linux-kernel - Re: [RFC v3] mm: add page preemption

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Date:   Sun, 3 Nov 2019 09:06:49 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>,
        Jan Kara <jack@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v3] mm: add page preemption

On Sun, Nov 03, 2019 at 07:57:27PM +0800, Hillf Danton wrote:
> The cpu preemption feature makes a task able to preempt other tasks
> of lower priorities for cpu. It has been around for a while.
> 
> This work introduces task prio into page reclaiming in order to add
> the page preemption feature that makes a task able to preempt other
> tasks of lower priorities for page.
> 
> No page will be reclaimed on behalf of tasks of lower priorities
> under pp, a two-edge feature that functions only under memory
> pressure, laying a barrier to pages flowing to lower prio, and the
> nice syscall is what users need to fiddle with it for instance if
> they have a bunch of workloads to run in datacenter, and some
> difficulty predicting the runtime working set size for every
> individual workload which is sensitive to jitters in lru pages.
> 
> Currently pages are reclaimed without prio taken into account; pages
> can be reclaimed from tasks of lower priorities on behalf of
> higher-prio tasks and vice versa.
> 
> s/and vice versa/only/ is what we need to make pp by definition, but
> it could not make a sense without prio introduced; otherwise we can
> simply skip deactivating the lru pages based on prio comprison, and
> work is done.
> 
> The introduction consists of two parts. On the page side, we have to
> store the page owner task's prio in page, which needs an extra room the
> size of the int type in the page struct. That room sounds impossible
> without inflating the page struct size, and is walked around by making
> pp depend on CONFIG_64BIT.

... and !MEMCG.  Which means that your work is uninteresting because all
the distros turn on CONFIG_MEMCG.

You still haven't given us any numbers.  Or a workload which actually
benefits from this patch.