Re: [Q] Default SLAB allocator

From: Ezequiel Garcia
Date: Tue Oct 16 2012 - 14:49:09 EST

On Tue, Oct 16, 2012 at 3:44 PM, Tim Bird <tim.bird@xxxxxxxxxxx> wrote:
> On 10/16/2012 11:27 AM, Ezequiel Garcia wrote:
>> On Tue, Oct 16, 2012 at 3:07 PM, Tim Bird <tim.bird@xxxxxxxxxxx> wrote:
>>> 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?
>> Yes, we have some numbers:
>> 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.
> The detail I'm interested in is the amount of wastage for a
> "common" workload, for each of the SLxB systems. Are we talking a
> few K, or 10's or 100's of K? It sounds like it's all from short strings.
> Are there other things using the 32-byte kmalloc cache, that waste
> a lot of memory (in aggregate) as well?

A more "Common" workload is one of the next items on my queue.

> Does your tool indicate a specific callsite (or small set of callsites)
> where these small allocations are made? It sounds like it's in the filesystem
> and would be content-driven (by the length of filenames)?

That's right. And, IMHO, the problem is precisely that the allocation
size is content-driven.

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