On 8 January 2015 at 10:12, Miklos Szeredi <mszeredi@xxxxxxx> wrote: > On Thu, 2015-01-08 at 16:25 +0800, Fam Zheng wrote: >> Applications could use epoll interface when then need to poll a big number of >> files in their main loops, to achieve better performance than ppoll(2). Except >> for one concern: epoll only takes timeout parameters in microseconds, rather >> than nanoseconds. >> >> That is a drawback we should address. For a real case in QEMU, we run into a >> scalability issue with ppoll(2) when many devices are attached to guest, in >> which case many host fds, such as virtual disk images and sockets, need to be >> polled by the main loop. As a result we are looking at switching to epoll, but >> the coarse timeout precision is a trouble, as explained below. >> >> We're already using prctl(PR_SET_TIMERSLACK, 1) which is necessary to implement >> timers in the main loop; and we call ppoll(2) with the next firing timer as >> timeout, so when ppoll(2) returns, we know that we have more work to do (either >> handling IO events, or fire a timer callback). This is natual and efficient, >> except that ppoll(2) itself is slow. >> >> Now that we want to switch to epoll, to speed up the polling. However the timer >> slack setting will be effectively undone, because that way we will have to >> round up the timeout to microseconds honoring timer contract. But consequently, >> this hurts the general responsiveness. >> >> Note: there are two alternatives, without changing kernel: >> >> 1) Leading ppoll(2), with the epollfd only and a nanosecond timeout. It won't >> be slow as one fd is polled. No more scalability issue. And if there are >> events, we know from ppoll(2)'s return, then we do the epoll_wait(2) with >> timeout=0; otherwise, there can't be events for the epoll, skip the following >> epoll_wait and just continue with other work. >> >> 2) Setup and add a timerfd to epoll, then we do epoll_wait(..., timeout=-1). >> The timerfd will hopefully force epoll_wait to return when it timeouts, even if >> no other events have arrived. This will inheritly give us timerfd's precision. >> Note that for each poll, the desired timeout is different because the next >> timer is different, so that, before each epoll_wait(2), there will be a >> timerfd_settime syscall to set it to a proper value. >> >> Unfortunately, both approaches require one more syscall per iteration, compared >> to the original single ppoll(2), cost of which is unneglectable when we talk >> about nanosecond granularity. > > Please consider adding a "flags" argument to the new syscall (and > returning EINVAL if non-zero). See this article, which shows that > extended syscalls almost always want flags, and they often get it only > on the second try: > > http://lwn.net/Articles/585415/ Yes, please ensure that the new call gets a flags argument. We've made this mistake way too many times in the past. Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
