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Date:   Thu, 12 Oct 2023 13:49:58 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Arjan Van De Ven <arjan@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Hildenbrand <david@redhat.com>,
        Johannes Weiner <jweiner@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Michal Hocko <mhocko@suse.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Matthew Wilcox <willy@infradead.org>,
        Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH 09/10] mm, pcp: avoid to reduce PCP high unnecessarily
Message-ID: <20231012124958.dj5ug5hih3joa542@techsingularity.net>
References: <20230920061856.257597-1-ying.huang@intel.com>
 <20230920061856.257597-10-ying.huang@intel.com>
 <20231011140949.rwsqfb57vyuub6va@techsingularity.net>
 <87lec8ffij.fsf@yhuang6-desk2.ccr.corp.intel.com>
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On Thu, Oct 12, 2023 at 03:48:04PM +0800, Huang, Ying wrote:
> "
> On a 2-socket Intel server with 224 logical CPU, we run 8 kbuild
> instances in parallel (each with `make -j 28`) in 8 cgroup.  This
> simulates the kbuild server that is used by 0-Day kbuild service.
> With the patch, The number of pages allocated from zone (instead of
> from PCP) decreases 21.4%.
> "
> 
> I also showed the performance number for each step of optimization as
> follows (copied from the above patchset V2 link).
> 
> "
> 	build time   lock contend%	free_high	alloc_zone
> 	----------	----------	---------	----------
> base	     100.0	      13.5          100.0            100.0
> patch1	      99.2	      10.6	     19.2	      95.6
> patch3	      99.2	      11.7	      7.1	      95.6
> patch5	      98.4	      10.0	      8.2	      97.1
> patch7	      94.9	       0.7	      3.0	      19.0
> patch9	      94.9	       0.6	      2.7	      15.0  <--	this patch
> patch10	      94.9	       0.9	      8.8	      18.6
> "
> 
> Although I think the patch is helpful via avoiding the unnecessary
> pcp->high decaying, thus reducing the zone lock contention.  There's no
> visible benchmark score change for the patch.
> 

Thanks!

Given that it's another PCP field with an update in a relatively hot
path, I would suggest dropping this patch entirely if it does not affect
performance. It has the risk of being a magical heuristic that we forget
later whether it's even worthwhile.

-- 
Mel Gorman
SUSE Labs