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

From: Paul Bolle
Date: Thu Jul 25 2013 - 02:51:20 EST

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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