RFR (M): 8159422: Very high Concurrent Mark mark stack contention

Thomas Schatzl thomas.schatzl at oracle.com
Tue Jul 19 15:38:22 UTC 2016


Hi all,

  can I have reviews for this change that removes the global (heavy-
weight) lock when accessing the global mark stack?


The change converts the lock and high-water mark based management of
the global mark stack into lock-free high-water mark and free list
based one.erts the lock and high-water mark based management of the
global mark stack into lock-free high-water mark and free list based
one.

In the previous review for JDK-8160897 I already mentioned that the
global lock when pushing/popping elements from the global mark stack is
very problematic particularly when there are many marking threads in
the system.

Overall, particularly at the end of marking (both in the concurrent
phases as well as during remark) this behavior represents a significant
bottleneck.

Particularly if there is a lot of traffic from and to the mark stack
(to be addressed by JDK-8057003), this results in marking not
completing quickly enough.

There is some some customer application on a 1 TB heap (with up to 80%
full at times) where this results in lunch-break like length full gc
pauses when concurrent marking does not complete in time.

Overall, together with JDK-8057003, this change reduces marking times
from >500 seconds to manageable 10-30s. :) (at 100 concurrent marking
threads, more could be used)
Microbenchmarks like the one from JDK-8057003 also basically scale
linearly with the number of threads then.

This change will also help improve the time to safepoint significantly;
because if there is a safepoint request while draining the mark stacks,
it will now yield much earlier.

There is one drawback, internal management reduces the usable mark
stack by around .1 percent. Since the follow-up, JDK-8057003 reduces
mark stack usage by quite a bit, this has been considered acceptable.

This is an enhancement, which is waiting for final approval.

CR:
https://bugs.openjdk.java.net/browse/JDK-8159422
Webrev:
http://cr.openjdk.java.net/~tschatzl/8159422/webrev/
Testing:
jprt, nightly run, several vm.gc runs, internal benchmarks

Thanks,
  Thomas



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