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From: Yosry Ahmed <yosryahmed@google.com>
Date: Thu, 6 Jun 2024 10:41:52 -0700
Message-ID: <CAJD7tkb2b0+4_m0gb8DKSTtRwtC2GMa9NF5RuGKhXJARYHK0gw@mail.gmail.com>
Subject: Re: kswapd0: page allocation failure: order:0, mode:0x820(GFP_ATOMIC),
 nodemask=(null),cpuset=/,mems_allowed=0 (Kernel v6.5.9, 32bit ppc)
To: Takero Funaki <flintglass@gmail.com>
Cc: Erhard Furtner <erhard_f@mailbox.org>, Yu Zhao <yuzhao@google.com>, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, 
	Johannes Weiner <hannes@cmpxchg.org>, Nhat Pham <nphamcs@gmail.com>, 
	Chengming Zhou <chengming.zhou@linux.dev>, Sergey Senozhatsky <senozhatsky@chromium.org>, 
	Minchan Kim <minchan@kernel.org>, "Vlastimil Babka (SUSE)" <vbabka@kernel.org>
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On Thu, Jun 6, 2024 at 10:14=E2=80=AFAM Takero Funaki <flintglass@gmail.com=
> wrote:
>
> 2024=E5=B9=B46=E6=9C=886=E6=97=A5(=E6=9C=A8) 8:42 Yosry Ahmed <yosryahmed=
@google.com>:
>
> > I think there are multiple ways to go forward here:
> > (a) Make the number of zpools a config option, leave the default as
> > 32, but allow special use cases to set it to 1 or similar. This is
> > probably not preferable because it is not clear to users how to set
> > it, but the idea is that no one will have to set it except special use
> > cases such as Erhard's (who will want to set it to 1 in this case).
> >
> > (b) Make the number of zpools scale linearly with the number of CPUs.
> > Maybe something like nr_cpus/4 or nr_cpus/8. The problem with this
> > approach is that with a large number of CPUs, too many zpools will
> > start having diminishing returns. Fragmentation will keep increasing,
> > while the scalability/concurrency gains will diminish.
> >
> > (c) Make the number of zpools scale logarithmically with the number of
> > CPUs. Maybe something like 4log2(nr_cpus). This will keep the number
> > of zpools from increasing too much and close to the status quo. The
> > problem is that at a small number of CPUs (e.g. 2), 4log2(nr_cpus)
> > will actually give a nr_zpools > nr_cpus. So we will need to come up
> > with a more fancy magic equation (e.g. 4log2(nr_cpus/4)).
> >
>
> I just posted a patch to limit the number of zpools, with some
> theoretical background explained in the code comments. I believe that
> 2 * CPU linearly is sufficient to reduce contention, but the scale can
> be reduced further. All CPUs are trying to allocate/free zswap is
> unlikely to happen.
>  How many concurrent accesses were the original 32 zpools supposed to
> handle? I think it was for 16 cpu or more. or nr_cpus/4 would be
> enough?

We use 32 zpools on machines with 100s of CPUs. Two zpools per CPU is
an overkill imo.

I have further comments that I will leave on the patch, but I mainly
think this should be driven by real data, not theoretical possibility
of lock contention.

>
> --
>
> <flintglass@gmail.com>