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

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Date:	Fri, 26 Oct 2012 09:27:58 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Ni zhan Chen <nizhan.chen@...il.com>,
	YingHang Zhu <casualfisher@...il.com>,
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

On Fri, Oct 26, 2012 at 11:25:44AM +1100, Dave Chinner 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....

Good point!

.... but wait .... this patch removes file->f_ra.ra_pages in all other
places too, so there will be no file->f_ra.ra_pages to be reset here... 

Thanks,
Fengguang
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