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

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Message-ID: <e314748f-b4df-c65d-7acc-45c21abf31ce@intel.com>
Date:   Thu, 11 May 2023 07:23:51 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Huang Ying <ying.huang@...el.com>, linux-mm@...ck.org
Cc:     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>,
        Michal Hocko <mhocko@...e.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 5/10/23 23:56, Huang Ying wrote:
> 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. 

A few anecdotes for why I think _some_ people will like this:

Some Intel hardware has a "RAM" caching mechanism.  It either caches
DRAM in High-Bandwidth Memory or Persistent Memory in DRAM.  This cache
is direct-mapped and can have lots of collisions.  One way to prevent
collisions is to chop up the physical memory into cache-sized zones and
let users choose to allocate from one zone.  That fixes the conflicts.

Some other Intel hardware a ways to chop a NUMA node representing a
single socket into slices.  Usually one slice gets a memory controller
and its closest cores.  Intel calls these approaches Cluster on Die or
Sub-NUMA Clustering and users can select it from the BIOS.

In both of these cases, users have reported scalability improvements.
We've gone as far as to suggest the socket-splitting options to folks
today who are hitting zone scalability issues on that hardware.

That said, those _same_ users sometimes come back and say something
along the lines of: "So... we've got this app that allocates a big hunk
of memory.  It's going slower than before."  They're filling up one of
the chopped-up zones, hitting _some_ kind of undesirable reclaim
behavior and they want their humpty-dumpty zones put back together again
... without hurting scalability.  Some people will never be happy. :)

Anyway, _if_ you do this, you might also consider being able to
dynamically adjust a CPU's zonelists somehow.  That would relieve
pressure on one zone for those uneven allocations.  That wasn't an
option in the two cases above because users had ulterior motives for
sticking inside a single zone.  But, in your case, the zones really do
have equivalent performance.