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. --b.
