linux-kernel - Re: CLONE_IO documentation

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Date:	Thu, 20 Nov 2008 06:52:23 -0500
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Jens Axboe" <jens.axboe@...cle.com>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	linux-man@...r.kernel.org
Subject: Re: CLONE_IO documentation

On Thu, Nov 20, 2008 at 2:48 AM, Jens Axboe <jens.axboe@...cle.com> wrote:
> On Wed, Nov 19 2008, Michael Kerrisk wrote:
>> Hi Jens,
>>
>> Following up after a long time on this:
>>
>> On Mon, Apr 14, 2008 at 12:13 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
>> > On Mon, Apr 14 2008, Michael Kerrisk wrote:
>> >> Hi Jens,
>> >>
>> >> Could you supply some text describing CLONE_IO suitable for inclusion
>> >> in the clone.2 man page?
>> >> ( http://www.kernel.org/doc/man-pages/online/pages/man2/clone.2.html
>> >> ).  In that text it would be helpful to explain what an "I/O context"
>> >> is.
>> >
>> > Sure, I'll see if I can come up with something. Or perhaps you can help
>> > me a bit, being the writer ;-)
>> >
>> > If the CLONE_IO flag is set, the process will share the same io context.
>> > The I/O context is the I/O scope of the disk scheduler. So if you think
>> > of the I/O context as what the I/O scheduler uses to map to a process,
>> > when CLONE_IO is set multiple processes will map to the same I/O context
>> > and will be treated as one by the I/O scheduler. What this means is that
>> > they get to share disk time. For the anticipatory and CFQ scheduler, if
>> > process A and process B share I/O context, they will be allowed to
>> > interleave their disk access. So if you have several threads doing I/O
>> > on behalf of the same process (aio_read(), for instance), they should
>> > set CLONE_IO to get better I/O performance with CFQ and AS.
>> >
>> > A man page should not mention the specific schedulers, just mention that
>> > it'll improve the information available to the kernel and the
>> > performance of the app for the scenario described. In practice, it'll
>> > only really apply to CFQ and AS. For deadline and noop, they'll be
>> > essentially zero difference as they have no concept of I/O contexts.
>>
>> I took your text as a base but did some reworking, so *please check
>> the following carefully*,  and let me know if there are things to
>> change and/or add:
>>
>>        CLONE_IO (since Linux 2.4.25)
>>               If  CLONE_IO  is set, then the new process shares an I/O
>>               context with the calling process.  If this flag  is  not
>>               set,  then (as with fork(2)) the new process has its own
>>               I/O context.
>>
>>               The I/O context is the I/O scope of the  disk  scheduler
>>               (i.e, what the I/O scheduler uses to model scheduling of
>>               a process's I/O).  If processes share the same I/O  con-
>>               text,  they are treated as one by the I/O scheduler.  As
>>               a consequence, they get to share disk  time.   For  some
>>               I/O  schedulers,  if two processes share an I/O context,
>>               they will be allowed to interleave  their  disk  access.
>>               If  several  threads are doing I/O on behalf of the same
>>               process (aio_read(3), for instance), they should  employ
>>               CLONE_IO to get better I/O performance.
>>
>>               If  the  kernel  is not configured with the CONFIG_BLOCK
>>               option, this flag is a no-op.
>>
>> The patch against clone.2 is below.
>
> That looks good,

Okay -- thanks.

> but you typoed the kernel version - it should read
> 'since 2.6.25' :-)

Will fix; thanks.

Cheers,

Michael
-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git
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