Re: [RFC PATCH] zram: support asynchronous GC for lazy slot freeing

Sergey Senozhatsky
2026-04-16T07:41:31Z

On (26/04/14 13:49), Xueyuan Chen wrote:
> On Sun, Apr 12, 2026 at 07:48:48PM +0800, Kairui Song wrote:
> [...]
> >What is making this slot_free so costly? zs_free?
>
> Yes, I've captured some perf data on RK3588 cpu2:
>
> - 3.79% 0.42% zram [zram] [k] slot_free
> - 89.04% slot_free
> - 65.40% zs_free
> + 77.29% free_zspage
> + 21.75% kmem_cache_free
> 0.68% __kern_my_cpu_offset
> + 13.19% _raw_spin_unlock
> + 4.86% _raw_read_unlock
> 4.75% obj_free
> + 4.72% _raw_read_lock
> 3.64% fix_fullness_group
> + 2.02% _raw_spin_lock
> + 1.31% kmem_cache_free
>
> It's clear that zs_free is the primary hotspot, accounting for ~65.40%
> of the total slot_free cycles. Beyond that, have some read and spin lock
> in slot_free.

Just a random thought, if zs_free() is costly then it likely also affects
zswap, which makes me wonder if doing something on the zsmalloc side is a
"batter" way forward.