Re: [PATCH] Documentation/admin-guide/sysctl/vm.rst adding the importance of NUMA-node count to documentation

[Vratislav Bendel <vbendel@redhat.com>]

On Fri, Apr 5, 2024 at 6:49 PM Matthew Cassell <mcassell411@gmail.com> wrote:
>
> If any bits are set in node_reclaim_mode (tunable via
> /proc/sys/vm/zone_reclaim_mode) within get_pages_from_freelist(), then
> page allocations start getting early access to reclaim via the
> node_reclaim() code path when memory pressure increases. This behavior
> provides the most optimization for multiple NUMA node machines. The above
> is mentioned in:
>
> Commit 9eeff2395e3cfd05c9b2e6 ("[PATCH] Zone reclaim: Reclaim logic")
> states "Zone reclaim is of particular importance for NUMA machines. It
> can be more beneficial to reclaim a page than taking the performance
> penalties that come with allocating a page on a REMOTE zone."
>
> While the pros/cons of staying on node versus allocating remotely are
> mentioned in commit histories and mailing lists. It isn't specifically
> mentioned in Documentation/ and isn't possible with a lone node. Imagine a
> situation where CONFIG_NUMA=y (the default on most major distributions)
> and only a single NUMA node exists. The latter is an oxymoron
> (single-node == uniform memory access). Informing the user via vm.rst that
> the most bang for their buck is when multiple nodes exist seems helpful.
>

I agree that the documentation could be improved to better express the
implications
and relevance of setting zone_reclaim_mode bits.

Though I would suggest to go a step further and also elaborate on
those "additional actions",
for example something like:
"The page allocator will attempt to reclaim memory within the zone,
depending on the bits set,
before looking for free pages in other zones, namely on remote memory nodes."

> Signed-off-by: Matthew Cassell <mcassell411@gmail.com>
> ---
>  Documentation/admin-guide/sysctl/vm.rst | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index c59889de122b..10270548af2a 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -1031,7 +1031,8 @@ Consider enabling one or more zone_reclaim mode bits if it's known that the
>  workload is partitioned such that each partition fits within a NUMA node
>  and that accessing remote memory would cause a measurable performance
>  reduction.  The page allocator will take additional actions before
> -allocating off node pages.
> +allocating off node pages. Keep in mind enabling bits in zone_reclaim_mode
> +makes the most sense for topologies consisting of multiple NUMA nodes.
>
>  Allowing zone reclaim to write out pages stops processes that are
>  writing large amounts of data from dirtying pages on other nodes. Zone
> --
> 2.34.1
>