IBM User Guide for Java V7 on Windows > Troubleshooting and support > Using diagnostic tools > Garbage Collector diagnostic data > -Xtgc tracing

-Xtgc:excessiveGC

This trace shows statistics for garbage collection cycles.

After a garbage collection cycle has completed, a trace entry is produced:

excessiveGC: gcid="10" intimems="122.269" outtimems="1.721" \
             percent="98.61" averagepercent="37.89"

This trace shows how much time was spent performing garbage collection and how much time was spent out of garbage collection. In this example, garbage collection cycle 10 took 122.269 ms to complete and 1.721 ms passed between collections 9 and 10. These statistics show that garbage collection accounted for 98.61% of the time from the end of collection 9 to the end of collection 10. The average time spent in garbage collection is 37.89%.

When the average time in garbage collection reaches 95%, extra trace entries are produced:

excessiveGC: gcid="65" percentreclaimed="1.70" freedelta="285728" \
             activesize="16777216" currentsize="16777216" maxiumumsize="16777216"

This trace shows how much garbage was collected. In this example, 285728 bytes were reclaimed by garbage collection 65, which accounts for 1.7% of the total heap size. The example also shows that the heap has expanded to its maximum size (see -Xmx in Garbage Collector command-line options).

When the average time in garbage collection reaches 95% and the percentage of free space reclaimed by a collection drops below 3%, another trace entry is produced:

excessiveGC: gcid="65" percentreclaimed="1.70" minimum="3.00" excessive gc raised

The JVM will then throw an OutOfMemoryError.

Parent: -Xtgc tracing

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