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

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Date:	Thu, 25 Oct 2012 09:48:46 +0800
From:	Ni zhan Chen <nizhan.chen@...il.com>
To:	YingHang Zhu <casualfisher@...il.com>,
	Fengguang Wu <fengguang.wu@...el.com>
CC:	Dave Chinner <david@...morbit.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 10/25/2012 08:17 AM, YingHang Zhu wrote:
> On Thu, Oct 25, 2012 at 4:19 AM, Dave Chinner <david@...morbit.com> wrote:
>> On Wed, Oct 24, 2012 at 07:53:59AM +0800, YingHang Zhu wrote:
>>> Hi Dave,
>>> On Wed, Oct 24, 2012 at 6:47 AM, Dave Chinner <david@...morbit.com> wrote:
>>>> On Tue, Oct 23, 2012 at 08:46:51PM +0800, Ying Zhu wrote:
>>>>> Hi,
>>>>>    Recently we ran into the bug that an opened file's ra_pages does not
>>>>> synchronize with it's backing device's when the latter is changed
>>>>> with blockdev --setra, the application needs to reopen the file
>>>>> to know the change,
>>>> or simply call fadvise(fd, POSIX_FADV_NORMAL) to reset the readhead
>>>> window to the (new) bdi default.
>>>>
>>>>> which is inappropriate under our circumstances.
>>>> Which are? We don't know your circumstances, so you need to tell us
>>>> why you need this and why existing methods of handling such changes
>>>> are insufficient...
>>>>
>>>> Optimal readahead windows tend to be a physical property of the
>>>> storage and that does not tend to change dynamically. Hence block
>>>> device readahead should only need to be set up once, and generally
>>>> that can be done before the filesystem is mounted and files are
>>>> opened (e.g. via udev rules). Hence you need to explain why you need
>>>> to change the default block device readahead on the fly, and why
>>>> fadvise(POSIX_FADV_NORMAL) is "inappropriate" to set readahead
>>>> windows to the new defaults.
>>> Our system is a fuse-based file system, fuse creates a
>>> pseudo backing device for the user space file systems, the default readahead
>>> size is 128KB and it can't fully utilize the backing storage's read ability,
>>> so we should tune it.
>> Sure, but that doesn't tell me anything about why you can't do this
>> at mount time before the application opens any files. i.e.  you've
>> simply stated the reason why readahead is tunable, not why you need
>> to be fully dynamic.....
> We store our file system's data on different disks so we need to change ra_pages
> dynamically according to where the data resides, it can't be fixed at mount time
> or when we open files.
> The abstract bdi of fuse and btrfs provides some dynamically changing
> bdi.ra_pages
> based on the real backing device. IMHO this should not be ignored.

And how to tune ra_pages if one big file distribution in different 
disks, I think Fengguang Wu can answer these questions,

Hi Fengguang,

>>> The above third-party application using our file system maintains
>>> some long-opened files, we does not have any chances
>>> to force them to call fadvise(POSIX_FADV_NORMAL). :(
>> So raise a bug/feature request with the third party.  Modifying
>> kernel code because you can't directly modify the application isn't
>> the best solution for anyone. This really is an application problem
>> - the kernel already provides the mechanisms to solve this
>> problem...  :/
> Thanks for advice, I will consult the above application's developers
> for more information.
> Now from the code itself should we merge the gap between the real
> device's ra_pages and the file's?
> Obviously the ra_pages is duplicated, otherwise each time we run into this
> problem, someone will do the same work as I have done here.
>
> Thanks,
>           Ying Zhu
>> Cheers,
>>
>> Dave.
>> --
>> Dave Chinner
>> david@...morbit.com
> --
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