death by swap [LWN.net]

death by swap

Posted Nov 18, 2009 16:47 UTC (Wed) by pimlottc (guest, #44833)
In reply to: death by swap by jabby
Parent article: Toward a smarter OOM killer

STOP USING the old "TWICE RAM" guideline!

You know, I've been hearing this lately, but the problem is there seems to be no consensus on what the guideline should be. Some swear by no swap at all, while others say running without at least some is dangerous. No one seems to agree on what an appropriate amount is. Until there is a new accepted rule of thumb, everyone will keep using the old one, even if it's wrong.

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death by swap

Posted Nov 18, 2009 18:37 UTC (Wed) by dlang (✭ supporter ✭, #313) [Link]

there used to be a technical reason for twice ram.

nowdays it depends on your system and how you use it.

if you use the in-kernel suspend code, the act of suspending will write all your ram into the swap space, so swap must be > ram

if you don't use the in-kernel suspend code you need as much swap as you intend to use. How much swap you are willing to use depends very much on your use case. for most people a little bit of swap in use doesn't hurt much to use and by freeing up additional ram results in a overall faster system. for other people the unpredictable delays in applications due to the need to pull things from swap is unacceptable. In any case, having a lot of swap activity if pretty much unacceptable for anyone.

note that if you disable overcommit you need more swap or allocations (like when a large program forks) will fail so you need additional swap space > max memory footprint of any process you intend to allow to fork (potentially multiples of this). With overcommit disabled I could see you needing swap significantly higher than 2x ram in some conditions.

my recommendation is that if you are using a normal hard drive (usually including the SSD drives that emulate normal hard drives), allocate a 2G swap partition and leave overcommit enabled (and that's probably a lot larger than you will ever use)

if you are using a system that doesn't have a normal hard drive (usually this sort of thing has no more than a few gig of flash as it's drive) you probably don't want any swap, and definantly want to leave overcommit on.

death by swap

Posted Nov 19, 2009 16:46 UTC (Thu) by nye (guest, #51576) [Link]

>my recommendation is that if you are using a normal hard drive (usually including the SSD drives that emulate normal hard drives), allocate a 2G swap partition and leave overcommit enabled (and that's probably a lot larger than you will ever use)

FWIW, I agree, except that I'd make it a file instead of a partition - it's just as fast, and it leaves some flexibility just in case.

I use a 2GB swapfile on machines ranging from 256MB to 8GB of RAM - it may be overkill but that much disk space costs next to nothing. I wouldn't want to set it higher, because if I'm really using swap to that extent, the machine's probably past the point of usability anyway.