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@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. That looks good, but you typoed the kernel version - it should read 'since 2.6.25' :-) -- Jens Axboe -- 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
