On Thu, Nov 20, 2008 at 2:48 AM, Jens Axboe <jens.axboe@xxxxxxxxxx> 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@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, 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 man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html -- 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
