Re: Slow, persistent memory leak in 2.6.20

[Jan Engelhardt <jengelh@computergmbh.de>]

On Aug 26 2007 12:49, Fred Tyler wrote:
>>
>> So I guess you are not seeing any memory leak at all, but just the regular
>> caching?
>
>I certainly hope that is the case, but until I try it on the
>production machine tonight I won't know for sure.

Note that not all kernels have the 'drop_caches' control file.
So there is not much you can do there.

>If this is indeed a
>leak, it's pretty slow, and it takes a week or so before you can even
>start noticing it on the graphs

Well if it helps, you can accelerate the 'problem', by issuing, for example:

(1)
	dd_rescue /dev/sda /dev/null -m $[4*1048576*1024]

for reading 4 GB from disk straight and populating 'buffers'.

(2)
	cat /some/big/big/big/file >/dev/null

for reading X GB from disk and populating 'cache'.

and then you'll see.  Also note that a kernel leak will eventually lead 
to very low buffers/cached values (the ones to the far right) even when 
large amounts of data are read (using either dd_rescue/cat as mentioned 
above), because, of course, the leak clogs up memory.

             total       used       free     shared    buffers     cached
Mem:        775792     493724     282068          0          8     308416
-/+ buffers/cache:     185300     590492
Swap:       795136         60     795076


>I can say with absolute certainty that something very similar was
>happening in 2.6.12 (compare the graphs in my original email), and in
>2.6.12 it would inevitably lead to the server running entirely out of
>memory, to the point where applications could no longer allocate
>memory and the server would have to be rebooted.

>The symptoms were almost identical in that case: I'd shut down
>virtually every application on the server, but the memory would still
>be almost entirely in use. I understand there's kernel caching, but if
>the kernel caching occurs at the expense of any other applications
>being able to access memory, then there's a real problem. (I actually
>still have one 2.6.12 machine running, but drop_caches doesn't appear
>to exist on it so I can't test it there. Is there an analogue?)

If you see an Out Of Memory notice in dmesg, you'll know there is a 
leak even if everything was shut down.

>
>Anyway, I'll post the results from the 2.6.20 server as soon as I have
>them. Should be late tonight.
>
>Thanks.
>

	Jan
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