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

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Date:	Fri, 26 Oct 2012 16:02:40 +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 03:47:19PM +0800, Ni zhan Chen wrote:
> On 10/26/2012 03:36 PM, Fengguang Wu wrote:
> >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;
> 
> Yeah, I mean why the 4th readahead size will be 0(disabled)? What's
> the original value of ra->ra_pages? How can guarantee the 4th shrink
> readahead size can be 0?

Ah OK, I'm talking about the typical case. The default readahead size
is 128k, which will become 0 after / 256. The reasonable good ra size
for hard disks is 1MB=256pages, which also becomes 1page after 4 errors.

Thanks,
Fengguang
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