Messages in this thread | | | Date | Fri, 7 Oct 2016 17:45:29 -0400 | From | Waiman Long <> | Subject | Re: [RFC PATCH-tip v4 02/10] locking/rwsem: Stop active read lock ASAP |
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On 10/06/2016 05:47 PM, Dave Chinner wrote: > 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.
Thanks for the update. I will change the test when I update this patch.
Cheers, Longman
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