linux-kernel - Re: [PATCH v6 25/29] memcg/sl[au]b: shrink dead caches

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Date:	Thu, 8 Nov 2012 23:31:16 +0100
From:	Glauber Costa <glommer@...allels.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Christoph Lameter <cl@...ux.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH v6 25/29] memcg/sl[au]b: shrink dead caches

On 11/08/2012 08:21 PM, Andrew Morton wrote:
> On Thu, 8 Nov 2012 17:15:36 +0000
> Christoph Lameter <cl@...ux.com> wrote:
> 
>> On Wed, 7 Nov 2012, Andrew Morton wrote:
>>
>>> What's up with kmem_cache_shrink?  It's global and exported to modules
>>> but its only external caller is some weird and hopelessly poorly
>>> documented site down in drivers/acpi/osl.c.  slab and slob implement
>>> kmem_cache_shrink() *only* for acpi!  wtf?  Let's work out what acpi is
>>> trying to actually do there, then do it properly, then killkillkill!
>>
>> kmem_cache_shrink is also used internally. Its simply releasing unused
>> cached objects.
> 
> Only in slub.  It could be removed outright from the others and
> simplified in slub.
> 
>>> Secondly, as slab and slub (at least) have the ability to shed cached
>>> memory, why aren't they hooked into the core cache-shinking machinery.
>>> After all, it's called "shrink_slab"!
>>
>> Because the core cache shrinking needs the slab caches to free up memory
>> from inodes and dentries. We could call kmem_cache_shrink at the end of
>> the shrink passes in vmscan. The price would be that the caches would have
>> to be repopulated when new allocations occur.
> 
> Well, the shrinker shouldn't strips away all the cache.  It will perform
> a partial trim, the magnitude of which increases with perceived
> external memory pressure.
> 
> AFACIT, this is correct and desirable behaviour for shrinking
> slab's internal caches.
> 

I believe calling this from shrink_slab() is not a bad idea at all. If
you're all in favour, I'll cook a patch for this soon

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