Date: Fri, 26 Oct 2012 15:09:36 +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
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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)

Thanks,
Fengguang
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