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

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Date:	Wed, 17 Oct 2012 21:13:37 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tim Bird <tim.bird@...sony.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 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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