> -----Original Message----- > From: Michael Kerrisk (man-pages) [mailto:mtk.manpages@xxxxxxxxx] > > > Since we are on the subject, I also believe that the man page for epoll_wait > > should state whether sequence of the signaled fd's is guaranteed to be > > correlated to the order in which they were specified. > > There is not such a guarantee, as far as I know. > > > As you know, there are > > many situations where programmer might want to wait on, say, eventfd > that > > says that thread should exit, plus four other semaphores, etc. If the four > > other semaphores guard queues that are, in general, never empty, then on > > each loop, the semaphores will be triggered. If then, on one of the loops, > > the eventfd for "exit" is triggered, there is no guarantee, according to the > > man page, that the "exit" eventfd will be returned in the triggered-set. It > > would seem natural that, if the "exit" eventfd is added to the first > > position with epoll_ctl, then of any trigger-set where "exit" eventfd is > > set, that "exit" eventfd ~must~ be included as a matter of explicit > > prioritization. Guaranteeing a correlation between specified order in > > epoll_ctrl and yielded order in epoll_wait would allow the programmer to > > specify maxevents=1 to epoll_wait, at which point, the "exit" eventfd, if > > triggered, would preempt any of the semaphores. > > > > Or...maybe I am missing something else? > > The only guarantee that I know of is that if multiple FDs a ready and > epoll_wait() requests fewer events than there are FDs ready, then > successive calls will cycle though the list of ready FDs. This helps avoid > starvation. I am starting a new thread since my last question was answered sufficiently to get my work done. I believe that epoll_wait should guarantee that it will yield the triggered fd's in the order in which they were specified to epoll_ctl. Just yesterday, I had a situation where I needed this guarantee. I had four queues that are annotated with semaphores. I intended to use epoll_wait to wait on the semaphores, in addition to several other fd's. When my thread unblocks, I need two of the semaphores to take priority over the other two. Normally, I would specify those two fd's first in my array to epoll_ctl. As is known, WaitForMultipleObjects, under Windows, yields only 1 fd per invocation, and that fd is the first fd in the array that was specified to WaitForMultipleObjects that has become triggered. This gives the programmer a convenient to specifying arbitration (prioritizations) among simultaneously triggered fd's. With epoll_wait, if say, 4 fd's are triggered, without a guarantee to honor order-of-specification, the programmer would effectively have to sort the triggered fd's in the order that makes sense for his/her application to determine if one or more "critical" fd's have been triggered. -Nut -- 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
