On Tue, Jun 14, 2016 at 06:48:05PM -0400, 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% > > The write-only microbench also showed improvement because some read > locking was done by the XFS code.
How does a reader only micro-bench react? I'm thinking the extra atomic might hurt a bit.
