Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

From: Chengming Zhou
Date: Tue Feb 13 2024 - 09:31:34 EST

On 2024/2/13 06:42, Yosry Ahmed wrote:
> On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
> <zhouchengming@xxxxxxxxxxxxx> wrote:
>> All zswap entries will take a reference of zswap_pool when
>> zswap_store(), and drop it when free. Change it to use the
>> percpu_ref is better for scalability performance.
>> Testing kernel build in tmpfs with memory.max=2GB
>> (zswap shrinker and writeback enabled with one 50GB swapfile).
>> mm-unstable zswap-global-lru
>> real 63.20 63.12
>> user 1061.75 1062.95
>> sys 268.74 264.44
> Are these numbers from a single run or the average of multiple runs?

The average of 5 runs. And I just checked/compared each run result,
the improvement is stable. So yes, it should be a real performance gain.

> It just seems that the improvement is small, and percpu refcnt is
> slightly less intuitive (and uses a bit more memory), so let's make
> sure there is a real performance gain first.

Right, percpu_ref use a bit more memory which should be ok for our use case,
since we almost have only one zswap_pool to be using. The performance gain is
for zswap_store/load hotpath.

> It would also be useful to mention how many threads/CPUs are being used here.

My bad, the testing uses 32 threads on a 128 CPUs x86-64 machine.