RFR(M): 8151085: Change G1 concurrent timer and tracer measuring time

sangheon sangheon.kim at oracle.com
Fri Mar 11 19:48:26 UTC 2016

Hi all,

Could I have some reviews for this change?

Concurrent gc timer and tracer are mainly used from concurrent mark 
thread except when abort happens. This abort results in a race condition 
between VM thread and concurrent mark thread.
So I propose to do timer and tracer related works only from concurrent 
mark thread.

Measurements are changed like below:

Start timer and tracer
	When Young Initial Mark GC starts from VM thread
	When actually starts concurrent cycle from concurrent mark thread
End timer and tracer
	1. At the end of concurrent cycle
2. When abort happens 	At the end of concurrent cycle

* I think this will be okay as even though we abort, we continue working 
for concurrent stuffs to finish concurrent work cycle. And we were just 
not measuring them.
Heap summary (before-gc)
	At the beginning of Young Initial Mark GC.
But concurrent mark thread is not working at that time. 	At the start of 
concurrent cycle.
Heap summary (after-gc)
	1. When abort happens before cleanup: sent when abort happens.
2. When abort happens after cleanup or normal case(w/o abort): sent at 
the end of cleanup. 	At the end of concurrent cycle which means it will 
include all minor clearing after 'Pause Cleanup'.

This proposal also includes related changes like below:
1. Removed the patch for JDK-8149834(race between VM thread and 
concurrent mark thread for concurrent gc timer): 8149834 was making 
related functions atomically while this proposal is changing not to 
happen that race.
2. Removed G1CollectorState::_concurrent_cycle_started: no longer needed 
as related logics are running from concurrent mark thread.

Currently 'total concurrent time' is not same as 'sum of concurrent 
phases' and the reason is that the concurrent timer is started when 
young initial mark gc happens. Secondly, concurrent mark thread is not 
counting the time for waiting VMThread to finish VM 
I am planning to enhance this after '8151614: Improve logging in 
concurrent mark code' is pushed.

CR: https://bugs.openjdk.java.net/browse/JDK-8151085
Webrev: http://cr.openjdk.java.net/~sangheki/8151085/webrev.00
Testing: JPRT, RBT(hotspot_all, testlist: running now), local reproducer 
for JDK-8149834

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