linux-kernel - Re: [RFC 0/6] mm: improve page allocator scalability via splitting zones

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Message-ID: <ZF0ET82ajDbFrIw/@dhcp22.suse.cz>
Date:   Thu, 11 May 2023 17:05:51 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Huang Ying <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Arjan Van De Ven <arjan@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Johannes Weiner <jweiner@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC 0/6] mm: improve page allocator scalability via splitting
 zones

On Thu 11-05-23 14:56:01, Huang Ying wrote:
> The patchset is based on upstream v6.3.
> 
> More and more cores are put in one physical CPU (usually one NUMA node
> too).  In 2023, one high-end server CPU has 56, 64, or more cores.
> Even more cores per physical CPU are planned for future CPUs.  While
> all cores in one physical CPU will contend for the page allocation on
> one zone in most cases.  This causes heavy zone lock contention in
> some workloads.  And the situation will become worse and worse in the
> future.
> 
> For example, on an 2-socket Intel server machine with 224 logical
> CPUs, if the kernel is built with `make -j224`, the zone lock
> contention cycles% can reach up to about 12.7%.
> 
> To improve the scalability of the page allocation, in this series, we
> will create one zone instance for each about 256 GB memory of a zone
> type generally.  That is, one large zone type will be split into
> multiple zone instances.  Then, different logical CPUs will prefer
> different zone instances based on the logical CPU No.  So the total
> number of logical CPUs contend on one zone will be reduced.  Thus the
> scalability is improved.

It is not really clear to me why you need a new zone for all this rather
than partition free lists internally within the zone? Essentially to
increase the current two level system to 3: per cpu caches, per cpu
arenas and global fallback.

I am also missing some information why pcp caches tunning is not
sufficient.
-- 
Michal Hocko
SUSE Labs