> On Wed, 2018-08-08 at 17:28 -0400, J. Bruce Fields wrote: > > On Wed, Aug 08, 2018 at 04:09:12PM -0400, J. Bruce Fields wrote: > > > On Wed, Aug 08, 2018 at 03:54:45PM -0400, J. Bruce Fields wrote: > > > > On Wed, Aug 08, 2018 at 11:51:07AM +1000, NeilBrown wrote: > > > > > If you have a many-core machine, and have many threads all > > > > > wanting to briefly lock a give file (udev is known to do this), > > > > > you can get quite poor performance. > > > > > > > > > > When one thread releases a lock, it wakes up all other threads > > > > > that are waiting (classic thundering-herd) - one will get the > > > > > lock and the others go to sleep. > > > > > When you have few cores, this is not very noticeable: by the > > > > > time the 4th or 5th thread gets enough CPU time to try to claim > > > > > the lock, the earlier threads have claimed it, done what was needed, and > released. > > > > > With 50+ cores, the contention can easily be measured. > > > > > > > > > > This patchset creates a tree of pending lock request in which > > > > > siblings don't conflict and each lock request does conflict with its parent. > > > > > When a lock is released, only requests which don't conflict with > > > > > each other a woken. > > > > > > > > Are you sure you aren't depending on the (incorrect) assumption > > > > that "X blocks Y" is a transitive relation? > > > > > > > > OK I should be able to answer that question myself, my patience > > > > for code-reading is at a real low this afternoon.... > > > > > > In other words, is there the possibility of a tree of, say, > > > exclusive locks with (offset, length) like: > > > > > > (0, 2) waiting on (1, 2) waiting on (2, 2) waiting on (0, 4) > > > > > > and when waking (0, 4) you could wake up (2, 2) but not (0, 2), > > > leaving a process waiting without there being an actual conflict. > > > > After batting it back and forth with Jeff on IRC.... So do I > > understand right that when we wake a waiter, we leave its own tree of > > waiters intact, and when it wakes if it finds a conflict it just adds > > it lock (with tree of waiters) in to the tree of the conflicting lock? > > > > If so then yes I think that depends on the transitivity > > assumption--you're assuming that finding a conflict between the root > > of the tree and a lock proves that all the other members of the tree > > also conflict. > > > > So maybe this example works. (All locks are exclusive and written > > (offset, length), X->Y means X is waiting on Y.) > > > > process acquires (0,3) > > 2nd process requests (1,2), is put to sleep. > > 3rd process requests (0,2), is put to sleep. > > > > The tree of waiters now looks like (0,2)->(1,2)->(0,3) > > > > (0,3) is unlocked. > > A 4th process races in and locks (2,2). > > The 2nd process wakes up, sees this new conflict, and waits on > > (2,2). Now the tree looks like (0,2)->(1,2)->(2,2), and (0,2) > > is waiting for no reason. > > > > That seems like a legit problem. > > One possible fix might be to have the waiter on (1,2) walk down the entire > subtree and wake up any waiter that is waiting on a lock that doesn't conflict > with the lock on which it's waiting. > > So, before the task waiting on 1,2 goes back to sleep to wait on 2,2, it could > walk down its entire fl_blocked subtree and wake up anything waiting on a lock > that doesn't conflict with (2,2). > > That's potentially an expensive operation, but: > > a) the task is going back to sleep anyway, so letting it do a little extra work > before that should be no big deal > > b) it's probably still cheaper than waking up the whole herd Yea, I think so. Now here's another question... How does this new logic play with Open File Description Locks? Should still be ok since there's a thread waiting on each of those. Frank
