linux-kernel - Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra

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Date:	Fri, 26 Oct 2012 11:55:50 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	YingHang Zhu <casualfisher@...il.com>
Cc:	Dave Chinner <david@...morbit.com>, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Ni zhan Chen <nizhan.chen@...il.com>
Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

On Fri, Oct 26, 2012 at 11:38:11AM +0800, YingHang Zhu wrote:
> On Fri, Oct 26, 2012 at 8:25 AM, Dave Chinner <david@...morbit.com> wrote:
> > On Thu, Oct 25, 2012 at 10:58:26AM +0800, Fengguang Wu wrote:
> >> Hi Chen,
> >>
> >> > But how can bdi related ra_pages reflect different files' readahead
> >> > window? Maybe these different files are sequential read, random read
> >> > and so on.
> >>
> >> It's simple: sequential reads will get ra_pages readahead size while
> >> random reads will not get readahead at all.
> >>
> >> Talking about the below chunk, it might hurt someone that explicitly
> >> takes advantage of the behavior, however the ra_pages*2 seems more
> >> like a hack than general solution to me: if the user will need
> >> POSIX_FADV_SEQUENTIAL to double the max readahead window size for
> >> improving IO performance, then why not just increase bdi->ra_pages and
> >> benefit all reads? One may argue that it offers some differential
> >> behavior to specific applications, however it may also present as a
> >> counter-optimization: if the root already tuned bdi->ra_pages to the
> >> optimal size, the doubled readahead size will only cost more memory
> >> and perhaps IO latency.
> >>
> >> --- a/mm/fadvise.c
> >> +++ b/mm/fadvise.c
> >> @@ -87,7 +86,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
> >>                 spin_unlock(&file->f_lock);
> >>                 break;
> >>         case POSIX_FADV_SEQUENTIAL:
> >> -               file->f_ra.ra_pages = bdi->ra_pages * 2;
> >
> > I think we really have to reset file->f_ra.ra_pages here as it is
> > not a set-and-forget value. e.g.  shrink_readahead_size_eio() can
> > reduce ra_pages as a result of IO errors. Hence if you have had io
> > errors, telling the kernel that you are now going to do  sequential
> > IO should reset the readahead to the maximum ra_pages value
> > supported....
> If we unify file->f_ra.ra_pages and its' bdi->ra_pages, then the error-prone
> device's readahead can be directly tuned or turned off with blockdev
> thus affect all files
> using the device and without bring more complexity...

It's not really feasible/convenient for the end users to hand tune
blockdev readahead size on IO errors. Even many administrators are
totally unaware of the readahead size parameter.

Thanks,
Fengguang
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