The CMS and G1

CMS and G1

What is GC?

• Object life cycle auto management
• Makes developer focus on the coding
• Stack auto management
  

Summary

Generation

• New Generation(Yong Generation)
  • Eden
  • Survivor(0/1)
• Old Generation(Tenured Generation)
• PerGen(Permanent Generation, Metaspace)

Garbage Collection way

• Mark - Copy
• Mark - Sweep(CMS)
• Mark - Reorganize(G1)

GC Type

• Serial(Default for client jvm)

  • Single thread
  • The stack size less equal 100M
  • -XX:+UseSerialGC

• Throughput(Default for server jvm)

  • Will stop-the-world(时空停顿) during Minor GC and Full GC
  • Use paralle threads to collect the gabage
  • -XX:+UseParallelGC

• CMS

  • Will stop-the-world(时空停顿) during Minor GC but not on Full GC
  • Use concurrent threads to collect the gabage on Old Generation
  • Will shotly stop-the-world(时空停顿) during scanning & marking time on Old Generation
  • Will down grade to Serial GC when the Old Generation is too fragmentation
  • -XX:+UseConcMarkSweepGC

• G1

  • Have good performance on the super large stack(over 4GB)
  • Split the Old Generation to 2048 region
  • Consume the CPU
  • Because of the region, G1 nearly do not need to do the Full GC
  • -XX:+UseG1GC

Algorithm

• Throughput

• CMS

• CMS Steps

  • YGC
  • Concurrent Cycle
    • Initial mark(stop-the-world, find all GC object)
    • Concurrent marking(Do not stop-the-world, just give the object a mark)
    • Concurrent preclean(Prepare to sweep)
    • Remarking
      • Concurrent abortable preclean()
    • Sweep
    • Concurrent reset

• G1

• G1 Steps
  • YGC
  • Concurrent Cycle
    • Initial makr(Find the G1, stop-the-world)
    • Root region scan
    • Concurrent marking(stop-the-world for a very shot time, just give the object a mark)
    • Remarking(stop-the-world for a very shot time)
    • Concurrent clean up
    • Mixed GC(Minor GC + clean the Old Generation region + collection active data into another region + start another Concurrent cycle)

Optimize

• The Code

  • Write the code as less as possible
  • Use the most suitable variable type
  • Reduce the unecessay new Object
  • Delayed loading
  • Clean as quick as possible(Null)
    • ArrayList(Removed)

    

  • Do not use System.gc() unless you really need the space and do not care about the time

• The JVM configuration

  • -XX:ConcGCThreads
  • -XX:CMSInitiatingOccupancyFraction=N(Default is 70) + -XX:+UseCMSInitiatingOccupancyOnly(Default is false)
  • -XX:InitiatingHeapOccupancyPercent=N
  • -XX:G1MixedGCLiveThresholdPercent=N
  • --XX:G1ReservePercent=n

Errors

• Concurrent mode failure

  • When YG prepare to sweep but no space for the OG
  • When YG prepare to sweep but OG is too fragment

• Java Heap Space
  java.lang.OutOfMemoryError: Java heap space

• Metaspace(PerGen Space)

• GC overhead limit exceeded

The End

• Can you put up with Full GC pause time?
• Could default setting meet your requirement?
• Did the pause time meet your expectation?
  • GC time pause time too long?
• Conncurrent model failture?
• Upgrade failture?

Reference:
[1]:Java Performance: The Definitive Guide
[2]:http://blog.jobbole.com/109170