Re: [PATCH v7 2/2] mm: support large folios swap-in for sync io devices

[Barry Song <21cnbao@gmail.com>]

On Thu, Aug 22, 2024 at 1:31 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Wed, Aug 21, 2024 at 03:45:40PM GMT, hanchuanhua@oppo.com wrote:
> > From: Chuanhua Han <hanchuanhua@oppo.com>
> >
> >
> > 3. With both mTHP swap-out and swap-in supported, we offer the option to enable
> >    zsmalloc compression/decompression with larger granularity[2]. The upcoming
> >    optimization in zsmalloc will significantly increase swap speed and improve
> >    compression efficiency. Tested by running 100 iterations of swapping 100MiB
> >    of anon memory, the swap speed improved dramatically:
> >                 time consumption of swapin(ms)   time consumption of swapout(ms)
> >      lz4 4k                  45274                    90540
> >      lz4 64k                 22942                    55667
> >      zstdn 4k                85035                    186585
> >      zstdn 64k               46558                    118533
>
> Are the above number with the patch series at [2] or without? Also can
> you explain your experiment setup or how can someone reproduce these?

Hi Shakeel,

The data was recorded after applying both this patch (swap-in mTHP) and
patch [2] (compressing/decompressing mTHP instead of page). However,
without the swap-in series, patch [2] becomes useless because:

If we have a large object, such as 16 pages in zsmalloc:
do_swap_page will happen 16 times:
1. decompress the whole large object and copy one page;
2. decompress the whole large object and copy one page;
3. decompress the whole large object and copy one page;
....
16.  decompress the whole large object and copy one page;

So, patchset [2] will actually degrade performance rather than
enhance it if we don't have this swap-in series. This swap-in
series is a prerequisite for the zsmalloc/zram series.

We reproduced the data through the following simple steps:
1. Collected anonymous pages from a running phone and saved them to a file.
2. Used a small program to open and read the file into a mapped anonymous
memory.
3.  Do the belows in the small program:
swapout_start_time
madv_pageout()
swapout_end_time

swapin_start_time
read_data()
swapin_end_time

We calculate the throughput of swapout and swapin using the difference between
end_time and start_time. Additionally, we record the memory usage of zram after
the swapout is complete.

>
> > [2] https://lore.kernel.org/all/20240327214816.31191-1-21cnbao@gmail.com/
>

Thanks
Barry