On Wed, Aug 08 2018, 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. Ahhh... I see what you are getting at. When lock requests are added individually, they will always be blocked by all ancestors in the tree. But when they are added as a group, that might not be the case. So we might need to re-add every request individually. In the (common) case where a lock request is blocked across its whole range, we can just attach the whole tree beneath the blocker. In other cases we need a finer grained approach. I'll have a look and see how to make the code work for this case. Thanks for the thorough review! NeilBrown > > 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. > > ? > > --b.
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