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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Barry Song <baohua@kernel.org> writes:

> On Tue, Jun 23, 2026 at 3:12 PM Ritesh Harjani <ritesh.list@gmail.com> wrote:
>>
>> Barry Song <baohua@kernel.org> writes:
>>
>> > On Fri, Jun 19, 2026 at 12:41 PM Ritesh Harjani (IBM)
>> > <ritesh.list@gmail.com> 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.
>

Sure, I will update the changelog in the next version with more details
on the benchmark ran. Mainly planning to provide the link to your commit
for the e.g. microbenchmark code we ran along with few other details.

> w/ that, please feel free to add:
>
> Reviewed-by: Barry Song <baohua@kernel.org>

Thanks Barry for the review.

-ritesh