From: Ni zhan Chen <nizhan.chen@gmail.com>
To: YingHang Zhu <casualfisher@gmail.com>,
Fengguang Wu <fengguang.wu@intel.com>
Cc: Dave Chinner <david@fromorbit.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: Thu, 25 Oct 2012 09:48:46 +0800 [thread overview]
Message-ID: <50889A7E.8010104@gmail.com> (raw)
In-Reply-To: <CAA9v8mExDX1TYgCrRfYuh82SnNmNkqC4HjkmczSnz3Ca4zT_qw@mail.gmail.com>
On 10/25/2012 08:17 AM, YingHang Zhu wrote:
> On Thu, Oct 25, 2012 at 4:19 AM, Dave Chinner <david@fromorbit.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@fromorbit.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@fromorbit.com
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
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>
next prev parent reply other threads:[~2012-10-25 1:49 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 [this message]
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
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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