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/ Thanks, Miklos P.S. stray apostrophes in To: and Cc: lines seems to be causing trouble. -- 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
