Messages in this thread | | | Date | Tue, 25 Jun 2019 00:24:56 +0000 | From | Eric Wong <> | Subject | Re: [PATCH v5 00/14] epoll: support pollable epoll from userspace |
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Roman Penyaev <rpenyaev@suse.de> wrote: > Hi all,
+cc Jason Baron
> ** Limitations
<snip>
> 4. No support for EPOLLEXCLUSIVE > If device does not pass pollflags to wake_up() there is no way to > call poll() from the context under spinlock, thus special work is > scheduled to offload polling. In this specific case we can't > support exclusive wakeups, because we do not know actual result > of scheduled work and have to wake up every waiter.
Lacking EPOLLEXCLUSIVE support is probably a showstopper for common applications using per-task epoll combined with non-blocking accept4() (e.g. nginx).
Fwiw, I'm still a weirdo who prefers a dedicated thread doing blocking accept4 for distribution between tasks (so epoll never sees a listen socket). But, depending on what runtime/language I'm using, I can't always dedicate a blocking thread, so I recently started using EPOLLEXCLUSIVE from Perl5 where I couldn't rely on threads being available.
If I could dedicate time to improving epoll; I'd probably add writev() support for batching epoll_ctl modifications to reduce syscall traffic, or pick-up the kevent()-like interface started long ago: https://lore.kernel.org/lkml/1393206162-18151-1-git-send-email-n1ght.4nd.d4y@gmail.com/ (but I'm not sure I want to increase the size of the syscall table).
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