linux-kernel - Re: [PATCH] ext4: slab caches set to SLAB_MEM

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Message-ID: <CADDb1s1pLddZ4TQciL=yO-17WwQDyLmzP1E8U0UsyiQqez8Q8A@mail.gmail.com>
Date:	Thu, 17 Nov 2011 10:41:20 +0530
From:	Amit Sahrawat <amit.sahrawat83@...il.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Theodore Tso <tytso@....edu>, Namjae Jeon <linkinjeon@...il.com>,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: slab caches set to SLAB_MEM_SPREAD flags.

Hi All,

To share with you all, this was picked as part of some review process.
We looked for the details regarding the introduction of
SLAB_MEM_SHARED(http://linux.derkeiler.com/Mailing-Lists/Kernel/2006-02/msg01409.html
- provide the slab cache infrastructure to support cpuset memory
spreading), and further the changes were introduced across all
filesystems - http://lwn.net/Articles/173654/.
Since, we do not have direct access to NUMA architecture machine, so
could not think of verifying the changes - but looking at the
respective changes - the changes in this patch does seems relevant
keeping in sync with all changes.
And, Yes David - these changes will make sense in case of CONFIG_SLAB.

Please share your opinion on the above introductions irrespective of
the code changes done in this patch.

Thanks & Regards,
Amit Sahrawat

On Thu, Nov 17, 2011 at 3:22 AM, David Rientjes <rientjes@...gle.com> wrote:
> On Wed, 16 Nov 2011, Theodore Tso wrote:
>
>> On Nov 16, 2011, at 10:04 AM, Namjae Jeon wrote:
>>
>> > If slab caches set to SLAB_MEM_SPREAD flags, The allocation is spread
>> > evenly over all the memory nodes instead of favoring allocation on the
>> > node local to current cpu.
>>
>> And why do you think this is a good thing?   For mballoc in particular,
>> the data structures are used immediately and then freed immediately ---
>> on the local node, so using a non-local memory just makes things worse
>> in a NUMA system.
>>
>
> I don't think this has the effect that Namjae thinks it does: this is only
> useful for CONFIG_SLAB and when you have cpusets enabled with
> cpuset.memory_spread_slab set.
>
> To test how useful it is, you should enable CONFIG_SLAB and then mount
> cpusets, set cpuset.memory_spread_slab, and create an MPOL_INTERLEAVE
> mempolicy over all online nodes.  This will have the same effect as adding
> SLAB_MEM_SPREAD to these slab caches (it just doesn't require the
> mempolicy) and will be able to quantify the effects without any changes to
> the kernel at all.
>
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