Changli Gao wrote: > On Wed, Apr 28, 2010 at 9:21 PM, Jamie Lokier <jamie@xxxxxxxxxxxxx> wrote: > > Changli Gao wrote: > >> > >> fs/eventpoll.c: 1443. > >> wait.flags |= WQ_FLAG_EXCLUSIVE; > >> __add_wait_queue(&ep->wq, &wait); > > > > The same thing about assumptions applies here. The userspace process > > may be waiting for an epoll condition to get access to a resource, > > rather than being a worker thread interchangeable with others. > > Oh, the lines above are the current ones. So the assumptions applies > and works here. No, because WQ_FLAG_EXCLUSIVE doesn't have your LIFO semantic at the moment. Your patch changes the behaviour of epoll, though I don't know if it matters. Perhaps all programs which have multiple tasks waiting on the same epoll fd are "interchangeable worker thread" types anyway :-) > > For example, userspace might be using a pipe as a signal-safe lock, or > > signal-safe multi-token semaphore, and epoll to wait for that pipe. > > > > WQ_FLAG_EXCLUSIVE means there is no point waking all tasks, to avoid a > > pointless thundering herd. It doesn't mean unfairness is ok. > > The users should not make any assumption about the waking up sequence, > neither LIFO nor FIFO. Correct, but they should be able to assume non-starvation (eventual progress) for all waiters. It's one of those subtle things, possibly a unixy thing: Non-RT tasks should always make progress when the competition is just other non-RT tasks, even if the progress is slow. Starvation can spread out beyond the starved process, to cause priority inversions in other tasks that are waiting on a resource locked by the starved process. Among other things, that can cause higher priority tasks, and RT priority tasks, to block permanently. Very unpleasant. > > The LIFO idea _might_ make sense for interchangeable worker-thread > > situations - including userspace. It would make sense for pipe > > waiters, socket waiters (especially accept), etc. > > Yea, and my following patches are for socket waiters. Occasionally unix socketpairs are occasionally used in the above ways too. I'm not against your patch, but I worry that starvation is a new semantic, and it may have a significant effect on something - either in the kernel, or in userspace which is harder to check. > > Do you have any measurements which showing the LIFO mode performing > > better than FIFO, and by how much? > > I didn't do any test yet. But some work done by LSE project years ago > showed that it is better. > > http://lse.sourceforge.net/io/aionotes.txt > > " Also in view of > better cache utilization the wake queue mechanism is LIFO by default. > (A new exclusive LIFO wakeup option has been introduced for this purpose)" I suspect it's possible to combine LIFO-ish and FIFO-ish queuing to prevent starvation while getting some of the locality benefit. Something like add-LIFO and increment a small counter in the next wait entry, but never add in front of an entry whose counter has reached MAX_LIFO_WAITERS? :-) -- Jamie -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
