How much swap?

[david thornton]

Cringe.

Do any of you gals/guys examine memory usage on long lived linux boxes? 
(I used the term long lived in the windows sense of the term , ie more 
than 2 weeks :P )

98% of boxes I've managed have never used swap. Never.

of the 2% that I have seen use swap:

1% are Oracle boxes often running more than one instance . I'm pretty 
green on the oracle side but what I undestand Oracle is a good OS 
citizen. You tell it how much you expect to use ( using some solid / 
simple math) it then allocates as much as it needs , for now. Then as 
needs rise , it uses upto as much as you originally told it to use. 
Note: This might exceed Physical memory.. result? Swap. and that's ok in 
that situ. Things run smoothly.

1% are boxes in serious trouble about to fall over go boom. Hullo poor 
servlets/jsp in tomcat containers?

A colloary to this observation: physical memory usage run at 99.XX % 
most of time ... this is due to agreesive disk caching as far as  I know.

So, I've done servers with the 2XRAM swap and servers with nearly zero ( 
512 MB) (I'd never do that on an Oracle machine). The time the extra 
swap buys me on a faulting app is negligble.

So what do you guys/gals see? Does swap help you when things go pear shaped?

David


ted leslie wrote:

>i have lately set up some systems with 10-12-16 GB of ram and even
>embarking on larger.
>SUSE 10  seems to put the swap at 2GB (i guess as a max) once you are up
>to these type of RAM numbers (this is in its standard install
>configuration recommendation).
>So I am thinking that there is generally never a need to go more the 2GB
>of HD swap? (at least in Novell's opinion)
>Now with ram being so darn cheap, how about a 16 GB system,
>but put 2GB of that into a RAM based FS and mount that as your SWAP :)
>kinda defeats the purpose, but at least Linux will have its swap
>space :)
>
>
>
>
>
>-tl
>
>On Wed, 2006-10-25 at 15:09 -0400, Robert Brockway wrote:
>  
>
>>On Wed, 25 Oct 2006, Ian Petersen wrote:
>>
>>    
>>
>>>Chances are, there are a bunch of processes on your system that spend
>>>most of their time idling.  If you have no swap, these idle processes
>>>_must_ reside in real RAM, which eats away at the amount of RAM that
>>>can be used for caching and buffering.  On the other hand, if you
>>>allow the kernel to swap out mostly-idle processes, it has more room
>>>for these speed-enhancing uses of physical memory.
>>>      
>>>
>>Hi Ian.  You took the words right out of my mouth.  Ram being cheap is not 
>>sufficient reason not to put swap in the system for exactly the reason 
>>that you mention.
>>
>>Allowing unused apps to swap and thus allowing the system to reclaim the 
>>memory and use it for cache and buffering is an overall win for 
>>performance, even after factoring in the need to swap apps back in from 
>>time to time.
>>
>>As for the original poster's query - the old "swap must equal 2x ram" 
>>formula was derived from the swapping algorithms used on some versions of 
>>Unix in the past.  This is not and has never been necessary on Linux[1].
>>
>>Basically the amount of ram+swap should be higher than you would 
>>reasonably expect the system to demand.  On systems with lots of ram (say 
>>4GB) I'll throw in a few GB of swap (disk is cheap too).  Watch out for 
>>runaway processes that gobble lots of memory - having swap can prevent the 
>>OOM killer[2] coming in to play and the OOM killer is nobody's friend.
>>
>>On a side note I highly recommend setting per process limits.  See 
>>limits.conf and ulimit.
>>
>>[1] Actually there was one kernel which had a bug which meant that if swap 
>>was not at least 2x ram the system would not swap at all.  This was 
>>quickly fixed and was entirely unintentional.
>>
>>[2] Out Of Memory killer.  ROARRR!!!!
>>
>>Rob
>>
>>    
>>
>
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