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

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Message-ID: <507F1BF4.6040209@am.sony.com>
Date:	Wed, 17 Oct 2012 13:58:28 -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 10/17/2012 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'm still interested in the dynamics of the slab sizes
and how it impacts performance, how much memory is wasted, etc.
I think there are a few "power-of-two-and-a-half" kmalloc
slabs (e.g. kmalloc-192).  Are these configurable anywhere?

Anyway, I greatly appreciate the discussion.

> First time I ran linux, years ago, it was on 486SX machines with 8M of
> memory (or maybe less, I dont remember exactly). But I no longer use
> this class of machines with recent kernels.

I ran a web server on an 8M machine that had an uptime of over 2 years,
but that was in the mid-90's.  Ahhh - those were the days...
 -- Tim

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

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