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.... --b. > > Testing shows that lock-acquisitions-per-second is now fairly stable even > as number of contending process goes to 1000. Without this patch, > locks-per-second drops off steeply after a few 10s of processes. > > There is a small cost to this extra complexity. > At 20 processes running a particular test on 72 cores, the lock > acquisitions per second drops from 1.8 million to 1.4 million with > this patch. For 100 processes, this patch still provides 1.4 million > while without this patch there are about 700,000. > > NeilBrown > > --- > > NeilBrown (4): > fs/locks: rename some lists and pointers. > fs/locks: allow a lock request to block other requests. > fs/locks: change all *_conflict() functions to return bool. > fs/locks: create a tree of dependent requests. > > > fs/cifs/file.c | 2 - > fs/locks.c | 142 +++++++++++++++++++++++++-------------- > include/linux/fs.h | 5 + > include/trace/events/filelock.h | 16 ++-- > 4 files changed, 103 insertions(+), 62 deletions(-) > > -- > Signature
