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From: Ni zhan Chen <nizhan.chen@gmail.com>
To: Fengguang Wu <fengguang.wu@intel.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
Date: Fri, 26 Oct 2012 15:47:19 +0800	[thread overview]
Message-ID: <508A4007.5080906@gmail.com> (raw)
In-Reply-To: <20121026073630.GA12886@localhost>

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?

Regards,
Chen

>
> 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
>


  reply	other threads:[~2012-10-26  7:47 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-10-23 12:46 [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state Ying Zhu
2012-10-23 13:21 ` Ni zhan Chen
     [not found]   ` <CAA9v8mGMa3SDD1OLTG_wdhCGx7K-0kvSV1+MRi9uCGTz6zZaLg@mail.gmail.com>
2012-10-23 13:41     ` YingHang Zhu
2012-10-24  1:02       ` Ni zhan Chen
2012-10-24  1:33         ` YingHang Zhu
2012-10-23 22:47 ` Dave Chinner
2012-10-23 23:53   ` YingHang Zhu
2012-10-24 20:19     ` Dave Chinner
2012-10-25  0:17       ` YingHang Zhu
2012-10-25  1:48         ` Ni zhan Chen
2012-10-25  1:50         ` Dave Chinner
2012-10-25  2:04           ` YingHang Zhu
2012-10-25  2:12             ` Ni zhan Chen
2012-10-25  2:31               ` YingHang Zhu
2012-10-25  2:58               ` Fengguang Wu
2012-10-25  3:12                 ` YingHang Zhu
2012-10-26  0:25                 ` Dave Chinner
2012-10-26  1:27                   ` Fengguang Wu
2012-10-26  2:30                     ` Ni zhan Chen
2012-10-26  3:28                       ` YingHang Zhu
2012-10-26  3:51                         ` Ni zhan Chen
2012-10-26  4:35                           ` YingHang Zhu
2012-10-26  6:58                       ` Fengguang Wu
2012-10-26  7:03                         ` Ni zhan Chen
2012-10-26  7:09                           ` Fengguang Wu
2012-10-26  7:19                             ` Ni zhan Chen
2012-10-26  7:36                               ` Fengguang Wu
2012-10-26  7:47                                 ` Ni zhan Chen [this message]
2012-10-26  8:02                                   ` Fengguang Wu
2012-10-26  8:08                                     ` Ni zhan Chen
2012-10-26  8:13                                     ` YingHang Zhu
2012-10-26  2:25                   ` Ni zhan Chen
2012-10-26  3:38                   ` YingHang Zhu
2012-10-26  3:55                     ` Fengguang Wu
2012-10-26  5:00                       ` YingHang Zhu
2012-10-25  2:38             ` Fengguang Wu
2012-10-25  3:08               ` YingHang Zhu

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