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

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Message-ID: <20250529231432.GB1271329@cmpxchg.org>
Date: Thu, 29 May 2025 19:14:32 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Matthew Wilcox <willy@...radead.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	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>, Barry Song <21cnbao@...il.com>,
	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 Thu, May 29, 2025 at 05:24:23PM -0400, Liam R. Howlett wrote:
> * Johannes Weiner <hannes@...xchg.org> [250529 17:14]:
> > 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:
> 
> Can you clarify this benchmark please?
>
> Is this running 'git log', then stream IO, then running 'git log' again?

Yes, but it's running git log twice first. On the vanilla kernel this
is the number of references when we usually activate.

You can substitute any sequence of commands that would interact with
the git objects repeatedly before a pause where programmer thinks.

You can probably get similar mmapIO patterns with sqlite, lmdb, etc.

Periodically running executables and scripts are another case. They
tend to be less latency-sensitive I suppose, but would still
unnecessarily eat into the available IO bandwidth.