Practical overview of garbage collection in different JVMs with emphasis on tuning, large heaps and minimizing pause times UPDATED 13 Oct 2011

1. Garbage collection in JVM and specifics of huge heaps Alexey Ragozin [email_address] Oct 2011

2. OVERVIEW OF GARBAGE COLLECTION

6. Garbage collection Generational approach

9. Oracle’s HotSpot JVM

11. Oracle’s JRockit JVM http:// aragozin.blogspot.com/2011/07/jrockit-gc-in-action.html -Xgc: option Generational Mark Sweep/Compact genconcon or gencon Yes concurrent incremental singleconcon or singlecon No concurrent incremental genconpar Yes concurrent parallel singleconpar No concurrent parallel genparpar or genpar Yes parallel parallel singleparpar or singlepar No parallel parallel genparcon Yes parallel incremental singleparcon No parallel incremental

14. YOUNG COLLECTION

17. How young collector works Collecting root references

18. How young collection works? Coping live objects

19. How young collection works? Collection finished

20. Card marking write barrier

26. Tuning young collection Research mode

28. OLD SPACE COLLECTION

33. Cost structure of pauses (CMS) Summary of pauses

34. Patching OpenJDK Vicious loop http://aragozin.blogspot.com/2011/07/openjdk-patch-cutting-down-gc-pause.html void ClearNoncleanCardWrapper :: do_MemRegion ( MemRegion mr ) { … while ( cur_entry >= limit ) { HeapWord * cur_hw = _ct -> addr_for ( cur_entry ); if ((* cur_entry != CardTableRS :: clean_card_val ()) && clear_card ( cur_entry )) { // Continue the dirty range by opening the // dirty window one card to the left. start_of_non_clean = cur_hw ; } else { // We hit a &quot;clean&quot; card; process any non-empty // &quot;dirty&quot; range accumulated so far. if ( start_of_non_clean < end_of_non_clean ) { const MemRegion mrd ( start_of_non_clean , end_of_non_clean ); _dirty_card_closure -> do_MemRegion ( mrd ); } // Reset the dirty window, while continuing to look // for the next dirty card that will start a // new dirty window. end_of_non_clean = cur_hw ; start_of_non_clean = cur_hw ; } cur_entry --; } … }

35. Patching OpenJDK Vicious loop - Optimized void ClearNoncleanCardWrapper :: do_MemRegion ( MemRegion mr ) { ... while ( cur_entry >= limit ) { HeapWord * cur_hw = _ct -> addr_for ( cur_entry ); if ((* cur_entry != CardTableRS :: clean_card_val ()) && clear_card ( cur_entry )) { start_of_non_clean = cur_hw ; cur_entry --; } else { if ( start_of_non_clean < end_of_non_clean ) { const MemRegion mrd ( start_of_non_clean , end_of_non_clean ); _dirty_card_closure -> do_MemRegion ( mrd ); } // fast forward via continuous range of clean cards // hardcoded 64 bit version if (((( jlong ) cur_entry ) & 7 ) == 0 ) { jbyte * cur_row = cur_entry - 8 ; while ( cur_row >= limit ) { if (*(( jlong *) cur_row ) == (( jlong )- 1 ) /* hardcoded row of 8 clean cards */ ) { cur_row -= 8 ; } else { break ; } } cur_entry = cur_row + 7 ; HeapWord * last_hw = _ct -> addr_for ( cur_row + 8 ); end_of_non_clean = last_hw ; start_of_non_clean = last_hw ; } else { end_of_non_clean = cur_hw ; start_of_non_clean = cur_hw ; cur_entry --; } } ...

36. Patching OpenJDK Serial collector gain http://aragozin.blogspot.com/2011/07/openjdk-patch-cutting-down-gc-pause.html

37. Patching OpenJDK CMS collector gain http://aragozin.blogspot.com/2011/07/openjdk-patch-cutting-down-gc-pause.html

38. CMS, PLABs and fragmentation

39. CMS, PLABs and fragmentation

40. CMS, PLABs and fragmentation

41. CMS, PLABs and fragmentation

42. JRockit’s gencon Hidden pauses

44. OTHER ASPECTS OF GC IN JAVA

48. SURVIVING HUGE HEAP CONCLUSION

51. MOVING OUT OF HEAP