Re: [PATCH] ext4: slab caches set to SLAB_MEM_SPREAD flags.

[NamJae Jeon]

2011/11/17 Amit Sahrawat <amit.sahrawat83@xxxxxxxxx>:
> 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@xxxxxxxxxx> 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? Â 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. ÂThis 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.
Regarding ted's question,
We prevent one node being concentrating workload from redundant
alloc/free by balancing workload to other nodes in performance side.
and we can avoid reclaim probability a little by allocation from only
local node also.
slab caches for extents io of btrfs is set to SLAB_MEM_SPREAD flags
with recliam flags.
>>
>
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