Looking at how selrecord() / selwakeup() and their Linux counterparts poll_wait() and wake_up() are used, i noticed the following:
- linux tends to call wake_up() unconditionally at the beginning of the poll handler - FreeBSD tends to call selrecord() only when it detects a blocking situation, and this also requires something (a lock or a retry; the lock in selinfo is not good for this) to avoid the race between the blocking_test..selrecord pair and the selwakeup(). FreeBSD could call selrecord unconditionally (and save the extra lock/retry), but selrecord is expensive as it queues the thread on the struct selinfo, and this takes a lock. I wonder if we could use the same optimization as Linux: as soon as pollscan/selscan detects a non-blocking fd, make selrecord a no-op (which is probably as simple as setting SELTD_RESCAN; and since it only goes up we do not need to lock to check it). This way, we would pay at most for one extra selrecord per poll/select. Even more interesting, it could simplify the logic and locking in poll handlers. As an example, in sys/uipc_socket.c :: sopoll_generic() we could completely decouple the locks on so_snd and so_rcv. comments ? Note that it is only an optimization, so we could write poll handlers in the selrecord-then-test style even without it. cheers luigi _______________________________________________ firstname.lastname@example.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"