On Thu, 01/08 09:57, Andy Lutomirski wrote: > On Thu, Jan 8, 2015 at 1:12 AM, 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. > > I'd like to see a more ambitious change, since the timer isn't the > only problem like this. Specifically, I'd like a syscall that does a > list of epoll-related things and then waits. The list of things could > include, at least: > > - EPOLL_CTL_MOD actions: level-triggered epoll users are likely to > want to turn on and off their requests for events on a somewhat > regular basis. This sounds good to me. > > - timerfd_settime actions: this allows a single syscall to wait and > adjust *both* monotonic and real-time wakeups. I'm not sure, doesn't this break orthogonality between epoll and timerfd? > > Would this make sense? It could look like: > > int epoll_mod_and_pwait(int epfd, > struct epoll_event *events, int maxevents, > struct epoll_command *commands, int ncommands, > const sigset_t *sigmask); What about flags? Fam -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
