linux-kernel - Re: Re: [PATCH] mm: memcg: Use larger chunks for proactive reclaim

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Date: Fri, 2 Feb 2024 11:15:31 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Efly Young <yangyifei03@...ishou.com>
Cc: hannes@...xchg.org, akpm@...ux-foundation.org, android-mm@...gle.com, 
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	mhocko@...nel.org, muchun.song@...ux.dev, roman.gushchin@...ux.dev, 
	shakeelb@...gle.com, tjmercier@...gle.com, yuzhao@...gle.com
Subject: Re: Re: [PATCH] mm: memcg: Use larger chunks for proactive reclaim

On Fri, Feb 02, 2024 at 01:02:47PM +0800, Efly Young <yangyifei03@...ishou.com> wrote:
> > Looking at the code, I'm not quite sure if this can be read this
> > literally. Efly might be able to elaborate, but we do a full loop of
> > all nodes and cgroups in the tree before checking nr_to_reclaimed, and
> > rely on priority level for granularity. So request size and complexity
> > of the cgroup tree play a role. I don't know where the exact factor
> > two would come from.
> 
> I'm sorry that this conclusion may be arbitrary. It might just only suit
> for my case. In my case, I traced it loop twice every time before checking
> nr_reclaimed, and it reclaimed less than my request size(1G) every time.
> So I think the upper bound is 2 * request. But now it seems that this is
> related to cgroup tree I constucted and my system status and my request
> size(a relatively large chunk). So there are many influencing factors,
> a specific upper bound is not accurate.

Alright, thanks for the background.

> > IMO it's more accurate to phrase it like this:
> > 
> > Reclaim tries to balance nr_to_reclaim fidelity with fairness across
> > nodes and cgroups over which the pages are spread. As such, the bigger
> > the request, the bigger the absolute overreclaim error. Historic
> > in-kernel users of reclaim have used fixed, small request batches to
> > approach an appropriate reclaim rate over time. When we reclaim a user
> > request of arbitrary size, use decaying batches to manage error while
> > maintaining reasonable throughput.

Hm, decay...
So shouldn't the formula be
  nr_pages = delta <= SWAP_CLUSTER_MAX ? delta : (delta + 3*SWAP_CLUSTER_MAX) / 4
where
  delta = nr_to_reclaim - nr_reclaimed
?
(So that convergence for smaller deltas is same like original- and other
reclaims while conservative factor is applied for effectivity of higher
user requests.)

Thanks,
Michal

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