Don't read from timerfd and eventfd on each EventLoop tick (#9192)

Motivation:

We do not need to issue a read on timerfd and eventfd when the EventLoop wakes up if we register these as Edge-Triggered. This removes the overhead of 2 syscalls and so helps to reduce latency.

Modifications:

- Ensure we register the timerfd and eventfd with EPOLLET flag
- If eventfd_write fails with EAGAIN, call eventfd_read and try eventfd_write again as we only use it as wake-up mechanism.

Result:

Less syscalls and so reducing overhead.

Co-authored-by: Carl Mastrangelo <carl@carlmastrangelo.com>
This commit is contained in:
Norman Maurer 2019-05-31 06:59:39 +02:00 committed by GitHub
parent ede7251ecb
commit f6cf681f90
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2 changed files with 29 additions and 10 deletions

View File

@ -116,10 +116,27 @@ static jint netty_epoll_native_timerFd(JNIEnv* env, jclass clazz) {
}
static void netty_epoll_native_eventFdWrite(JNIEnv* env, jclass clazz, jint fd, jlong value) {
jint eventFD = eventfd_write(fd, (eventfd_t) value);
uint64_t val;
if (eventFD < 0) {
netty_unix_errors_throwChannelExceptionErrorNo(env, "eventfd_write() failed: ", errno);
for (;;) {
jint ret = eventfd_write(fd, (eventfd_t) value);
if (ret < 0) {
// We need to read before we can write again, let's try to read and then write again and if this
// fails we will bail out.
//
// See http://man7.org/linux/man-pages/man2/eventfd.2.html.
if (errno == EAGAIN) {
if (eventfd_read(fd, &val) == 0 || errno == EAGAIN) {
// Try again
continue;
}
netty_unix_errors_throwChannelExceptionErrorNo(env, "eventfd_read(...) failed: ", errno);
} else {
netty_unix_errors_throwChannelExceptionErrorNo(env, "eventfd_write(...) failed: ", errno);
}
}
break;
}
}

View File

@ -98,12 +98,16 @@ class EpollEventLoop extends SingleThreadEventLoop {
this.epollFd = epollFd = Native.newEpollCreate();
this.eventFd = eventFd = Native.newEventFd();
try {
Native.epollCtlAdd(epollFd.intValue(), eventFd.intValue(), Native.EPOLLIN);
// It is important to use EPOLLET here as we only want to get the notification once per
// wakeup and don't call eventfd_read(...).
Native.epollCtlAdd(epollFd.intValue(), eventFd.intValue(), Native.EPOLLIN | Native.EPOLLET);
} catch (IOException e) {
throw new IllegalStateException("Unable to add eventFd filedescriptor to epoll", e);
}
this.timerFd = timerFd = Native.newTimerFd();
try {
// It is important to use EPOLLET here as we only want to get the notification once per
// wakeup and don't call read(...).
Native.epollCtlAdd(epollFd.intValue(), timerFd.intValue(), Native.EPOLLIN | Native.EPOLLET);
} catch (IOException e) {
throw new IllegalStateException("Unable to add timerFd filedescriptor to epoll", e);
@ -402,12 +406,10 @@ class EpollEventLoop extends SingleThreadEventLoop {
private void processReady(EpollEventArray events, int ready) {
for (int i = 0; i < ready; i ++) {
final int fd = events.fd(i);
if (fd == eventFd.intValue()) {
// consume wakeup event.
Native.eventFdRead(fd);
} else if (fd == timerFd.intValue()) {
// consume wakeup event, necessary because the timer is added with ET mode.
Native.timerFdRead(fd);
if (fd == eventFd.intValue() || fd == timerFd.intValue()) {
// Just ignore as we use ET mode for the eventfd and timerfd.
//
// See also https://stackoverflow.com/a/12492308/1074097
} else {
final long ev = events.events(i);