Re: [PATCH v4 3/3] powerpc: Kconfig: Enable THP_SWAP on Book3S64

[Barry Song]

On Tue, Jun 23, 2026 at 3:12 PM Ritesh Harjani <ritesh.list@xxxxxxxxx> wrote:
>
> Barry Song <baohua@xxxxxxxxxx> writes:
>
> > On Fri, Jun 19, 2026 at 12:41 PM Ritesh Harjani (IBM)
> > <ritesh.list@xxxxxxxxx> wrote:
> >>
> >> THP_SWAP avoids splitting of a transparent huge folio into 32 smaller
> >> 64K folios (Radix-64K pagesize / 2M PMD) or into 256 smaller 64K folios
> >> (Hash-64K pagesize / 16M PMD), during swapout. This improves the
> >> swapping performance since all the bookking & I/O submission happens
> >> once per large folio. More details at [1].
> >>
> >> PowerPC Book3S64 could not enable this before because PMD_ORDER is
> >> selected at runtime depending upon the chosen MMU. The earlier patches
> >> in this series turn SWAPFILE_CLUSTER into a runtime value and introduce
> >> an ARCH_MAX_PMD_ORDER upperbound override for SWAP_NR_ORDERS. With those
> >> changes, we can now enable THP SWAP for Book3S64.
> >>
> >> This increases bandwidth throughput with zram backend for swapout by
> >> 40-50% with Radix and 100-130% with Hash (Tested by Sayali)
> >
> > Thanks!
> >
> > I am curious about the contents of the anonymous memory being tested
> > and the compression algorithm used by zram.
> >
>
> I am sure it was derived from your microbenchmark itself which you had
> shared here (so repetitive pattern) with default zram compression
> algorithm. Thanks for that :)
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d0637c505f
>
> I think I got your point - I can mention that it was a microbenchmark
> similar to yours and not a real world workload test. Is this what you
> meant here?

Yep. Please make it clear in the changelog what kind of workload was
used, as different data can result in completely different compression
ratios and compression/decompression costs. Consequently, the reported
swap-out and swap-in performance improvements can vary significantly as
well.

w/ that, please feel free to add:

Reviewed-by: Barry Song <baohua@xxxxxxxxxx>