Re: [PATCH v0 0/2] mm: swap: Gather swap entries and batch async release

[Chris Li <chrisl@kernel.org>]

On Tue, Sep 9, 2025 at 9:15 AM Chris Li <chrisl@kernel.org> wrote:
>
> On Tue, Sep 9, 2025 at 2:24 AM Barry Song <21cnbao@gmail.com> wrote:
> > I feel the cover letter does not clearly describe where the bottleneck
> > occurs or where the performance gains originate. To be honest, even
> > the versions submitted last year did not present the bottleneck clearly.
> >
> > For example, is this due to lock contention (in which case we would
> > need performance data to see how much CPU time is spent waiting for
> > locks), or simply because we can simultaneously zap present and
> > non-present PTEs?
>
> I have done some long tail analysis of the zswap page fault a while
> back, before zswap converting to xarray. For the zswap page fault, in
> the long tail a good chunk is a bath free swap slot. The breakdown
> inside  shows a huge chunk is the clear_shadow() followed by
> memsw_uncharge(). I will post the link to the breakdown image once it
> is available.

Here is a graph, high level breakdown shows the batch free swap slot
contribute to the long tail:
https://services.google.com/fh/files/misc/zswap-breakdown.png

The detail breakdown inside bath free swap slots:
https://services.google.com/fh/files/misc/zswap-breakdown-detail.png

Those data are on pretty old data, before zswap uses the xarray.

Now the batch freeing the swap entries is gone. I am wondering if the
new kernel shows any bottleneck for Lei's zram test case.

Hi Lei, Please report back on your new findings. In this case, with
removal of swap slot cache, the performance profile will likely be
very different. Let me know if you have difficulties running the
latest kernel on your test bench.

Chris