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

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Date:	Tue, 16 Oct 2012 21:16:08 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ezequiel Garcia <elezegarcia@...il.com>
Cc:	Tim Bird <tim.bird@...sony.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 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

# 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.

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




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