linux-kernel - Re: [RFC PATCH] mm/readahead: readahead aggressively if read drops in willneed range

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Date: Tue, 30 Jan 2024 09:12:59 +1100
From: Dave Chinner <david@...morbit.com>
To: Mike Snitzer <snitzer@...nel.org>
Cc: Ming Lei <ming.lei@...hat.com>, Matthew Wilcox <willy@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Don Dutile <ddutile@...hat.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, linux-block@...r.kernel.org
Subject: Re: [RFC PATCH] mm/readahead: readahead aggressively if read drops
 in willneed range

On Mon, Jan 29, 2024 at 12:19:02PM -0500, Mike Snitzer wrote:
> While I'm sure this legacy application would love to not have to
> change its code at all, I think we can all agree that we need to just
> focus on how best to advise applications that have mixed workloads
> accomplish efficient mmap+read of both sequential and random.
> 
> To that end, I heard Dave clearly suggest 2 things:
> 
> 1) update MADV/FADV_SEQUENTIAL to set file->f_ra.ra_pages to
>    bdi->io_pages, not bdi->ra_pages * 2
> 
> 2) Have the application first issue MADV_SEQUENTIAL to convey that for
>    the following MADV_WILLNEED is for sequential file load (so it is
>    desirable to use larger ra_pages)
> 
> This overrides the default of bdi->io_pages and _should_ provide the
> required per-file duality of control for readahead, correct?

I just discovered MADV_POPULATE_READ - see my reply to Ming
up-thread about that. The applicaiton should use that instead of
MADV_WILLNEED because it gives cache population guarantees that
WILLNEED doesn't. Then we can look at optimising the performance of
MADV_POPULATE_READ (if needed) as there is constrained scope we can
optimise within in ways that we cannot do with WILLNEED.

-Dave.
-- 
Dave Chinner
david@...morbit.com