Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

Ni zhan Chen
2012-10-26T08:08:31Z

On 10/26/2012 04:02 PM, Fengguang Wu wrote:
> 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.

Then why default size is not set to reasonable size?

Regards,
Chen

>
> Thanks,
> Fengguang
>