Re: [RFC PATCH 0/2] mm: zsmalloc: make shrinker compaction budget-aware

[Chris Wiseman]

On (26/11/02 08:47), Sergey Senozhatsky wrote:
> Adding Chris for visibility

Thanks for looping me in, Sergey.

I tested your CONFIG_ZSMALLOC_SHRINKER=n patch in ARM64 QEMU (7.3.0+, 16
boots per arm) by fragmenting zram0, triggering a manual compaction pass
at pressure onset via /sys/block/zram0/compact, driving ~240 MB of
swap-outs under memory pressure, and running a post-swap manual
compaction pass.

Your theory holds up: with the shrinker enabled, the kernel compacted
2,218 pages (~9 MB) in the middle of the swap-out, only for incoming
swap-outs to immediately reuse those pages.  With
CONFIG_ZSMALLOC_SHRINKER=n, mid-swap compaction dropped to 0 and zram
pool memory was identical both during the swap-out (120.9 MB vs
121.0 MB) and after manual compaction (110.9 MB vs 111.2 MB).

I also tested adding the unlocked zs_can_compact() pre-check in
__zs_compact() on top of CONFIG_ZSMALLOC_SHRINKER=n, which cuts each
userspace /sys/block/zram0/compact scan from 97 us to 27 us:

                                  Shrinker ON   Shrinker OFF   Shrinker OFF + pre-check
  -------------------------------------------------------------------------------------
  Pages compacted mid-swap              2,218              0                          0
  zram mem_used_total (mid-swap)     121.0 MB       120.9 MB                   121.1 MB
  zram mem_used_total (post-compact) 111.2 MB       110.9 MB                   111.1 MB
  /sys/block/zram0/compact scan time    96 us          97 us                      27 us

Representative /sys/block/zram0/mm_stat snapshots:

BASE (CONFIG_ZSMALLOC_SHRINKER=y)
  pre_onset:         184553472  58747279   82350080  0   82374656  0      0   0   0
  onset_compact:     184553472  58747279   60456960  0   82374656  0   5345   0   0
  after_swap_out:    334196736 107289761  120946688  0  120946688  0   7570  29  29
  post_compact:      334749696 107472834  110907392  0  120946688  0  10016  29  29

PATCHED (CONFIG_ZSMALLOC_SHRINKER=n)
  pre_onset:         184553472  58747279   82350080  0   82374656  0      0   0   0
  onset_compact:     184553472  58747279   60456960  0   82374656  0   5345   0   0
  after_swap_out:    334372864 107347183  120922112  0  120922112  0   5345  29  29
  post_compact:      334995456 107550158  111009792  0  120922112  0   7767  29  29