94 lines
3.1 KiB
Diff
94 lines
3.1 KiB
Diff
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From b137f071988d9b01c05cb693a56d4359f19cdb2c Mon Sep 17 00:00:00 2001
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From: Andy Pan <i@andypan.me>
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Date: Wed, 17 Apr 2024 10:36:47 +0000
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Subject: [PATCH] Avoid calling read(2) on eventfd on each event-loop wakeup
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Register the eventfd with EPOLLET to enable edge-triggered notification
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where we don't need to read the data from the eventfd for every wakeup
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event.
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When the eventfd counter reaches the maximum value of the unsigned 64-bit,
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we rewind the counter and retry again. This optimization saves one system
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call on each event-loop wakeup, which eliminates the extra latency for epoll
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as the EVFILT_USER filter does for the kqueue.
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---
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event.c | 38 ++++++++++++++++++++++++--------------
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1 file changed, 24 insertions(+), 14 deletions(-)
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diff --git a/event.c b/event.c
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index 7a42b73..e37e588 100644
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--- a/event.c
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+++ b/event.c
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@@ -211,7 +211,7 @@ int event_debug_mode_on_ = 0;
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* to be shared across threads (if thread support is enabled).
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*
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* When and if evthreads are initialized, this variable will be evaluated,
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- * and if set to something other than zero, this means the evthread setup
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+ * and if set to something other than zero, this means the evthread setup
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* functions were called out of order.
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*
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* See: "Locks and threading" in the documentation.
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@@ -2523,13 +2523,30 @@ evthread_notify_base_default(struct event_base *base)
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static int
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evthread_notify_base_eventfd(struct event_base *base)
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{
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+ int efd = base->th_notify_fd[0];
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ev_uint64_t msg = 1;
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- int r;
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- do {
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- r = write(base->th_notify_fd[0], (void*) &msg, sizeof(msg));
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- } while (r < 0 && errno == EAGAIN);
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+ ev_uint64_t val;
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- return (r < 0) ? -1 : 0;
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+ int ret;
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+ for (;;) {
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+ ret = eventfd_write(efd, (eventfd_t) msg);
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+ if (ret < 0) {
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+ // When EAGAIN occurs, the eventfd counter hits the maximum value of the unsigned 64-bit.
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+ // We need to first drain the eventfd and then write again.
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+ //
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+ // Check out https://man7.org/linux/man-pages/man2/eventfd.2.html for details.
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+ if (errno == EAGAIN) {
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+ // It's ready to retry.
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+ if (eventfd_read(efd, &val) == 0 || errno == EAGAIN) {
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+ continue;
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+ }
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+ }
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+ // Unknown error occurs.
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+ ret = -1;
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+ }
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+ break;
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+ }
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+ return ret;
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}
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#endif
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@@ -3582,14 +3599,7 @@ event_set_mem_functions(void *(*malloc_fn)(size_t sz),
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static void
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evthread_notify_drain_eventfd(evutil_socket_t fd, short what, void *arg)
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{
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- ev_uint64_t msg;
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- ev_ssize_t r;
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struct event_base *base = arg;
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-
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- r = read(fd, (void*) &msg, sizeof(msg));
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- if (r<0 && errno != EAGAIN) {
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- event_sock_warn(fd, "Error reading from eventfd");
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- }
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EVBASE_ACQUIRE_LOCK(base, th_base_lock);
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base->is_notify_pending = 0;
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EVBASE_RELEASE_LOCK(base, th_base_lock);
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@@ -3667,7 +3677,7 @@ evthread_make_base_notifiable_nolock_(struct event_base *base)
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/* prepare an event that we can use for wakeup */
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event_assign(&base->th_notify, base, base->th_notify_fd[0],
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- EV_READ|EV_PERSIST, cb, base);
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+ EV_READ|EV_PERSIST|EV_ET, cb, base);
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/* we need to mark this as internal event */
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base->th_notify.ev_flags |= EVLIST_INTERNAL;
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--
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2.27.0
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