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

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Date:	Wed, 17 Oct 2012 11:45:23 -0700
From:	Tim Bird <tim.bird@...sony.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Ezequiel Garcia <elezegarcia@...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 10/16/2012 12:16 PM, Eric Dumazet wrote:
> On Tue, 2012-10-16 at 15:27 -0300, Ezequiel Garcia wrote:
> 
>> Yes, we have some numbers:
>>
>> http://elinux.org/Kernel_dynamic_memory_analysis#Kmalloc_objects
>>
>> Are they too informal? I can add some details...
>>
>> They've been measured on a **very** minimal setup, almost every option
>> is stripped out, except from initramfs, sysfs, and trace.
>>
>> On this scenario, strings allocated for file names and directories
>> created by sysfs
>> are quite noticeable, being 4-16 bytes, and produce a lot of fragmentation from
>> that 32 byte cache at SLAB.
>>
>> Is an option to enable small caches on SLUB and SLAB worth it?
> 
> Random small web server :
> 
> # free
>              total       used       free     shared    buffers     cached
> Mem:       7884536    5412572    2471964          0     155440    1803340
> -/+ buffers/cache:    3453792    4430744
> Swap:      2438140      51164    2386976

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... :-)

> # 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.

> (Waste on bigger objects is probably more important by orders of magnitude)

Maybe.

I need to run some measurements on systems that are more similar to what
we're deploying in products.  I'll see if I can share them.
 -- Tim

=============================
Tim Bird
Architecture Group Chair, CE Workgroup of the Linux Foundation
Senior Staff Engineer, Sony Network Entertainment
=============================

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