Jason Baron <jbaron@xxxxxxxxxx> wrote: > On 02/09/2015 05:45 PM, Andy Lutomirski wrote: > > On Mon, Feb 9, 2015 at 1:32 PM, Jason Baron <jbaron@xxxxxxxxxx> wrote: > >> On 02/09/2015 03:18 PM, Andy Lutomirski wrote: > >>> On 02/09/2015 12:06 PM, Jason Baron wrote: > >>>> Epoll file descriptors that are added to a shared wakeup source are always > >>>> added in a non-exclusive manner. That means that when we have multiple epoll > >>>> fds attached to a shared wakeup source they are all woken up. This can > >>>> lead to excessive cpu usage and uneven load distribution. > >>>> > >>>> This patch introduces two new 'events' flags that are intended to be used > >>>> with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event > >>>> source in an exclusive manner such that the minimum number of threads are > >>>> woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can > >>>> also be added to the 'events' flag, such that we round robin around the set > >>>> of waiting threads. > >>>> > >>>> An implementation note is that in the epoll wakeup routine, > >>>> 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful > >>>> wakeup, only when there are current waiters. The idea is to use this additional > >>>> heuristic in order minimize wakeup latencies. > >>> I don't understand what this is intended to do. > >>> > >>> If an event has EPOLLONESHOT, then this only one thread should be woken regardless, right? If not, isn't that just a bug that should be fixed? > >>> > >> hmm...so with EPOLLONESHOT you basically get notified once about an event. If i have multiple epoll fds (say 1 per-thread) attached to a single source in EPOLLONESHOT, then all threads will potentially get woken up once per event. Then, I would have to re-arm all of them. So I don't think this addresses this particular usecase...what I am trying to avoid is this mass wakeup or thundering herd for a shared event source. > > Now I understand. Why are you using multiple epollfds? > > > > --Andy > > So the multiple epollfds is really a way to partition the set of > events. Otherwise, I have all the threads contending on all the events > that are being generated. So I'm not sure if that is scalable. I wonder if EPOLLONESHOT + epoll_wait with a sufficiently large maxevents value is sufficient for you. All events would be shared, so they can migrate between threads(*). Each thread takes a largish set of events on every epoll_wait call and doesn't call epoll_wait again until it's done with the whole set it got. You'll hit more contention on EPOLL_CTL_MOD with shared events and a single epoll, but I think it's a better goal to make that lock-free. (*) Too large a maxevents will lead to head-of-line blocking, but from what I'm inferring, you already risk that with multiple epollfds and separate threads working on them. Do you have a userland use case to share? > In the use-case I'm trying to describe, I've partitioned a large set > of the events, but there may still be some event sources that we wish > to share among all of the threads (or even subsets of them), so as not > to overload any one in particular. > More specifically, in the case of a single listen socket, its natural > to call accept() on the thread that has been woken up, but without > doing round robin, you quickly get into a very unbalanced load, and in > addition you waste a lot of cpu doing unnecessary wakeups. There are > other approaches to solve this, specifically using SO_REUSEPORT, which > creates a separate socket per-thread and gets one back to the > separately partitioned events case previously described. However, > SO_REUSEPORT, I believe is very specific to tcp/udp, and in addition > does not have knowledge of the threads that are actively waiting as > the epoll code does. Did you try my suggestion of using a dedicated thread (or thread pool) which does nothing but loop on accept() + EPOLL_CTL_ADD? Those dedicated threads could do its own round-robin in userland to pick a different epollfd to call EPOLL_CTL_ADD on. -- 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
