Power your JVM with Effective Garbage Collection Tuning

SiliconIndia Java Conference 29th October, 2010

Vivekanand Jha

Its Science

Measure>Understand>>Tune>>Measure>>Understand>>Retune ….

Agenda

• Some background on JVM GC Architecture

• Demystifying JVM tuning and GC tuning

• Reading the logs

Different GC schemes

GC in JDK 7 & other exotic GC algorithms

• G1 or Garbage first algorithm, the holy grail of Garbage collection?

• Pauseless garbage collection from Azul.

• Garbage collection with hard time guarantee in Sun’s premium product JAVA RTS

GC Tuning

• Tuning the Young Generation

• Tuning Parallel GC

• Tuning CMS

• Monitoring GC

Dream GC

• Low GC Overhead. ie. High Throughput

• Low GC pause times

• Space efficiency (Less fragmentation)

Wisdom droplets

• Supersize it

• Minimize live object for minor collections

• Size the Tenured generation to steady state space requirement

• WHEN IN DOUBT, SUPERSIZE IT!!

Unnecessary Information

• -Xmx<size> : max heap size• ● young generation + old generation• > -Xms<size> : initial heap size• ● young generation + old generation• > -Xmn<size> : young generation size• > Applications with emphasis on performance

tend• to set -Xms and -Xmx to the same value• > When -Xms != -Xmx, heap growth or shrinking• requires a Full GC

How to get your object profile in YG

• Monitor tenuring distribution with

-XX:+PrintTenuringDistribution

• Desired survivor size 6684672 bytes, new threshold 8 (max 8)

- age 1: 2315488 bytes, 2315488 total

- age 2: 19528 bytes, 2335016 total

- age 3: 96 bytes, 2335112 total

- age 4: 32 bytes, 2335144 total

Importance of GC Threads

Challenges with the CMS

CMS-Too early[ParNew 390868K->296358K(773376K), 0.1882258 secs][CMS-initial-mark 298458K(773376K), 0.0847541 secs][ParNew 401318K->306863K(773376K), 0.1933159 secs][CMS-concurrent-mark: 0.787/0.981 secs][CMS-concurrent-preclean: 0.149/0.152 secs][CMS-concurrent-abortable-preclean: 0.105/0.183 secs][CMS-remark 374049K(773376K), 0.0353394 secs][ParNew 407285K->312829K(773376K), 0.1969370 secs][ParNew 405554K->311100K(773376K), 0.1922082 secs][ParNew 404913K->310361K(773376K), 0.1909849 secs][ParNew 406005K->311878K(773376K), 0.2012884 secs][CMS-concurrent-sweep: 2.179/2.963 secs][CMS-concurrent-reset: 0.010/0.010 secs][ParNew 387767K->292925K(773376K), 0.1843175 secs][CMS-initial-mark 295026K(773376K), 0.0865858 secs][ParNew 397885K->303822K(773376K), 0.1995878 secs]

CMS-Too Late

[ParNew 742993K->648506K(773376K), 0.1688876 secs][ParNew 753466K->659042K(773376K), 0.1695921 secs][CMS-initial-mark 661142K(773376K), 0.0861029 secs][Full GC 645986K->234335K(655360K), 8.9112629 secs][ParNew 339295K->247490K(773376K), 0.0230993 secs][ParNew 352450K->259959K(773376K), 0.1933945 secs]

-XX:+ExplicitGCInvokesConcurrent

Requires a post 6 JVM

-XX:+ExplicitGCInvokesConcurrentAndUnloadClasses

Monitoring GC

You need at least:● -XX:+PrintGCTimeStamps(Add -XX:+PrintGCDateStamps if you must)● -XX:+PrintGCDetailsPreferred over -verbosegc as it's more detailed> Also useful:● -Xloggc:<file> Separates GC logging output from applicationoutput

Its Art!