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

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Date:	Fri, 26 Oct 2012 10:30:05 +0800
From:	Ni zhan Chen <nizhan.chen@...il.com>
To:	Fengguang Wu <fengguang.wu@...el.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 10/26/2012 09:27 AM, Fengguang Wu wrote:
> 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...

In his patch,

  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.

>
> Thanks,
> Fengguang
>

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