap in the windows world.
for win'9x: use up ram until almost gone then start allocating swap space in anticipation of actually using it. should memory allocation still be increasing then actually use swap space. reverse the order when freeing memory.
i had 384 megs ram at the time and as long as i used less than about 350 megs total the system wouldn't be in swap.
for win 2k & xp: (when within physical ram limits) whatever amount of memory is requested, allocate between 60-80% to ram and the rest to the swapfile. even the disk cache partially goes to swap! i didn't believe it at first but all one has to do is look at the numbers in the task manager's memory/cpu window. at first i figured that all i'd need to do is throw in some more ram and the disk thrashing and absolute crawl would go away. i put in a gigabyte of ram (i never allocate more than 700 megs at most and the total system memory usage on bootup is 100 megs). even with the extra ram the problem stayed the same.
turning off swap gives me consistent fast performance, and since the disk cache isn't swapped (partially) i get 2x the throughput i had with a swapfile on large file copy operations
machine tested: duron 1.3ghz, 1 gig pc133 ram, 2x 80 gig wd800jb hdd.. os win2000 & winxp running newsbin which allocates disgusting amounts of ram in a large header grab (yeah i could have used a test program but why do that when newsbin is a real-world test for me). the os and applications are on different drives on their own ide chains
with swapfile enabled (size=1.5x system ram).
allocation time: unaffected, only the time to perform task reqested
memory de-allocation time: (by either quitting app or selecting another group) 23 MINUTES of constant disk thrashing
with swapfile DISabled
allocation time: unaffected, only the time to perform task reqested
memory de-allocation time: (by either quitting app or selecting another group) 2 seconds
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There are two "theorems" quoted: The first says that no matter what, if you have a size X of RAM used by the OS, and you add a size Y swap disk, you get better OS performance than if you only had X RAM.
The second "theorem" says: if you have X RAM + Y swap disk, then add Y RAM and use that instead as the swap disk, then you get *faster* performance.
The naysaysers now say that the second statement is misleading. Why? Because with X+Y RAM and Z swap disk, you'd get better performance again.
I think this betrays an underlying assumption which I'm not sure is true, namely: X+Y RAM managed by the OS any way it likes is always better managed then X RAM managed by the OS any way it likes and Y RAM reserved for swap operations.
In fact, let us suppose that the OS memory management is not optimal, ie when the OS manages X+Y amount of RAM, it does so suboptimally. Then it is possible that a different memory management scheme, e.g. X RAM used normally + Y RAM used exclusively for swap, may turn out to better use the available total RAM.
So the theoretical question is this: is Linux's memory management sufficiently optimal that with an ordinary set of applications running, it can always make better use of X+Y amount of RAM than if it always reserved Y for swap? Alternatively, under what kind of running application mix is it true that reserving Y amount for swap yields a better memory management algorithm than using X+Y fully?
Here's a real-life example of why swap is useful. One machine I manage has a gig of ram. At the time of purchase, that seemed quite reasonable. But the users are working on a project that takes 2 gig of ram. So currently it's using a gig of the swap. Yes, that's bad, and I'll be adding a second gig to it in a few days (it's in the mail). But in the mean-time, that swap space is really handy. It means the users can get their work done! Think of the first 256M of swap as being for speed. If you're regularly using more than that, then it's time to order more ram. But it's nice to have the spare gig of ram for odd jobs, or while you're waiting to install it.
I'm no expert, but I think a lot of these arguments could be resolved if people took advantage of the ulimit constraints. If you can limit how much a program can get out of control, then there's no longer a concern for a single user sending the server into swap hell. One of my current projects is to figure out reasonable limits.
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The 2.6 kernel now has a swappiness setting in /proc where you can tell the kernel avoid swapping please (set it to zero) or swap like mad (set it to 100). Therefore you can tune your system to your specific needs. It'd be nice if they had a similar control for filesystem cache.
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I don't know if Linux works this way, but...
UNIX kernels have assumed the availability of swap for nearly 35 years. You cannot remove this major architecutural feature without unintended side effects.
Once in a while I'll do something like 'grep -r "oops" /big/filetree'. The fact of the matter is that I'm probably only reading any of that data ONCE, and it's not going to all fit in memory anyways, so I don't even gain anything if I run the grep a second time.
In a situation like that, I'd like to have some sort of 'nocache' directive that says 'Don't waste the cache with this'.
Something else that might help would be to have some sort of 'minprog' directive which would tell the swapper that a certain amount of space is reserved for 'program' data (i.e. code (including shared libs) and data), -- and that that memory shouldn't be swapped out in favour of something otherwise being read from disk. I think that this might avoid the situation that I sometimes run into of a large program (mozilla/gimp) being unresponsive after I do some other disk-intensive task (like the aformentioned recursive grep).
Things like the OS enforcing things like the RSS rlimit hints would also help. (I hadn't previously realized that it didn't).
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My boss started worrying that we weren't going to be able to deliver what the company had contracted to deliver. He was the antithesis of a PHB and so he sat down and in a few hours wrote a small driver to emulate the overall task the project had to accomplish. No detail, just broad brush emulation. He was able to demonstrate with a few lines of code that nothing we could do would hit the delivery spec. Burroughs responded by doubling the amount of RAM on the box as well as installing RAM that was twice as fast as what they had initially delivered. The combination enabled us to turn off swapping and deliver a working product.
Fast forward to 2004 and I'm working on Excel spreadsheets that have 60-70 sheets in a workbook. Saving the book is a bitch - 15-20 second wait after I hit ctrl-S. Every so often, Excel just goes away as it performs a prophylactic background save just in case Excel dies. 15-20 second pauses because the software has become so bloated that saving a 2-3 meg document is an excuse to flog the poor drive into a seek frenzy. The drive, which was about 4 years old, finally gave up the ghost. Its replacement has an 8 meg cache separate from the 512meg Windows manages - that "little" 8 meg junk of RAM belongs to hard drive alone. Night and day performance difference. The Excel swap frenzies that were induced by a simple ctrl-s are gone. 3 meg documents save in under a second - just what you'd expect from a drive that has a transfer speed in excess of 60 mbytes/sec.
My sense is that swap has always been a kludge. It's an attempt to squeeze more data into a machine that has only so much space. The working set graphs look pretty but they seldom describe what is happening day to day. Trading 2 nanosecond response for a 5 millisecond seek is seldom going to be a good trade. Bottom line from that OS class 35 years ago? Keep your working set size less than your physical memory and your machine will remain responsive. Just what the old IBM Geezers were saying in the first place.
Whilst that's strictly true, some modern languages use generational garbage collectors that segregate objects in memory according to age. Only when an age group gets full do they sweep through an age group, and move any surviving objects up to the next age group.
This heuristic works exceptionally well, and runs fantastically quickly, and triggers significant swapping hardly ever.
There are some circumstances where it runs slowly, but in the worst case the performance is similar to simply doing a full garbage collection. These situations are pretty rare; objects generally segregate very well into young/old or young/middle aged/old categories- the vast majority of objects die very young.
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"Gravity is only a theory, not a fact!"Adding RAM always helps. No one ever says that swap is BETTER than RAM. Having X+Y RAM is better than X RAM + Y swap. However, having X+Y RAM plus Z swap is better yet.
Sure, add more RAM. But swap will always be useful, because there's always some stuff which is better off on the disk, because it hasn't been used in forever, and until your RAM is larger than your HD, you'll get better milage out of that RAM if you use it as a cache.