linux-kernel - Re: [PATCH 1/9] mm, page_alloc: rip out ZONELIST_ORDER

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Date:   Wed, 19 Jul 2017 11:33:49 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>, linux-api@...r.kernel.org
Subject: Re: [PATCH 1/9] mm, page_alloc: rip out ZONELIST_ORDER_ZONE

On 07/14/2017 09:59 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> Supporting zone ordered zonelists costs us just a lot of code while
> the usefulness is arguable if existent at all. Mel has already made
> node ordering default on 64b systems. 32b systems are still using
> ZONELIST_ORDER_ZONE because it is considered better to fallback to
> a different NUMA node rather than consume precious lowmem zones.
> 
> This argument is, however, weaken by the fact that the memory reclaim
> has been reworked to be node rather than zone oriented. This means
> that lowmem requests have to skip over all highmem pages on LRUs already
> and so zone ordering doesn't save the reclaim time much. So the only
> advantage of the zone ordering is under a light memory pressure when
> highmem requests do not ever hit into lowmem zones and the lowmem
> pressure doesn't need to reclaim.
> 
> Considering that 32b NUMA systems are rather suboptimal already and
> it is generally advisable to use 64b kernel on such a HW I believe we
> should rather care about the code maintainability and just get rid of
> ZONELIST_ORDER_ZONE altogether. Keep systcl in place and warn if
> somebody tries to set zone ordering either from kernel command line
> or the sysctl.
> 
> Cc: <linux-api@...r.kernel.org>
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Found some leftovers to cleanup:

include/linux/mmzone.h:
extern char numa_zonelist_order[];
#define NUMA_ZONELIST_ORDER_LEN 16      /* string buffer size */

Also update docs?
Documentation/sysctl/vm.txt:zone.  Specify "[Zz]one" for zone order.
Documentation/admin-guide/kernel-parameters.txt:
numa_zonelist_order= [KNL, BOOT] Select zonelist order for NUMA.
Documentation/vm/numa:a default zonelist order based on the sizes of the
various zone types relative
Documentation/vm/numa:default zonelist order may be overridden using the
numa_zonelist_order kernel

Otherwise,
Acked-by: Vlastimil Babka <vbabka@...e.cz>