Date: Fri, 26 Oct 2012 15:36:30 +0800
From: Fengguang Wu <fengguang.wu@intel.com>
To: Ni zhan Chen <nizhan.chen@gmail.com>
Cc: Dave Chinner <david@fromorbit.com>, YingHang Zhu <casualfisher@gmail.com>,
        akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state
Message-ID: <20121026073630.GA12886@localhost>
References: <CAA9v8mEULAEHn8qSsFokEue3c0hy8pK8bkYB+6xOtz_Tgbp0vw@mail.gmail.com>
 <50889FF1.9030107@gmail.com>
 <20121025025826.GB23462@localhost>
 <20121026002544.GI29378@dastard>
 <20121026012758.GA6282@localhost>
 <5089F5AD.5040708@gmail.com>
 <20121026065855.GA9179@localhost>
 <508A35B0.30106@gmail.com>
 <20121026070936.GA12282@localhost>
 <508A399D.6000506@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <508A399D.6000506@gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1831
Lines: 46

On Fri, Oct 26, 2012 at 03:19:57PM +0800, Ni zhan Chen wrote:
> On 10/26/2012 03:09 PM, Fengguang Wu wrote:
> >On Fri, Oct 26, 2012 at 03:03:12PM +0800, Ni zhan Chen wrote:
> >>On 10/26/2012 02:58 PM, Fengguang Wu wrote:
> >>>>  static void shrink_readahead_size_eio(struct file *filp,
> >>>>                                         struct file_ra_state *ra)
> >>>>  {
> >>>>-       ra->ra_pages /= 4;
> >>>>+       spin_lock(&filp->f_lock);
> >>>>+       filp->f_mode |= FMODE_RANDOM;
> >>>>+       spin_unlock(&filp->f_lock);
> >>>>
> >>>>As the example in comment above this function, the read maybe still
> >>>>sequential, and it will waste IO bandwith if modify to FMODE_RANDOM
> >>>>directly.
> >>>Yes immediately disabling readahead may hurt IO performance, the
> >>>original '/ 4' may perform better when there are only 1-3 IO errors
> >>>encountered.
> >>Hi Fengguang,
> >>
> >>Why the number should be 1-3?
> >The original behavior is '/= 4' on each error.
> >
> >After 1 errors, readahead size will be shrinked by 1/4
> >After 2 errors, readahead size will be shrinked by 1/16
> >After 3 errors, readahead size will be shrinked by 1/64
> >After 4 errors, readahead size will be effectively 0 (disabled)
> 
> But from function shrink_readahead_size_eio and its caller
> filemap_fault I can't find the behavior you mentioned. How you
> figure out it?

It's this line in shrink_readahead_size_eio():

        ra->ra_pages /= 4;

That ra_pages will keep shrinking by 4 on each error. The only way to
restore it is to reopen the file, or POSIX_FADV_SEQUENTIAL.

Thanks,
Fengguang
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/