RFR 8065579: WB method to start G1 concurrent mark cycle should be introduced

Stefan Johansson stefan.johansson at oracle.com
Mon Nov 24 12:23:31 UTC 2014

Hi Leonid,

I think the changes for making it possible to trigger a marking cycle 
through the WB API looks good. There are however some problems with the 
   24         assertTrue(wb.g1StartConcMarkCycle());
   25         wb.fullGC();
   26         assertTrue(wb.g1StartConcMarkCycle());

Here the test assumes that it should be able to trigger a new marking 
cycle since the Full-GC have aborted the previous one, but it is 
possible that the concurrent mark thread hasn't yet finished the cycle 
even if it has been aborted. That is, during cycle will return true for 
some time after the concurrent mark has been aborted.
   28         while (wb.g1InConcurrentMark()) {
   29             assertFalse(wb.g1StartConcMarkCycle());
   30         }
   32         assertTrue(wb.g1StartConcMarkCycle());
   34         while (wb.g1InConcurrentMark()) {
   35             assertFalse(wb.g1StartConcMarkCycle());
   36         }

Here the problem is that we can't guarantee that the concurrent cycle 
won't finish just before the call to wb.g1StartConcMarkCycle(), and 
therefor the assert might fail.

I'm not sure how you want to improve the test to avoid these problems, 
the only really robust way I think is to only use the 
g1InConcurrentMark() call to wait until marking is finished and then 
assert that we can trigger a new cycle.


On 2014-11-20 16:13, Leonid Mesnik wrote:
> Hi
> Here is the fix which introduce WB method which stars concurrent mark 
> cycle explicitly. It is needed to make CM cycle related tests more stable.
> Changeset also includes test and very small fix of WB method 
> g1InConcurrentMark .
> Fix pass JPRT with hotspot testset.
> JIRA: https://bugs.openjdk.java.net/browse/JDK-8065579
> Webrev: http://cr.openjdk.java.net/~sjohanss/lmesnik/8065579/hotspot.00/

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