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

Dave Chinner
2024-01-29T22:13:08Z

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@fromorbit.com