linux-kernel - Re: [PATCH] mm: swap: async free swap slot cache entries

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Date: Sat, 23 Dec 2023 19:01:14 -0800 (PST)
From: David Rientjes <rientjes@...gle.com>
To: Chris Li <chrisl@...nel.org>
cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org, Wei Xu <weixugc@...gle.com>, 
    Yu Zhao <yuzhao@...gle.com>, Greg Thelen <gthelen@...gle.com>, 
    Chun-Tse Shao <ctshao@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>, 
    Yosry Ahmed <yosryahmed@...gle.com>, Brain Geffon <bgeffon@...gle.com>, 
    Minchan Kim <minchan@...nel.org>, Michal Hocko <mhocko@...e.com>, 
    Mel Gorman <mgorman@...hsingularity.net>, 
    Huang Ying <ying.huang@...el.com>, Nhat Pham <nphamcs@...il.com>, 
    Johannes Weiner <hannes@...xchg.org>, Kairui Song <kasong@...cent.com>, 
    Zhongkun He <hezhongkun.hzk@...edance.com>, 
    Kemeng Shi <shikemeng@...weicloud.com>, Barry Song <v-songbaohua@...o.com>, 
    Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] mm: swap: async free swap slot cache entries

On Sat, 23 Dec 2023, Chris Li wrote:

> > How do you quantify the impact of the delayed swap_entry_free()?
> >
> > Since the free and memcg uncharge are now delayed, is there not the
> > possibility that we stay under memory pressure for longer?  (Assuming at
> > least some users are swapping because of memory pressure.)
> >
> > I would assume that since the free and uncharge itself is delayed that in
> > the pathological case we'd actually be swapping *more* until the async
> > worker can run.
> 
> Thanks for raising this interesting question.
> 
> First of all, the swap_entry_free() does not impact "memory.current".
> It reduces "memory.swap.current". Technically it is the swap pressure
> not memory pressure that suffers the extra delay.
> 
> Secondly, we are talking about delaying up to 64 swap entries for a
> few microseconds.

What guarantees that the async freeing happens within a few microseconds?

> Where the swap slot cache itself delays the freeing
> of the entries for an arbitrary amount of time. It is not freed until
> the cache is full of 64 entries. This delay can be seconds or even
> minutes. Adding a few microseconds of  extra delay to existing seconds
> delay really makes no difference from the swap pressure point of view.
> 
> Chris
>