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When PRECISE_TIMERS is set with epoll, use timerfd for microsecond precision

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The epoll interface ordinarily gives us one-millisecond
precision, so on Linux it makes perfect sense to use the
CLOCK_MONOTONIC_COARSE timer.  But when the user has set the new
PRECISE_TIMER flag for an event_base (either by the
EVENT_BASE_FLAG_PRECISE_TIMER flag, or by the EVENT_PRECISE_TIMER
environment variable), they presumably want finer granularity.

On not-too-old Linuxes, we can achieve this using the Timerfd
mechanism, which accepts nanosecond granularity and understands
posix clocks.  It's a little more expensive than just calling
epoll_wait(), so we won't do it by default.
This commit is contained in:
Nick Mathewson 2012-04-26 16:22:03 -04:00
parent 7428c78a95
commit 26c75828b7
3 changed files with 95 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
configure.in
View File

@ -221,6 +221,7 @@ AC_CHECK_HEADERS([ \
sys/sendfile.h \
sys/socket.h \
sys/time.h \
sys/timerfd.h \
sys/uio.h \
sys/wait.h \
unistd.h \
@ -357,6 +358,7 @@ AC_CHECK_FUNCS([ \
strtok_r \
strtoll \
sysctl \
timerfd_create \
unsetenv \
usleep \
vasprintf \

84
epoll.c
View File

@ -47,6 +47,9 @@
#ifdef EVENT__HAVE_FCNTL_H
#include <fcntl.h>
#endif
#ifdef EVENT__HAVE_SYS_TIMERFD_H
#include <sys/timerfd.h>
#endif
#include "event-internal.h"
#include "evsignal-internal.h"
@ -57,10 +60,24 @@
#include "changelist-internal.h"
#include "time-internal.h"
#if defined(EVENT__HAVE_SYS_TIMERFD_H) && \
defined(EVENT__HAVE_TIMERFD_CREATE) && \
defined(HAVE_POSIX_MONOTONIC) && defined(TFD_NONBLOCK) && \
defined(TFD_CLOEXEC)
/* Note that we only use timerfd if TFD_NONBLOCK and TFD_CLOEXEC are available
and working. This means that we can't support it on 2.6.25 (where timerfd
was introduced) or 2.6.26, since 2.6.27 introduced those flags.
*/
#define USING_TIMERFD
#endif
struct epollop {
struct epoll_event *events;
int nevents;
int epfd;
#ifdef USING_TIMERFD
int timerfd;
#endif
};
static void *epoll_init(struct event_base *);
@ -147,8 +164,38 @@ epoll_init(struct event_base *base)
if ((base->flags & EVENT_BASE_FLAG_EPOLL_USE_CHANGELIST) != 0 ||
((base->flags & EVENT_BASE_FLAG_IGNORE_ENV) == 0 &&
evutil_getenv_("EVENT_EPOLL_USE_CHANGELIST") != NULL))
evutil_getenv_("EVENT_EPOLL_USE_CHANGELIST") != NULL)) {
base->evsel = &epollops_changelist;
}
#ifdef USING_TIMERFD
/*
The epoll interface ordinarily gives us one-millisecond precision,
so on Linux it makes perfect sense to use the CLOCK_MONOTONIC_COARSE
timer. But when the user has set the new PRECISE_TIMER flag for an
event_base, we can try to use timerfd to give them finer granularity.
*/
if ((base->flags & EVENT_BASE_FLAG_PRECISE_TIMER) &&
base->monotonic_timer.monotonic_clock == CLOCK_MONOTONIC) {
int fd;
fd = epollop->timerfd = timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK|TFD_CLOEXEC);
if (epollop->timerfd >= 0) {
struct epoll_event epev;
epev.data.fd = epollop->timerfd;
epev.events = EPOLLIN;
if (epoll_ctl(epollop->epfd, EPOLL_CTL_ADD, fd, &epev) < 0) {
event_warn("epoll_ctl(timerfd)");
close(fd);
epollop->timerfd = -1;
}
} else {
event_warn("timerfd_create");
}
} else {
epollop->timerfd = -1;
}
#endif
evsig_init_(base);
@ -509,6 +556,33 @@ epoll_dispatch(struct event_base *base, struct timeval *tv)
int i, res;
long timeout = -1;
#ifdef USING_TIMERFD
if (epollop->timerfd >= 0) {
struct itimerspec is;
is.it_interval.tv_sec = 0;
is.it_interval.tv_nsec = 0;
if (tv == NULL) {
/* No timeout; disarm the timer. */
is.it_value.tv_sec = 0;
is.it_value.tv_nsec = 0;
} else {
if (tv->tv_sec == 0 && tv->tv_usec == 0) {
/* we need to exit immediately; timerfd can't
* do that. */
timeout = 0;
}
is.it_value.tv_sec = tv->tv_sec;
is.it_value.tv_nsec = tv->tv_usec * 1000;
}
/* TODO: we could avoid unnecessary syscalls here by only
calling timerfd_settime when the top timeout changes, or
when we're called with a different timeval.
*/
if (timerfd_settime(epollop->timerfd, 0, &is, NULL) < 0) {
event_warn("timerfd_settime");
}
} else
#endif
if (tv != NULL) {
timeout = evutil_tv_to_msec_(tv);
if (timeout < 0 || timeout > MAX_EPOLL_TIMEOUT_MSEC) {
@ -542,6 +616,10 @@ epoll_dispatch(struct event_base *base, struct timeval *tv)
for (i = 0; i < res; i++) {
int what = events[i].events;
short ev = 0;
#ifdef USING_TIMERFD
if (events[i].data.fd == epollop->timerfd)
continue;
#endif
if (what & (EPOLLHUP|EPOLLERR)) {
ev = EV_READ | EV_WRITE;
@ -586,6 +664,10 @@ epoll_dealloc(struct event_base *base)
mm_free(epollop->events);
if (epollop->epfd >= 0)
close(epollop->epfd);
#ifdef USING_TIMERFD
if (epollop->timerfd >= 0)
close(epollop->timerfd);
#endif
memset(epollop, 0, sizeof(struct epollop));
mm_free(epollop);

11
test/test.sh
View File

@ -42,6 +42,7 @@ setup () {
eval "EVENT_NO$i=yes; export EVENT_NO$i"
done
unset EVENT_EPOLL_USE_CHANGELIST
unset EVENT_PRECISE_TIMER
}
announce () {
@ -112,16 +113,24 @@ do_test() {
unset EVENT_NO$1
if test "$2" = "(changelist)" ; then
EVENT_EPOLL_USE_CHANGELIST=yes; export EVENT_EPOLL_USE_CHANGELIST
elif test "$2" = "(timerfd)" ; then
EVENT_PRECISE_TIMER=1; export EVENT_PRECISE_TIMER
elif test "$2" = "(timerfd+changelist)" ; then
EVENT_EPOLL_USE_CHANGELIST=yes; export EVENT_EPOLL_USE_CHANGELIST
EVENT_PRECISE_TIMER=1; export EVENT_PRECISE_TIMER
fi
run_tests
}
announce "Running tests:"
do_test EPOLL "(timerfd)"
do_test EPOLL "(changelist)"
do_test EPOLL "(timerfd+changelist)"
for i in $BACKENDS; do
do_test $i
done
do_test EPOLL "(changelist)"
if test "$FAILED" = "yes"; then
exit 1