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Date: Wed, 30 Sep 2015 16:18:56 +0100
From: Mel Gorman <mgorman@techsingularity.net>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Vitaly Wool <vitalywool@gmail.com>, Joonsoo Kim <js1304@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, Rik van Riel <riel@redhat.com>,
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        Joonsoo Kim <iamjoonsoo.kim@lge.com>, Michal Hocko <mhocko@kernel.org>,
        Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 12/12] mm, page_alloc: Only enforce watermarks for
 order-0 allocations
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On Wed, Sep 30, 2015 at 04:43:00PM +0200, Vlastimil Babka wrote:
> >>Does a better job regarding what exactly? It does fix the CMA-specific
> >>issue, but so does this patch - without affecting allocation fastpaths by
> >>making them update another counter. But the issues discussed here are not
> >>related to that CMA problem.
> >
> >Let me disagree. Guaranteeing one suitable high-order page is not
> >enough, so the suggested patch does not work that well for me.
> >Existing broken watermark calculation doesn't work for me either, as
> >opposed to the one with my patch applied. Both solutions are related
> >to the CMA issue but one does make compaction work harder and cause
> >bigger latencies -- why do you think these are not related?
> 
> Well you didn't mention which issues you have with this patch. If you did
> measure bigger latencies and more compaction work, please post the numbers
> and details about the test.
> 

And very broadly watch out for decisions that force more reclaim/compaction
to potentially reduce latency in the future. It's trading definite overhead
now combined with potential reclaim of hot pages to reduce a *possible*
high-order allocation request in the future. It's why I think a series that
keeps more high-order pages free to reduce future high-order allocation
latency needs to be treated with care.

-- 
Mel Gorman
SUSE Labs