Re: kswapd0: page allocation failure: order:0, mode:0x820(GFP_ATOMIC), nodemask=(null),cpuset=/,mems_allowed=0 (Kernel v6.5.9, 32bit ppc)

[Yosry Ahmed]

On Thu, Jun 6, 2024 at 10:14 AM Takero Funaki <flintglass@xxxxxxxxx> wrote:
>
> 2024年6月6日(木) 8:42 Yosry Ahmed <yosryahmed@xxxxxxxxxx>:
>
> > 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@xxxxxxxxx>