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

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Message-ID: <CAGDaZ_qKg3x_ChdZck25P_XF78cJNeB_DJLg=ZtL3eZWSz3yXA@mail.gmail.com>
Date:	Wed, 17 Oct 2012 12:20:12 -0700
From:	Shentino <shentino@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tim Bird <tim.bird@...sony.com>,
	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 Wed, Oct 17, 2012 at 12:13 PM, Eric Dumazet <eric.dumazet@...il.com> 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 ?)
>
>> > # 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.
>
> Using SLUB on them might not be the best choice.
>
> 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.
>
> # size vmlinux
>    text    data     bss     dec     hex filename
> 10290631        1278976 1896448 13466055         cd79c7 vmlinux
>
>
> --
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Potentially stupid question

But is SLAB the one where all objects per cache have a fixed size and
thus you don't have any bookkeeping overhead for the actual
allocations?

I remember something about one of the allocation mechanisms being
designed for caches of fixed sized objects to minimize the need for
bookkeeping.
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