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Date:   Tue, 20 Oct 2020 21:33:31 +0100
From:   Mel Gorman <mgorman@suse.de>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH] mm: don't wake kswapd prematurely when watermark
 boosting is disabled
Message-ID: <20201020203331.GB32041@suse.de>
References: <20201020175833.397286-1-hannes@cmpxchg.org>
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On Tue, Oct 20, 2020 at 01:58:33PM -0400, Johannes Weiner wrote:
> On 2-node NUMA hosts we see bursts of kswapd reclaim and subsequent
> pressure spikes and stalls from cache refaults while there is plenty
> of free memory in the system.
> 
> Usually, kswapd is woken up when all eligible nodes in an allocation
> are full. But the code related to watermark boosting can wake kswapd
> on one full node while the other one is mostly empty. This may be
> justified to fight fragmentation, but is currently unconditionally
> done whether watermark boosting is occurring or not.
> 
> In our case, many of our workloads' throughput scales with available
> memory, and pure utilization is a more tangible concern than trends
> around longer-term fragmentation. As a result we generally disable
> watermark boosting.
> 
> Wake kswapd only woken when watermark boosting is requested.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

This makes sense to the extent that I would support adding

Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")

With or without that;

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs