Hi Jens, Following up after a long time on this: On Mon, Apr 14, 2008 at 12:13 PM, Jens Axboe <jens.axboe@xxxxxxxxxx> 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. Thanks, Michael --- a/man2/clone.2 +++ b/man2/clone.2 @@ -224,6 +223,36 @@ Calls to .BR umask (2) performed later by one of the processes do not affect the other process. .TP +.BR CLONE_IO " (since Linux 2.4.25)" +If +.B 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 +.BR fork (2)) +the new process has its own I/O context. + +.\" The following based on text from Jens Axboe +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 context, +they are treated as one by the I/O scheduler. +As a consequence, they get to share disk time. +For some I/O schedulers, +.\" the anticipatory and CFQ scheduler +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 +.RB ( aio_read (3), +for instance), they should employ +.BR CLONE_IO +to get better I/O performance. +.\" with CFQ and AS. + +If the kernel is not configured with the +.B CONFIG_BLOCK +option, this flag is a no-op. +.TP .BR CLONE_NEWIPC " (since Linux 2.4.19)" If .B CLONE_NEWIPC -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
