RE: [PATCH v7 00/15] zswap IAA compress batching

[Sridhar, Kanchana P]

Hi All,

> Performance testing (Kernel compilation, allmodconfig):
> =======================================================
> 
> The experiments with kernel compilation test, 32 threads, in tmpfs use the
> "allmodconfig" that takes ~12 minutes, and has considerable swapout/swapin
> activity. The cgroup's memory.max is set to 2G.
> 
> 
>  64K folios: Kernel compilation/allmodconfig:
>  ============================================
> 
>  -------------------------------------------------------------------------------
>                      mm-unstable               v7   mm-unstable            v7
>  -------------------------------------------------------------------------------
>  zswap compressor    deflate-iaa      deflate-iaa          zstd          zstd
>  -------------------------------------------------------------------------------
>  real_sec                 775.83           765.90        769.39        772.63
>  user_sec              15,659.10        15,659.14     15,666.28     15,665.98
>  sys_sec                4,209.69         4,040.44      5,277.86      5,358.61
>  -------------------------------------------------------------------------------
>  Max_Res_Set_Size_KB   1,871,116        1,874,128     1,873,200     1,873,488
>  -------------------------------------------------------------------------------
>  memcg_high                    0                0             0             0
>  memcg_swap_fail               0                0             0             0
>  zswpout             107,305,181      106,985,511    86,621,912    89,355,274
>  zswpin               32,418,991       32,184,517    25,337,514    26,522,042
>  pswpout                     272               80            94            16
>  pswpin                      274               69            54            16
>  thp_swpout                    0                0             0             0
>  thp_swpout_fallback           0                0             0             0
>  64kB_swpout_fallback        494                0             0             0
>  pgmajfault           34,577,545       34,333,290    26,892,991    28,132,682
>  ZSWPOUT-64kB          3,498,796        3,460,751     2,737,544     2,823,211
>  SWPOUT-64kB                  17                4             4             1
>  -------------------------------------------------------------------------------
> 
> [...]
>
> Summary:
> ========
> The performance testing data with usemem 30 processes and kernel
> compilation test show 61%-73% throughput gains and 27%-37% sys time
> reduction (usemem30) and 4% sys time reduction (kernel compilation) with
> zswap_store() large folios using IAA compress batching as compared to
> IAA sequential. There is no performance regression for zstd/usemem30 and a
> slight 1.5% sys time zstd regression with kernel compilation allmod
> config.

I think I know why kernel_compilation with zstd shows a regression whereas
usemem30 does not. It is because I lock/unlock the acomp_ctx mutex once
per folio. This can cause decomp jobs to wait for the mutex, which can cause
more compressions, and this repeats. kernel_compilation has 25M+ decomps
with zstd, whereas usemem30 has practically no decomps, but is
compression-intensive, because of which it benefits the once-per-folio lock
acquire/release.

I am testing a fix where I return zswap_compress() to do the mutex lock/unlock,
and expect to post v8 by end of the day. I would appreciate it if you can hold off
on reviewing only the zswap patches [14, 15] in my v7 and instead review the v8
versions of these two patches.

Thanks!
Kanchana