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

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Message-ID: <507DA245.9050709@am.sony.com>
Date:	Tue, 16 Oct 2012 11:07:01 -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 05:56 AM, Eric Dumazet wrote:
> On Tue, 2012-10-16 at 09:35 -0300, Ezequiel Garcia wrote:
> 
>> Now, returning to the fragmentation. The problem with SLAB is that
>> its smaller cache available for kmalloced objects is 32 bytes;
>> while SLUB allows 8, 16, 24 ...
>>
>> Perhaps adding smaller caches to SLAB might make sense?
>> Is there any strong reason for NOT doing this?
> 
> 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.

That's interesting...

It would be good to measure the performance/size tradeoff here.
I'm interested in very small systems, and it might be worth
the tradeoff, depending on how bad the performance is.  Maybe
a new config option would be useful (I can hear the groans now... :-)

Ezequiel - do you have any measurements of how much memory
is wasted by 32-byte kmalloc allocations for smaller objects,
in the tests you've been doing?
 -- Tim


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

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