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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <507DAB0F.30000@am.sony.com>
Date:	Tue, 16 Oct 2012 11:44:31 -0700
From:	Tim Bird <tim.bird@...sony.com>
To:	Ezequiel Garcia <elezegarcia@...il.com>
CC:	Eric Dumazet <eric.dumazet@...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 10/16/2012 11:27 AM, Ezequiel Garcia wrote:
> On Tue, Oct 16, 2012 at 3:07 PM, Tim Bird <tim.bird@...sony.com> 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:
> 
> 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.

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?

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)?

This might be an issue particularly for cameras, where all the generated
filenames are 8.3 (and will be for the foreseeable future)

> Is an option to enable small caches on SLUB and SLAB worth it?
I'll have to do some measurements to see.  I'm guessing the option
itself would be pretty trivial to implement?
 -- Tim

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/