RFR (jdk10): JDK-8180175: ObjectSynchronizer only needs to iterate in-use monitors

Roman Kennke rkennke at redhat.com
Thu May 11 10:08:20 UTC 2017


Hi,

ObjectSynchronizer::oops_do() currently iterates all monitor blocks,
even free ones. With -XX:+MonitorInUseLists, each thread has its own
in-use monitors list, and ObjectSynchronizer has a global gOmInUseList
for moribund threads. Iterating over in-use monitors only significantly
inmproves scanning time for monitors, because it avoids scanning free
blocks, improves caching (no padding, and better chances to have
thread-local oops in cache already).

Performance-wise it makes a very significant difference (running
gc-bench's roots.Sync test, which exaggerates synchronizer usage, with
Shenandoah):

baseline:
S: Thread Roots 37748 us
S: Synchronizer Roots 15115 us
UR: Thread Roots 24967 us
UR: Synchronizer Roots 11906 us

patched:
S: Thread Roots 40365 us
S: Synchronizer Roots 0 us
UR: Thread Roots 24459 us
UR: Synchronizer Roots 0 us

Testing: hotspot_gc, specjvm, jcstress -m quick

http://cr.openjdk.java.net/~rkennke/8180175/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/8180175/webrev.00/>

Roman




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