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work around epoll spurious notifications

Marc Alexander Lehmann 13 years ago
  1. 2
  2. 2
  3. 11
  4. 20


@ -5,6 +5,8 @@ WISH? monotonic clocks times/GetTickCount for coarse corrections?
- further optimise away the EPOLL_CTL_ADD/MOD combo in the epoll
backend by assuming the kernel event mask hasn't changed if
ADD fails with EEXIST.
- work around spurious event notification bugs in epoll by using
an 8-bit generation counter.
- use memset to initialise most arrays now and do away with the
init functions.
- expand time-out strategies into a "Be smart about timeouts" section.


@ -452,7 +452,7 @@ typedef struct
unsigned char events;
unsigned char reify;
unsigned char emask; /* the epoll backend stores the actual kernel mask in here */
unsigned char unused; /* currently unused padding */
unsigned char egen; /* generation counter to counter epoll bugs */
SOCKET handle;


@ -391,16 +391,19 @@ of shortcomings, such as silently dropping events in some hard-to-detect
cases and requiring a system call per fd change, no fork support and bad
support for dup.
Epoll is also notoriously buggy - embedding epoll fds should work, but
of course doesn't, and epoll just loves to report events for totally
I<different> file descriptors (even already closed ones) than registered
in the set (especially on SMP systems). Libev tries to counter these
spurious notifications by employing an additional generation counter and
comparing that against the events to filter out spurious ones.
While stopping, setting and starting an I/O watcher in the same iteration
will result in some caching, there is still a system call per such incident
(because the fd could point to a different file description now), so its
best to avoid that. Also, C<dup ()>'ed file descriptors might not work
very well if you register events for both fds.
Please note that epoll sometimes generates spurious notifications, so you
need to use non-blocking I/O or other means to avoid blocking when no data
(or space) is available.
Best performance from this backend is achieved by not unregistering all
watchers for a file descriptor until it has been closed, if possible,
i.e. keep at least one watcher active per fd at all times. Stopping and


@ -85,7 +85,8 @@ epoll_modify (EV_P_ int fd, int oev, int nev)
oldmask = anfds [fd].emask;
anfds [fd].emask = nev; = fd; /* use u64 to fully initialise the struct, for nicer strace etc. */
/* store the generation counter in the upper 32 bits */ = fd | ((uint64_t)++anfds [fd].egen << 32); = (nev & EV_READ ? EPOLLIN : 0)
| (nev & EV_WRITE ? EPOLLOUT : 0);
@ -96,7 +97,7 @@ epoll_modify (EV_P_ int fd, int oev, int nev)
/* if ENOENT then the fd went away, so try to do the right thing */
if (!nev)
goto dec_egen;
if (!epoll_ctl (backend_fd, EPOLL_CTL_ADD, fd, &ev))
@ -105,11 +106,18 @@ epoll_modify (EV_P_ int fd, int oev, int nev)
/* EEXIST means we ignored a previous DEL, but the fd is still active */
/* if the kernel mask is the same as the new mask, we assume it hasn't changed */
if (oldmask == nev || !epoll_ctl (backend_fd, EPOLL_CTL_MOD, fd, &ev))
if (oldmask == nev)
goto dec_egen;
if (!epoll_ctl (backend_fd, EPOLL_CTL_MOD, fd, &ev))
fd_kill (EV_A_ fd);
/* we didn't successfully call epoll_ctl, so decrement the generation counter again */
--anfds [fd].egen;
static void
@ -130,11 +138,15 @@ epoll_poll (EV_P_ ev_tstamp timeout)
struct epoll_event *ev = epoll_events + i;
int fd = ev->data.u64;
int fd = (uint32_t)ev->data.u64; /* mask out the lower 32 bits */
int got = (ev->events & (EPOLLOUT | EPOLLERR | EPOLLHUP) ? EV_WRITE : 0)
| (ev->events & (EPOLLIN | EPOLLERR | EPOLLHUP) ? EV_READ : 0);
int want = anfds [fd].events;
if (anfds [fd].egen != (unsigned char)(ev->data.u64 >> 32))
/*fprintf (stderr, "spurious notification fd %d, %d vs %d\n", fd, (int)(ev->data.u64 >> 32), anfds [fd].egen);*/
if (expect_false (got & ~want))
anfds [fd].emask = want;