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

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Message-ID: <508A4007.5080906@gmail.com>
Date:	Fri, 26 Oct 2012 15:47:19 +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 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
>

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