On Thu, Oct 06, 2016 at 11:17:18AM -0700, Davidlohr Bueso wrote: > On Thu, 18 Aug 2016, Waiman Long wrote: > > >Currently, when down_read() fails, the active read locking isn't undone > >until the rwsem_down_read_failed() function grabs the wait_lock. If the > >wait_lock is contended, it may takes a while to get the lock. During > >that period, writer lock stealing will be disabled because of the > >active read lock. > > > >This patch will release the active read lock ASAP so that writer lock > >stealing can happen sooner. The only downside is when the reader is > >the first one in the wait queue as it has to issue another atomic > >operation to update the count. > > > >On a 4-socket Haswell machine running on a 4.7-rc1 tip-based kernel, > >the fio test with multithreaded randrw and randwrite tests on the > >same file on a XFS partition on top of a NVDIMM with DAX were run, > >the aggregated bandwidths before and after the patch were as follows: > > > > Test BW before patch BW after patch % change > > ---- --------------- -------------- -------- > > randrw 1210 MB/s 1352 MB/s +12% > > randwrite 1622 MB/s 1710 MB/s +5.4% > > Yeah, this is really a bad workload to make decisions on locking > heuristics imo - if I'm thinking of the same workload. Mainly because > concurrent buffered io to the same file isn't very realistic and you > end up pathologically pounding on i_rwsem (which used to be until > recently i_mutex until Al's parallel lookup/readdir). Obviously write > lock stealing wins in this case. Except that it's DAX, and in 4.7-rc1 that used shared locking at the XFS level and never took exclusive locks. *However*, the DAX IO path locking in XFS has changed in 4.9-rc1 to match the buffered IO single writer POSIX semantics - the test is a bad test based on the fact it exercised a path that is under heavy development and so can't be used as a regression test across multiple kernels. If you want to stress concurrent access to a single file, please use direct IO, not DAX or buffered IO. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
