"-XX:ConcGCThreads=3" - Old GC Concurrent Threads

Java GC Tutorials - Herong's Tutorial Examples

∟Concurrent Mark-Sweep (CMS) Collector - "+XX:+UseConcMarkSweepGC"

∟"-XX:ConcGCThreads=3" - Old GC Concurrent Threads

This section demonstrates that the '-XX:ConcGCThreads=3' option can be used to control the number of threads used in 1 of the CMS concurrent phases: Mark phase.

In the last tutorial, we learned how to use the "-XX:ParallelGCThreads=6" JVM option to control the number threads used in Old GC parallel (stop-the-world) phases.

In this tutorial, let's try to use the "-XX:ConcGCThreads=5" JVM option to control the number threads used in Old GC concurrent phases.

herong> java -XX:+UseConcMarkSweepGC \
   -Xms90m -Xmx90m -XX:NewRatio=2 -XX:SurvivorRatio=1 \
   -XX:CMSInitiatingOccupancyFraction=20 \
   -XX:+UseCMSInitiatingOccupancyOnly \
   -XX:ParallelGCThreads=6 -XX:ConcGCThreads=5 \
   -Xlog:gc*=trace \
   GarbageCollection2 > output.txt

herong> more output.txt

GC(13) Pause Initial Mark
GC(13) checkpointRootsInitialWork
GC(13) eden 0x00000000fa600000-0x00000000fa700010-...
GC(13) _eden_chunk_index=1, _eden_chunk_capacity=1281
GC(13) _eden_chunk_array[0]=0x00000000fa700010
GC(13) survivor 0x00000000fb000000-0x00000000fb800080-...
GC(13) _survivor_chunk_index=0, _survivor_chunk_capacity=5120
GC(13) oops_do_marking_prologue
GC(13) WorkerManager::add_workers() : created_workers: 6
GC(13) GC Thread: using 6 out of 6 workers
...
GC(13) oops_do_marking_epilogue
GC(13) checkpointRootsInitialWork 0.316ms
GC(13) Pause Initial Mark 40M->40M(80M) 0.335ms
GC(13) User=0.00s Sys=0.00s Real=0.00s
   -- Initial Mark phase, Pause
   -- 6 threads used as specified by -XX:ParallelGCThreads=6

GC(13) Concurrent Mark
GC(13) WorkerManager::add_workers() : created_workers: 5
GC(13) CMS Thread: using 5 out of 5 workers
GC(13) Using 5 workers of 5 for marking
GC(13) Finished cms space scanning in 1th thread: 0.000 sec
GC(13) Finished cms space scanning in 2th thread: 0.000 sec
GC(13) Finished cms space scanning in 3th thread: 0.000 sec
GC(13) Finished cms space scanning in 4th thread: 0.000 sec
GC(13) Finished cms space scanning in 0th thread: 0.005 sec
GC(13) Finished work stealing in 0th thread: 0.000 sec
GC(13) Finished work stealing in 4th thread: 0.005 sec
GC(13) Finished work stealing in 3th thread: 0.005 sec
GC(13) Finished work stealing in 1th thread: 0.005 sec
GC(13) Finished work stealing in 2th thread: 0.005 sec
GC(13) Concurrent active time: 0.002ms
GC(13)  (CMS Concurrent Mark yielded 1 times)
GC(13) Concurrent Mark 5.709ms
GC(13) User=0.00s Sys=0.00s Real=0.01s
   -- Mark phase, Concurrent
   -- 5 threads used as specified by -XX:ConcGCThreads=5

GC(13) Concurrent Preclean
GC(13) Preclean SoftReferences 0.065ms
GC(13) Preclean WeakReferences 0.048ms
GC(13) Preclean FinalReferences 0.022ms
GC(13) Preclean PhantomReferences 0.036ms
GC(13) Concurrent Preclean 0.438ms
GC(13) User=0.00s Sys=0.00s Real=0.00s
   -- Preclean phase, Concurrent

GC(13) Concurrent Abortable Preclean
GC(13) Concurrent Abortable Preclean 16.231ms
GC(13) User=0.03s Sys=0.00s Real=0.02s
   -- Abortable Preclean phase, Concurrent

GC(13) Pause Remark
GC(13) YG occupancy: 8392 K (20480 K)
GC(13) checkpointRootsFinalWork
GC(13) eden 0x00000000fa600000-0x00000000fa7321b0-...
GC(13) _eden_chunk_index=2, _eden_chunk_capacity=1281
GC(13) _eden_chunk_array[0]=0x00000000fa700010
GC(13) _eden_chunk_array[1]=0x00000000fa7321b0
GC(13) survivor 0x00000000fb000000-0x00000000fb700070-...
GC(13) _survivor_chunk_index=0, _survivor_chunk_capacity=5120
GC(13) Rescan (parallel)
GC(13) oops_do_marking_prologue
GC(13) WorkerManager::add_workers() : created_workers: 6
GC(13) GC Thread: using 6 out of 6 workers
...
GC(13) Pause Remark 62M->62M(80M) 1.288ms
GC(13) User=0.00s Sys=0.00s Real=0.00s
   -- Remark phase, Pause
   -- 6 threads used as specified by -XX:ParallelGCThreads=6

GC(13) Concurrent Sweep
GC(13) size[3] :
GC(13) demand: 0, old_rate: 40633.671875, current_rate: 0.000000, ...
...
GC(13) Concurrent active time: 0.007ms
GC(13)  (CMS Concurrent Sweep yielded 1 times)
GC(13) Concurrent Sweep 7.251ms
   -- Sweep phase, Concurrent

GC(13) User=0.00s Sys=0.02s Real=0.01s
GC(13) Old: 32486K->32486K(61440K)
   -- GC #13 (Old GC) completed

As you can see, the "-XX:ConcGCThreads=5" option works as expected. The "Mark" and "Remark" phase used 5 threads.

Conclusion: CMS Old GC has 6 phases. But not all phases supports multiple threads:

            Phases  Threads  JVM Option         Concurrency

      Initial Mark        6  ParallelGCThreads  pause - stop-the-world
              Mark        5  ConcGCThreads      concurrent to app/ParNew
          Preclean        1                     concurrent to app/ParNew
Abortable Preclean	      1                     concurrent to app/ParNew
            Remark        6  ParallelGCThreads  pause stop-the-world
             Sweep        1                     concurrent to app/ParNew