In my last blog entry I gave a simple overview of how GC or garbage collection works in Java. In this post i list some of the command line options which i found very useful for GC tuning and performance measurement.
| Command | Details | |
| -verbose:gc | Prints information at every collection | |
-XX:+PrintGCDetails | Prints detailed info of GC | |
-XX:+PrintGCTimeStamps | Will additionally print a time stamp at the start of each collection | |
| -XX:MinHeapFreeRatio=<minimum> | If the percent of free space in a generation falls below <minimum>% the size of the generation will be expanded so as to have <minimum>% of the space free assuming the size of the generation has not already reached its limit. | |
-XX:MaxHeapFreeRatio=<maximum> | If the percent of free space exceeds <maximum>% the size of the generation will be shrunk so as to have only <maximum>% of the space free as long as shrinking the generation does not decrease it below the minimum size of the generation. | |
-XX:NewRatio=ratioValue | The young generation size is controlled by NewRatio. For example setting -XX:NewRatio=3 means that the ratio between the young and tenured generation is 1:3. | |
-XX:NewSize | Determines the lower limit of young gen size | |
-XX:MaxNewSize | Determines the uppper limit of young gen size | |
-XX:SurvivorRatio=ratioValue | Sets the ratio between each survivor space and eden to be 1:ratioValue. Each survivor space will be 1/(ratioValue+2) of the young generation (as there are two survivor spaces). | |
-XX:+PrintTenuringDistribution | Can be used to show this threshold and the ages of objects in the new generation. | |
-XX:ParallelGCThreads=<desired number> | Determines the number of threads dedicated to Parallel GC. | |
-XX:MaxGCPauseMillis=<nnn> | Determines the maximum pause time for GC | |
-XX:GCTimeRatio=<nnn> | Sets the ratio of garbage collection time to application time to 1 / (1 + <nnn>) For example -XX:GCTimeRatio=19 sets a goal of 5% of the total time for garbage collection. By default the goal for total time for garbage collection is 1%. | |
-XX:YoungGenerationSizeIncrement=<nnn > | Grows the size of young generation by nnn percent. | |
-XX:TenuredGenerationSizeIncrement=<nnn> | Grows the size of tenured generation by nnn percent. | |
-XX:MaxPermSize=<desired size> | Sets uppper limit of permanent gen size | |
-XX:+ TraceClassloading | Know what classes are being loaded | |
-XX:+ TraceClassUnloading | Know what classes are being unloaded | |
| -XX:MaxTenuringThreshold=n | To move an object that survives a young generation collection n times to the tenured generation. Note that the throughput collector does not use the MaxTenuringThreshold parameter. | |
-XX:+ DisableExplicitGC | Disable explicit GC | |
-XX:CMSInitiatingOccupancyFraction=<percent> | The command line flag CMS Initiating Occupancy Fraction can be used to set the level at which the collection is started. |
References: same as last blog entry on gc.
