Understanding Coffee Breaks

Visualizing Java memory internals and understanding GC

Transcript

1. Understanding   Coffee  Breaks   June  2013  
2. Run+me  Data  Areas   Method   Area   Heap   Java   Stacks   PC   Registers   Ma+ve   Method   Stacks  
3. Sizing  the  Heap   •  -­‐Xms  3670k   •  -­‐Xmx  64m   •  -­‐XX:MinHeapFreeRa+o    40   •  -­‐XX:MaxHeapFreeRa+o  70  
4. Memory  Genera+ons   •  Young  Genera+on   –  Eden  Space   –  Survior  Space   •  Old/Tenured  Genera+on   •  Permanent  Genera+on          Eden   Survivor  1   Survivor  2                                                Old  Gen       Perm   Gen       Na=ve   Heap   Young  Gen  
5. Sizing  Young  Genera+on   •  -­‐XX:NewRa+o      2   •  -­‐XX:NewSize      2228K   •  -­‐XX:MaxNewSize    not  limited   •  -­‐XX:SurvivorRa+o  32  
6. Visualizing     JVM  Heap  &  GC   Demo  
7. Garbage  Collec=on   Various  Algorithms  
8. Op+ons  with  GC   •  Serial  versus  Parallel   •  Concurrent  versus  Stop-­‐the-­‐world   •  Compac+ng  versus  Non-­‐compac+ng   versus  Copying  
9. Serial  Collector   -­‐XX:+UseSerialGC   •  Young  Gen   – Single  step  collec+on,  uses  only  1  CPU   – Stop  the  world   – Copy  compac+on  from  Eden  space  to   survivor  space    
10. •  Old  Gen   – Mark-­‐sweep  collec+on,  uses  only  1   CPU   – Stop  the  world   – Sliding  compac+on   Serial  Collector   -­‐XX:+UseSerialGC  
11. •  Applies  only  to  Young  Gen   – Parallel  version  of  Serial  Collector   – Uses  mul+ple  threads   Parallel  Collector   -­‐XX:+UseParallelGC  
12. •  Old  Gen   – Mark-­‐summary-­‐sweep  collec+on   – Uses  mul+ple  threads  (-­‐XX:ParallelGCThreads=n)   – Stop  the  world   – Logically  divided  into  fixed-­‐sized  regions   – Cleans  regions  with  more  garbage   – Copy  compac+on   – Lea  dense,  right  free   Parallel  Compac+ng  Collector   -­‐XX:+UseParallelOldGC  
13. •  Old  Gen   – Ini+al  mark,  stop  the  world   – Concurrent  mark   – Re-­‐mark,  stop  the  world   – Concurrent  Sweep   Concurrent  Mark-­‐Sweep     -­‐XX:+UseConcMarkSweepGC  
14. Concurrent  Mark-­‐Sweep     -­‐XX:+UseConcMarkSweepGC  
15. •  Old  Gen   – Low  latency  collec+on  (short  pauses)   – Low  throughput  (due  to  remarking)   – No  compac+on  (suffers  fragmenta+on)   – Causes  Floa+ng  garbage   – –XX:CMSIni+a+ngOccupancyFrac+on=n   •  Incremental  Mode   – –XX:+CMSIncrementalMode   Concurrent  Mark-­‐Sweep     -­‐XX:+UseConcMarkSweepGC  
16. •  Maximum  Pause  Time  Goal   – -­‐XX:MaxGCPauseMillis=n   •  Throughput  Goal   – -­‐XX:GCTimeRa+o=n  [1  :  (1+n)]   •  Footprint  Goal   – Reduces  the  footprint  un+l  only  one   of  the  above  goals  is  met   – Goal  Priori+es   – Maximum  Pause  Time,  Throughput,   Footprint  in  that  order   Behavior-­‐based  Tuning  
17. •  Ini+al  Mark  -­‐  Stop  the  World   •  Concurrent  Marking   •  Remark  -­‐  Stop  the  World  Event  -­‐   completely  empty  regions  are   reclaimed   •  Copying/Cleanup:    Regions  with   max  garbage  are  reclaimed  in  the   available  +me  –  concurrent  for   most  part   Garbage-­‐First  Collector   -­‐XX:+UseG1GC  
18. Ques=ons   shemanth@