Re: [patch 1/3] mm: vmscan: fix numa reclaim balance problem in kswapd

From: Rik van Riel
Date: Mon Jul 22 2013 - 15:47:19 EST


On 07/19/2013 04:55 PM, Johannes Weiner wrote:
When the page allocator fails to get a page from all zones in its
given zonelist, it wakes up the per-node kswapds for all zones that
are at their low watermark.

However, with a system under load and the free page counters being
per-cpu approximations, the observed counter value in a zone can
fluctuate enough that the allocation fails but the kswapd wakeup is
also skipped while the zone is still really close to the low
watermark.

When one node misses a wakeup like this, it won't be aged before all
the other node's zones are down to their low watermarks again. And
skipping a full aging cycle is an obvious fairness problem.

Kswapd runs until the high watermarks are restored, so it should also
be woken when the high watermarks are not met. This ages nodes more
equally and creates a safety margin for the page counter fluctuation.

By using zone_balanced(), it will now check, in addition to the
watermark, if compaction requires more order-0 pages to create a
higher order page.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

This patch alone looks like it could have the effect of increasing
the pressure on the first zone in the zonelist, keeping its free
memory above the low watermark essentially forever, without having
the allocator fall back to other zones.

However, your third patch fixes that problem, and missed wakeups
would still hurt, so...

Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>

--
All rights reversed
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/