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

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Date:	Fri, 26 Oct 2012 15:19:57 +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 03:09 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?

Regards,
Chen

>
> Thanks,
> Fengguang
>

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