mirror of /home/gitosis/repositories/libev.git
work around epoll spurious notifications
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2
Changes
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@ -5,6 +5,8 @@ WISH? monotonic clocks times/GetTickCount for coarse corrections?
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- further optimise away the EPOLL_CTL_ADD/MOD combo in the epoll
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backend by assuming the kernel event mask hasn't changed if
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ADD fails with EEXIST.
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- work around spurious event notification bugs in epoll by using
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an 8-bit generation counter.
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- use memset to initialise most arrays now and do away with the
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init functions.
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- expand time-out strategies into a "Be smart about timeouts" section.
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2
ev.c
2
ev.c
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@ -452,7 +452,7 @@ typedef struct
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unsigned char events;
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unsigned char reify;
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unsigned char emask; /* the epoll backend stores the actual kernel mask in here */
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unsigned char unused; /* currently unused padding */
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unsigned char egen; /* generation counter to counter epoll bugs */
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#if EV_SELECT_IS_WINSOCKET
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SOCKET handle;
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#endif
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11
ev.pod
11
ev.pod
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@ -391,16 +391,19 @@ of shortcomings, such as silently dropping events in some hard-to-detect
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cases and requiring a system call per fd change, no fork support and bad
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support for dup.
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Epoll is also notoriously buggy - embedding epoll fds should work, but
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of course doesn't, and epoll just loves to report events for totally
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I<different> file descriptors (even already closed ones) than registered
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in the set (especially on SMP systems). Libev tries to counter these
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spurious notifications by employing an additional generation counter and
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comparing that against the events to filter out spurious ones.
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While stopping, setting and starting an I/O watcher in the same iteration
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will result in some caching, there is still a system call per such incident
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(because the fd could point to a different file description now), so its
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best to avoid that. Also, C<dup ()>'ed file descriptors might not work
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very well if you register events for both fds.
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Please note that epoll sometimes generates spurious notifications, so you
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need to use non-blocking I/O or other means to avoid blocking when no data
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(or space) is available.
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Best performance from this backend is achieved by not unregistering all
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watchers for a file descriptor until it has been closed, if possible,
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i.e. keep at least one watcher active per fd at all times. Stopping and
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20
ev_epoll.c
20
ev_epoll.c
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@ -85,7 +85,8 @@ epoll_modify (EV_P_ int fd, int oev, int nev)
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oldmask = anfds [fd].emask;
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anfds [fd].emask = nev;
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ev.data.u64 = fd; /* use u64 to fully initialise the struct, for nicer strace etc. */
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/* store the generation counter in the upper 32 bits */
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ev.data.u64 = fd | ((uint64_t)++anfds [fd].egen << 32);
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ev.events = (nev & EV_READ ? EPOLLIN : 0)
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| (nev & EV_WRITE ? EPOLLOUT : 0);
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@ -96,7 +97,7 @@ epoll_modify (EV_P_ int fd, int oev, int nev)
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{
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/* if ENOENT then the fd went away, so try to do the right thing */
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if (!nev)
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return;
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goto dec_egen;
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if (!epoll_ctl (backend_fd, EPOLL_CTL_ADD, fd, &ev))
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return;
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@ -105,11 +106,18 @@ epoll_modify (EV_P_ int fd, int oev, int nev)
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{
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/* EEXIST means we ignored a previous DEL, but the fd is still active */
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/* if the kernel mask is the same as the new mask, we assume it hasn't changed */
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if (oldmask == nev || !epoll_ctl (backend_fd, EPOLL_CTL_MOD, fd, &ev))
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if (oldmask == nev)
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goto dec_egen;
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if (!epoll_ctl (backend_fd, EPOLL_CTL_MOD, fd, &ev))
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return;
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}
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fd_kill (EV_A_ fd);
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dec_egen:
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/* we didn't successfully call epoll_ctl, so decrement the generation counter again */
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--anfds [fd].egen;
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}
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static void
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@ -130,11 +138,15 @@ epoll_poll (EV_P_ ev_tstamp timeout)
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{
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struct epoll_event *ev = epoll_events + i;
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int fd = ev->data.u64;
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int fd = (uint32_t)ev->data.u64; /* mask out the lower 32 bits */
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int got = (ev->events & (EPOLLOUT | EPOLLERR | EPOLLHUP) ? EV_WRITE : 0)
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| (ev->events & (EPOLLIN | EPOLLERR | EPOLLHUP) ? EV_READ : 0);
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int want = anfds [fd].events;
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if (anfds [fd].egen != (unsigned char)(ev->data.u64 >> 32))
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/*fprintf (stderr, "spurious notification fd %d, %d vs %d\n", fd, (int)(ev->data.u64 >> 32), anfds [fd].egen);*/
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continue;
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if (expect_false (got & ~want))
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{
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anfds [fd].emask = want;
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