linux-kernel - Re: [DISCUSSION] proposed mctl() API

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Message-ID: <CAGsJ_4yKDqUu8yZjHSmWBz3CpQhU6DM0=EhibfTwHbTo+QWvZA@mail.gmail.com>
Date: Fri, 30 May 2025 19:52:28 +1200
From: Barry Song <21cnbao@...il.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Matthew Wilcox <willy@...radead.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	"Liam R . Howlett" <Liam.Howlett@...cle.com>, David Hildenbrand <david@...hat.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>, Arnd Bergmann <arnd@...db.de>, 
	Christian Brauner <brauner@...nel.org>, SeongJae Park <sj@...nel.org>, Usama Arif <usamaarif642@...il.com>, 
	Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org, 
	Pedro Falcato <pfalcato@...e.de>
Subject: Re: [DISCUSSION] proposed mctl() API

On Fri, May 30, 2025 at 9:14 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Thu, May 29, 2025 at 04:28:46PM +0100, Matthew Wilcox wrote:
> > Barry's problem is that we're all nervous about possibly regressing
> > performance on some unknown workloads.  Just try Barry's proposal, see
> > if anyone actually compains or if we're just afraid of our own shadows.
>
> I actually explained why I think this is a terrible idea. But okay, I
> tried the patch anyway.
>
> This is 'git log' on a hot kernel repo after a large IO stream:
>
>                                      VANILLA                      BARRY
> Real time                 49.93 (    +0.00%)         60.36 (   +20.48%)
> User time                 32.10 (    +0.00%)         32.09 (    -0.04%)
> System time               14.41 (    +0.00%)         14.64 (    +1.50%)
> pgmajfault              9227.00 (    +0.00%)      18390.00 (   +99.30%)
> workingset_refault_file  184.00 (    +0.00%)    236899.00 (+127954.05%)
>
> Clearly we can't generally ignore page cache hits just because the
> mmaps() are intermittent.

Hi Johannes,
Thanks!

Are you on v1, which lacks folio demotion[1], or v2, which includes it [2]?

[1] https://lore.kernel.org/linux-mm/20250412085852.48524-1-21cnbao@gmail.com/
[2] https://lore.kernel.org/linux-mm/20250514070820.51793-1-21cnbao@gmail.com/

>
> The whole point is to cache across processes and their various
> apertures into a common, long-lived filesystem space.
>
> Barry knows something about the relationship between certain processes
> and certain files that he could exploit with MADV_COLD-on-exit
> semantics. But that's not something the kernel can safely assume. Not
> without defeating the page cache for an entire class of file accesses.

Best Regards
Barry