From: Ni zhan Chen <nizhan.chen@gmail.com>
To: YingHang Zhu <casualfisher@gmail.com>
Cc: Dave Chinner <david@fromorbit.com>,
akpm@linux-foundation.org, Fengguang Wu <fengguang.wu@intel.com>,
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 10:12:01 +0800 [thread overview]
Message-ID: <50889FF1.9030107@gmail.com> (raw)
In-Reply-To: <CAA9v8mEULAEHn8qSsFokEue3c0hy8pK8bkYB+6xOtz_Tgbp0vw@mail.gmail.com>
On 10/25/2012 10:04 AM, YingHang Zhu wrote:
> On Thu, Oct 25, 2012 at 9:50 AM, Dave Chinner <david@fromorbit.com> wrote:
>> On Thu, Oct 25, 2012 at 08:17:05AM +0800, 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.
>> That doesn't make a whole lot of sense to me. let me try to get this
>> straight.
>>
>> There is data that resides on two devices (A + B), and a fuse
>> filesystem to access that data. There is a single file in the fuse
>> fs has data on both devices. An app has the file open, and when the
>> data it is accessing is on device A you need to set the readahead to
>> what is best for device A? And when the app tries to access data for
>> that file that is on device B, you need to set the readahead to what
>> is best for device B? And you are changing the fuse BDI readahead
>> settings according to where the data in the back end lies?
>>
>> It seems to me that you should be setting the fuse readahead to the
>> maximum of the readahead windows the data devices have configured at
>> mount time and leaving it at that....
> Then it may not fully utilize some device's read IO bandwidth and put too much
> burden on other devices.
>>> 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.
>> btrfs simply takes into account the number of disks it has for a
>> given storage pool when setting up the default bdi ra_pages during
>> mount. This is basically doing what I suggested above. Same with
>> the generic fuse code - it's simply setting a sensible default value
>> for the given fuse configuration.
>>
>> Neither are dynamic in the sense you are talking about, though.
> Actually I've talked about it with Fengguang, he advised we should unify the
But how can bdi related ra_pages reflect different files' readahead
window? Maybe these different files are sequential read, random read and
so on.
> ra_pages in struct bdi and file_ra_state and leave the issue that
> spreading data
> across disks as it is.
> Fengguang, what's you opinion about this?
>
> 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/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>
next prev parent reply other threads:[~2012-10-25 2:12 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
2012-10-25 1:50 ` Dave Chinner
2012-10-25 2:04 ` YingHang Zhu
2012-10-25 2:12 ` Ni zhan Chen [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=50889FF1.9030107@gmail.com \
--to=nizhan.chen@gmail.com \
--cc=akpm@linux-foundation.org \
--cc=casualfisher@gmail.com \
--cc=david@fromorbit.com \
--cc=fengguang.wu@intel.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).