linux-kernel - Re: [RFC][PATCH] update /proc/sys/vm/drop

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Date:	Wed, 15 Sep 2010 14:34:12 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	lnxninja@...ux.vnet.ibm.com
Subject: Re: [RFC][PATCH] update /proc/sys/vm/drop_caches documentation

Dave Hansen <dave@...ux.vnet.ibm.com> writes:

> On Wed, 2010-09-15 at 11:37 -0700, Eric W. Biederman wrote:
>> > I'm worried that there are users out there experiencing real problems
>> > that aren't reporting it because "workarounds" like this just paper over
>> > the issue.
>> 
>> For what it is worth.  I had a friend ask me about a system that had 50%
>> of it's memory consumed by slab caches.  20GB out of 40GB.  The kernel
>> was suse? 2.6.27 so it's old, but if you are curious.
>> /proc/sys/vm/drop_caches does nothing in that case. 
>
> Was it the reclaimable caches doing it, though?  The other really common
> cause is kmalloc() leaks.

It was reclaimable caches.  He kept seeing the cache sizes of the
problem caches shrink.  On an idle system he said he was seeing
about 16MB/min getting free or something like that.  Something
that would take hours and hours before things freed up.

I asked and my friend told me that according to slabtop the slab
with the most memory used kept changing dramatically and he could
not see a pattern.

So at least on one old kernel on one strange workload there was a problem.

Eric
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