Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

[Dave Chinner <david@fromorbit.com>]

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....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com