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From: Vratislav Bendel <vbendel@redhat.com>
Date: Sat, 27 Apr 2024 16:13:25 +0200
Message-ID: <CAHrQN1ximY20i9EiOgbmTQcgdEpfsZarjKczE15pyUp0Zc7=2g@mail.gmail.com>
Subject: Re: [PATCH] Documentation/admin-guide/sysctl/vm.rst adding the
 importance of NUMA-node count to documentation
To: Matthew Cassell <mcassell411@gmail.com>
Cc: corbet@lwn.net, akpm@linux-foundation.org, rppt@kernel.org, 
	linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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(plain text resend)

IMHO you went too encyclopedia-style this time. :)
My extra point was just to elaborate on what the "additional actions" means=
,
but I would suggest keeping it as concise as possible.
Also the bit values are already explained - no need to repeat that.


On Fri, Apr 12, 2024 at 10:48=E2=80=AFPM Matthew Cassell <mcassell411@gmail=
com> wrote:
>
> Thanks for the feedback. Here is a quick outline I came up with on your
> advice:
>
> [...] (original content)
>
> Keep in mind enabling bits in zone_reclaim_mode makes the most sense
> for topologies consisting of multiple NUMA nodes. In addition to vanilla
> zone_reclaim (clean and unmapped pages), there exist additional bits that
> expand which pages are eligible to be reclaimed and dictate scan_control
> policy during the reclaim process. The page allocator will attempt to rec=
laim
> memory locally in accordance with these bits before attempting to allocat=
e
> on remote nodes.
>
> Allow dirty pages to become candidates for memory reclaim::
>
>         echo 2 > /proc/sys/vm/zone_reclaim_mode
>
> [...] (original content)
>
> Allow mapped pages to become candidates for memory reclaim::
>
>         echo 4 > /proc/sys/vm/zone_reclaim_mode
>
> [...] (original content)
>
> I'm trying to balance between keeping the original content, being descrip=
tive,
> and not going into encyclopedia-mode. My motivation was to stress the imp=
ortance
> of NUMA-node count and describe the additional bits more per your advice.
> I added the echo snippets to better segue the aggressive options. Any tho=
ughts
> on the above?
>
> On Thu, Apr 11, 2024 at 2:54=E2=80=AFAM Vratislav Bendel <vbendel@redhat.=
com> wrote:
> >
> > On Fri, Apr 5, 2024 at 6:49=E2=80=AFPM Matthew Cassell <mcassell411@gma=
il.com> wrote:
> > >
> > > If any bits are set in node_reclaim_mode (tunable via
> > > /proc/sys/vm/zone_reclaim_mode) within get_pages_from_freelist(), the=
n
> > > page allocations start getting early access to reclaim via the
> > > node_reclaim() code path when memory pressure increases. This behavio=
r
> > > provides the most optimization for multiple NUMA node machines. The a=
bove
> > > is mentioned in:
> > >
> > > Commit 9eeff2395e3cfd05c9b2e6 ("[PATCH] Zone reclaim: Reclaim logic")
> > > states "Zone reclaim is of particular importance for NUMA machines. I=
t
> > > 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 specificall=
y
> > > mentioned in Documentation/ and isn't possible with a lone node. Imag=
ine a
> > > situation where CONFIG_NUMA=3Dy (the default on most major distributi=
ons)
> > > and only a single NUMA node exists. The latter is an oxymoron
> > > (single-node =3D=3D uniform memory access). Informing the user via vm=
rst that
> > > the most bang for their buck is when multiple nodes exist seems helpf=
ul.
> > >
> >
> > 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 n=
odes."
> >
> > > 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 performanc=
e
> > >  reduction.  The page allocator will take additional actions before
> > > -allocating off node pages.
> > > +allocating off node pages. Keep in mind enabling bits in zone_reclai=
m_mode
> > > +makes the most sense for topologies consisting of multiple NUMA node=
s.
> > >
> > >  Allowing zone reclaim to write out pages stops processes that are
> > >  writing large amounts of data from dirtying pages on other nodes. Zo=
ne
> > > --
> > > 2.34.1
> > >
> >
>