On Tue, 31 Oct 2017, Jason Baron wrote:
>The use of ep_call_nested() in ep_eventpoll_poll(), which is the .poll
>routine for an epoll fd, is used to prevent excessively deep epoll
>nesting, and to prevent circular paths. However, we are already preventing
>these conditions during EPOLL_CTL_ADD. In terms of too deep epoll chains,
>we do in fact allow deep nesting of the epoll fds themselves (deeper
>than EP_MAX_NESTS), however we don't allow more than EP_MAX_NESTS when
>an epoll file descriptor is actually connected to a wakeup source. Thus,
>we do not require the use of ep_call_nested(), since ep_eventpoll_poll(),
>which is called via ep_scan_ready_list() only continues nesting if there
>are events available. Since ep_call_nested() is implemented using a global
>lock, applications that make use of nested epoll can see large performance
>improvements with this change.
Improvements are quite obscene actually, such as for the following epoll_wait()
benchmark with 2 level nesting on a 80 core IvyBridge:
ncpus vanilla dirty delta
1 2447092 3028315 +23.75%
4 231265 2986954 +1191.57%
8 121631 2898796 +2283.27%
16 59749 2902056 +4757.07%
32 26837 2326314 +8568.30%
64 12926 1341281 +10276.61%
(http://linux-scalability.org/epoll/epoll-test.c)
Thanks,
Davidlohr
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