linux-kernel - Re: [Q] Default SLAB allocator

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Date:	Wed, 17 Oct 2012 18:05:16 -0300
From:	Ezequiel Garcia <elezegarcia@...il.com>
To:	Tim Bird <tim.bird@...sony.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"celinux-dev@...ts.celinuxforum.org" 
	<celinux-dev@...ts.celinuxforum.org>
Subject: Re: [Q] Default SLAB allocator

On Wed, Oct 17, 2012 at 5:58 PM, Tim Bird <tim.bird@...sony.com> wrote:
> On 10/17/2012 12:13 PM, Eric Dumazet wrote:
>> On Wed, 2012-10-17 at 11:45 -0700, Tim Bird wrote:
>>
>>> 8G is a small web server?  The RAM budget for Linux on one of
>>> Sony's cameras was 10M.  We're not merely not in the same ballpark -
>>> you're in a ballpark and I'm trimming bonsai trees... :-)
>>>
>>
>> Even laptops in 2012 have +4GB of ram.
>>
>> (Maybe not Sony laptops, I have to double check ?)
>>
>> Yes, servers do have more ram than laptops.
>>
>> (Maybe not Sony servers, I have to double check ?)
>
> I wouldn't know.  I suspect they are running 4GB+
> like everyone else.
>
>>>> # grep Slab /proc/meminfo
>>>> Slab:             351592 kB
>>>>
>>>> # egrep "kmalloc-32|kmalloc-16|kmalloc-8" /proc/slabinfo
>>>> kmalloc-32         11332  12544     32  128    1 : tunables    0    0    0 : slabdata     98     98      0
>>>> kmalloc-16          5888   5888     16  256    1 : tunables    0    0    0 : slabdata     23     23      0
>>>> kmalloc-8          76563  82432      8  512    1 : tunables    0    0    0 : slabdata    161    161      0
>>>>
>>>> Really, some waste on these small objects is pure noise on SMP hosts.
>>> In this example, it appears that if all kmalloc-8's were pushed into 32-byte slabs,
>>> we'd lose about 1.8 meg due to pure slab overhead.  This would not be noise
>>> on my system.
>> I said :
>>
>> <quote>
>> I would remove small kmalloc-XX caches, as sharing a cache line
>> is sometime dangerous for performance, because of false sharing.
>>
>> They make sense only for very small hosts
>> </quote>
>>
>> I think your 10M cameras are very tiny hosts.
>
> I agree.  Actually, I'm currently doing research for
> items with smaller memory footprints that this.  My current
> target is devices with 4M RAM and 8M NOR flash.
> Undoubtedly this is different than what a lot of other
> people are doing with Linux.
>
>> Using SLUB on them might not be the best choice.
> Indeed. :-)
>

I think the above assertion still needs some updated measurement.

  Is SLUB really a bad choice? Is SLAB the best choice? Or is this a
SLOB use case?

I've been trying to answer this questions, again focusing on
memory-constrained tiny hosts.
If anyone has some insight, it would very much like to hear it.

    Ezequiel
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