Re: [PATCH RFC] mm/readahead: improve randread performance with readahead disabled

[Yu Kuai]

Hi,

在 2025/07/14 9:42, Yu Kuai 写道:
> Hi,
>
> 在 2025/07/01 19:08, Yu Kuai 写道:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > We have a workload of random 4k-128k read on a HDD, from iostat we 
> > observed
> > that average request size is 256k+ and bandwidth is 100MB+, this is 
> > because
> > readahead waste lots of disk bandwidth. Hence we disable readahead and
> > performance from user side is indeed much better(2x+), however, from
> > iostat we observed request size is just 4k and bandwidth is just around
> > 40MB.
> >
> > Then we do a simple dd test and found out if readahead is disabled,
> > page_cache_sync_ra() will force to read one page at a time, and this
> > really doesn't make sense because we can just issue user requested size
> > request to disk.
> >
> > Fix this problem by removing the limit to read one page at a time from
> > page_cache_sync_ra(), this way the random read workload can get better
> > performance with readahead disabled.
> >
> > PS: I'm not sure if I miss anything, so this version is RFC
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   mm/readahead.c | 12 ++++++++++--
> >   1 file changed, 10 insertions(+), 2 deletions(-)
>
> Friendly ping ...

Friendly ping again ...

> > diff --git a/mm/readahead.c b/mm/readahead.c
> > index 20d36d6b055e..1df85ccba575 100644
> > --- a/mm/readahead.c
> > +++ b/mm/readahead.c
> > @@ -561,13 +561,21 @@ void page_cache_sync_ra(struct readahead_control 
> > *ractl,
> >        * Even if readahead is disabled, issue this request as readahead
> >        * as we'll need it to satisfy the requested range. The forced
> >        * readahead will do the right thing and limit the read to just the
> > -     * requested range, which we'll set to 1 page for this case.
> > +     * requested range.
> >        */
> > -    if (!ra->ra_pages || blk_cgroup_congested()) {
> > +    if (blk_cgroup_congested()) {
> >           if (!ractl->file)
> >               return;
> > +        /*
> > +         * If the cgroup is congested, ensure to do at least 1 page of
> > +         * readahead to make progress on the read.
> > +         */
> >           req_count = 1;
> >           do_forced_ra = true;
> > +    } else if (!ra->ra_pages) {
> > +        if (!ractl->file)
> > +            return;
> > +        do_forced_ra = true;
> >       }
> >       /* be dumb */
>
> .