Strange GC behaviour,

Dang Nhan Nguyen nhann at chalmers.se
Wed Dec 7 22:20:08 UTC 2011


Hi guys,

I am benchmarking CMS collector and found it strange that there are many objects left in remark phase.

When I check the issue again by only running foreground CMS (I changed the source code a bit to force CMS running in foreground), so there is no promotion during collecting  (so no objects marked during Preclean and Abortable Preclean phase). Even in this case, I found that there are still many objects left in remark phase (in lusearch and tomcat in Dacapo benchmark, they are 10 times and 1/3 of number of objects marked in concmarking phase, while in suflow, they are just about 100 objects/refs). Most (maybe ALL) of these objects marked in remark phase were processed by refProcessingWork(). In turn, this refProcessingWork() run in single thread mode. The performance was reduced a lot because of this.

My questions are:
1. What kinds of references were processed by refProcessingWork()? weak and soft ref? what are exactly they are?
2. While I only run CMS in stop-the-world, why are there still many objects left to be processed by remark, in particular here is refProcessingWork()? Why aren't these references/objects processed in concurrent marking phase?

Best,
-Nhan Nguyen
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