RFR: 8151556: Use the PreservedMarks* classes for the G1 preserved mark stacks

Tony Printezis tprintezis at twitter.com
Thu Mar 10 17:56:09 UTC 2016

Change to use the newly-introduced PreservedMarks* classes for preserving marks in G1:


G1 already had per-worker mark stacks so this change was mostly straightforward. A few comments / questions:

- The main change to the PreservedMarks* classes is to add the par_restore() method so that G1 continues doing that phase in parallel. I changed the PreservedMarks::restore() method to do what G1 was doing (i.e., keep popping entries from the stack until the stack is empty) instead of iterating over the stack, then clearing it. The latter was, for some reason, faster in the serial case (what we’ve had so far in PreservedMarks). However, it doesn’t seem to parallelize well. So, I opted to copy what G1 was doing so that at least there’s no regression in G1. I also changed the logic slightly: the parallel workers now claim tasks to do (using the SequentialSubTasksDone class) instead of only doing the one that corresponds to their worker id (what G1 was doing before). This doesn’t seem to penalize this phase (at least in the tests I ran) and it’s a bit safer (if one worker wakes up late, maybe another one will do the work).

- Changed the max cache size of the stacks to 0 so that the stacks do not cache any segments. Given that we don’t expect promotion / evacuation failures to happen very frequently, caching segments on these stacks is mostly wasted space. I also added asserts to ensure that there are no cached segments when the stacks are supposed to be empty.

- I wanted to have one method that has the logic to initialize an object’s mark word after promotion / evacuation failure. Both ParNew / ParallelGC use the RemoveForwardedPointerClosure, but G1 has its own closure (as it has to do some extra work). I introduced the init_forwarded_mark() method that’s called from both places. I could also make the G1 closure a subclass of RemoveForwardedPointerClosure and just call the do_object() on the parent. Or drop the idea of using the same method. Feedback welcome.

- Now that G1 doesn’t use the OopAndMarkOop and OopAndMarkOopStack classes directly I made them private to PreservedMarks.

- Small re-arranging of the #include declarations in preservedMarks.inline.hpp as I had put them before the #ifndef guard for some reason (oops! sorry…)

- As it was straightforward, I also call par_restore() from ParNew too. I opted to decide whether to call the serial or parallel cases in DefNew based on whether workers() is NULL or not. I can also do that with a virtual method on DefNew which is overwritten in ParNew. Your call. BTW, I can do this change on a separate CR too if it’s considered outside the scope of this one (but it was pretty trivial so I think it’s OK piggy-backing it here).




Tony Printezis | JVM/GC Engineer / VM Team | Twitter

tprintezis at twitter.com

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