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 [off-list ref] wrote:

On Wed, 16 Nov 2011, Theodore Tso wrote:

quoted

On Nov 16, 2011, at 10:04 AM, Namjae Jeon wrote:

quoted

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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