linux-kernel - Re: [PATCH v2 16/17] mm/slab: introduce new slab management type, OBJFREELIST

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Date:	Sat, 27 Feb 2016 02:06:21 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 16/17] mm/slab: introduce new slab management type, OBJFREELIST_SLAB

2016-02-27 1:21 GMT+09:00 Christoph Lameter <cl@...ux.com>:
> On Fri, 26 Feb 2016, js1304@...il.com wrote:
>
>> Although this idea can apply to all caches whose size is larger than
>> management array size, it isn't applied to caches which have a
>> constructor.  If such cache's object is used for management array,
>> constructor should be called for it before that object is returned to
>> user.  I guess that overhead overwhelm benefit in that case so this idea
>> doesn't applied to them at least now.
>
> Caches which have a constructor (or are used with SLAB_RCU_FREE) have a
> defined content even when they are free. Therefore they cannot be used
> for the freelist.

Yes, I know. I already handled it. I attach related hunk.

+static bool set_objfreelist_slab_cache(struct kmem_cache *cachep,
+                       size_t size, unsigned long flags)
+{
+       size_t left;
+
+       cachep->num = 0;
+
+       if (cachep->ctor || flags & SLAB_DESTROY_BY_RCU)
+               return false;

So, if there is ctor or RCU slabs, objfreelist will not be used.

>> For summary, from now on, slab management type is determined by
>> following logic.
>>
>> 1) if management array size is smaller than object size and no ctor, it
>>    becomes OBJFREELIST_SLAB.
>
> Also do not do this for RCU slabs.

Explained above.

>> 2) if management array size is smaller than leftover, it becomes
>>    NORMAL_SLAB which uses leftover as a array.
>>
>> 3) if OFF_SLAB help to save memory than way 4), it becomes OFF_SLAB.
>>    It allocate a management array from the other cache so memory waste
>>    happens.
>
> Wonder how many of these ugly off slabs are left after what you did here.

See below result.

>> TOTAL = OBJFREELIST + NORMAL(leftover) + NORMAL + OFF
>>
>> /Before/
>> 126 = 0 + 60 + 25 + 41
>>
>> /After/
>> 126 = 97 + 12 + 15 + 2

97 is the number of 1) type caches.
12 is the number of 2) type caches.
and so on...

>> Result shows that number of caches that doesn't waste memory increase
>> from 60 to 109.
>
> Great results.

Thanks. :)

>> v2: fix SLAB_DESTROTY_BY_RCU cache type handling
>
> Ok how are they handled now? Do not see that dealt with in the patch.

Explained above.

Thanks.