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SubjectRe: [Q] Default SLAB allocator
On 10/16/2012 12:16 PM, Eric Dumazet wrote:
> 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

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... :-)

> # 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.

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

Maybe.

I need to run some measurements on systems that are more similar to what
we're deploying in products. I'll see if I can share them.
-- Tim

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



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