垃圾收集

Question

垃圾回收（GC）信息的日志记录选项可帮助您调整堆大小或诊断与内存有关的任何其他问题，它是由什么完成的？

Answer 1

-verbose:gc is what you want.

Take a look at GCPortal for more related information.

JConsole can be used to monitor the garbage collection graphically, which can be useful sometimes for identifying trends etc.

Answer 2

In short

• -XX:PrintGCDetails - timings and space sizes for each GC phase
• -XX:+PrintGCDateStamps - add timestamps to gC logs
• -XX:+PrintReferenceGC - helps to find reference abuse
• -XX:+PrintPromotionFailure
• -XX:+PrintGCApplicationStoppedTime - report all safe points not just GC

More comprehensive list of GC options .

Answer 3

The garbage collection logging depends on the type of JVM you are using. There are different parameters to enable GC logging depending on the JVM being used. Below are parameters that need to be included to enable GC logging:

SUN JVM : -XX:+PrintGCDetails -XX:+PrintGCDateStamps (It will print both time and date) -verbose:gc -Xloggc:/path/gc.log

Note- -XX:+PrintGCTimeStamps (It will print the time after JVM restart in seconds and not the time stamp in hh:mm:ss format. This is somewhat difficult to interpret so it is recommended to use PrintGCDateStamps instead)

JRockit -Xverbose -XverboseTimeStamp -Xverboselog:/path/gc.log

IBMJ9 -verbose:gc

Answer 4

You may want to look at Java Flight Recorder, which gives you complete GC statistics with very less overhead. You can start JFR recording while the Application is running. The output file generated will have detailed info on the GC statistics. You have to open the output jfr file using Java Mission Control.

Answer 5

I used to work with these options to analyze GC performance for Sun/OpenJDK JVM:

# print détails GC info

-XX:+PrintGCDetails

# print date stamps at GC events in order to correlate them with applications issues

-XX:+PrintGCDateStamps

# print HEAP Usage (before GC, after GC, Young Gen, Old Gen, ...).

-XX:+PrintHeapAtGC

# print reference processing time during STW pause

-XX:+PrintReferenceGC

# print Young Gen décision made by JVM. helpful if you didn't set a size for Young Gen with NewRation option

-XX:+PrintAdaptiveSizePolicy

# add GC causes to logs (System.gc, promotion failure, concurrent mode failure, ...)

-XX:+PrintGCCause

# add details for promotion failure

-XX:+PrintPromotionFailure

# print distribution of java objects that populate the Youn Gen based on their age. Helpful to see if your Eden and survivor spaces are well sized

-XX:+PrintTenuringDistribution