Re: [patch 3/3] mm: page_alloc: fair zone allocator policy

[Paul Bolle <paul.bollee@gmail.com>]

On 07/23/2013 04:21 AM, Rik van Riel wrote:
> On 07/19/2013 04:55 PM, Johannes Weiner wrote:
>
>> @@ -1984,7 +1992,8 @@ this_zone_full:
>>           goto zonelist_scan;
>>       }
>>
>> -    if (page)
>> +    if (page) {
>> +        atomic_sub(1U << order, &zone->alloc_batch);
>>           /*
>>            * page->pfmemalloc is set when ALLOC_NO_WATERMARKS was
>>            * necessary to allocate the page. The expectation is
>
> Could this be moved into the slow path in buffered_rmqueue and
> rmqueue_bulk, or would the effect of ignoring the pcp buffers be
> too detrimental to keeping the balance between zones?
>
> It would be kind of nice to not have this atomic operation on every
> page allocation...

atomic operation will lock cache line or memory bus? And cmpxchg will 
lock cache line or memory bus? ;-)

>
> As a side benefit, higher-order buffered_rmqueue and rmqueue_bulk
> both happen under the zone->lock, so moving this accounting down
> to that layer might allow you to get rid of the atomics alltogether.
>
> I like the overall approach though. This is something Linux has needed
> for a long time, and could be extremely useful to automatic NUMA
> balancing as well...
>

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