Date: Wed, 30 Mar 2016 17:25:14 +0900
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
To: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 06/11] mm/slab: don't keep free slabs if free_objects
 exceeds free_limit
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On Mon, Mar 28, 2016 at 08:03:16PM -0500, Christoph Lameter wrote:
> On Mon, 28 Mar 2016, js1304@gmail.com wrote:
> 
> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >
> > Currently, determination to free a slab is done whenever free object is
> > put into the slab. This has a problem that free slabs are not freed
> > even if we have free slabs and have more free_objects than free_limit
> 
> There needs to be a better explanation here since I do not get why there
> is an issue with checking after free if a slab is actually free.

Okay. Consider following 3 objects free situation.

free_limt = 10
nr_free = 9

free(free slab) free(free slab) free(not free slab)

If we check it one by one, when nr_free > free_limit (at last free),
we cannot free the slab because current slab isn't a free slab.

But, if we check it lastly, we can free 1 free slab.

I will add more explanation on the next version.

> 
> > when processed slab isn't a free slab. This would cause to keep
> > too much memory in the slab subsystem. This patch try to fix it
> > by checking number of free object after all free work is done. If there
> > is free slab at that time, we can free it so we keep free slab as minimal
> > as possible.
> 
> Ok if we check after free work is done then the number of free slabs may
> be higher than the limit set and then we free the additional slabs to get
> down to the limit that was set?

Yes.

Thanks.