linux-kernel - Re: [patch] mm: page_alloc: exclude unreclaimable allocations from zone fairness policy

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Date:	Wed, 11 Dec 2013 22:47:19 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: page_alloc: exclude unreclaimable allocations from
 zone fairness policy

On Wed, Dec 11, 2013 at 01:09:16PM -0500, Johannes Weiner wrote:
> Dave Hansen noted a regression in a microbenchmark that loops around
> open() and close() on an 8-node NUMA machine and bisected it down to
> 81c0a2bb515f ("mm: page_alloc: fair zone allocator policy").  That
> change forces the slab allocations of the file descriptor to spread
> out to all 8 nodes, causing remote references in the page allocator
> and slab.
> 

The original patch was primarily concerned with the fair aging of LRU pages
of zones within a node. This patch uses GFP_MOVABLE_MASK which includes
__GFP_RECLAIMABLE meaning any slab created with SLAB_RECLAIM_ACCOUNT is still
getting the round-robin treatment. Those pages have a different lifecycle
to LRU pages and the shrinkers are only node aware, not zone aware.
While I get this patch probably helps this specific benchmark, was the
use of GFP_MOVABLE_MASK intentional or did you mean to use __GFP_MOVABLE?

Looking at the original patch again I think I made a major mistake when
reviewing it. Considering the effect of the following for NUMA machines

        for_each_zone_zonelist_nodemask(zone, z, zonelist,
                                                high_zoneidx, nodemask) {
		....
                if (alloc_flags & ALLOC_WMARK_LOW) {
                        if (zone_page_state(zone, NR_ALLOC_BATCH) <= 0)
				continue;
                        if (zone_reclaim_mode &&
                            !zone_local(preferred_zone, zone))
                                continue;
		}

Enabling zone_reclaim_mode sucks badly for workloads that are not paritioned
to fit within NUMA nodes. Consequently, I expect the common case it that
it's disabled by default due to small NUMA distances or manually disabled.

However, the effect of that block is that we allocate NR_ALLOC_BATCH
from local zones then fallback to batch allocating remote nodes! I bet
the numa_hit stats in /proc/vmstat have sucked recently. The original
problem was because the page allocator would try allocating from the
highest zone while kswapd reclaimed from it causing LRU-aging problems.
The problem is not the same between nodes. How do you feel about dropping
the zone_reclaim_mode check above and only round-robin in batches between
zones on the local node?

-- 
Mel Gorman
SUSE Labs
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