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

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Date:	Fri, 26 Oct 2012 15:36:30 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Ni zhan Chen <nizhan.chen@...il.com>
Cc:	Dave Chinner <david@...morbit.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 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
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