Java Garbage Collection Log messages

Question

I have configured java to dump garbage collection information into the logs  verbose GC   I am unsure of what the garbage collection entries in the logs mean  A sample of these entries are posted below  I ve searched around on Google and have not found solid explanations  I have some reasonable guesses  but I m looking for answers which provide strict definitions of what the numbers in the entries mean  backed up by credible sources  An automatic  1 to all answers which cite sun documentation  My questions are   What does PSYoungGen refer to  I assume it has something to do with the previous  younger   generation  but what exactly  What is the difference between the second triplet of numbers and the first  Why is a name PSYoungGen  specified for the first triplet of numbers but not the second  What does each number  memory size  in the triplet mean  For example in 109884K- gt 14201K 139904K   is the memory before GC 109884k and then it is reduced to 14201K  How is the third number relevant  Why would we require a second set of numbers    8109 128   GC  PSYoungGen  109884K- gt 14201K 139904K   691015K- gt 595332K 1119040K   0 0454530 secs  8112 111   GC  PSYoungGen  126649K- gt 15528K 142336K   707780K- gt 605892K 1121472K   0 0934560 secs  8112 802   GC  PSYoungGen  130344K- gt 3732K 118592K   720708K- gt 607895K 1097728K   0 0682690 secs

User · Answer

I just wanted to mention that one can get the detailed GC log with the

-XX:+PrintGCDetails

parameter. Then you see the PSYoungGen or PSPermGen output like in the answer.

Also -Xloggc:gc.log seems to generate the same output like -verbose:gc but you can specify an output file in the first.

Example usage:

java -Xloggc:./memory.log -XX:+PrintGCDetails Memory

To visualize the data better you can try gcviewer (a more recent version can be found on github).

Take care to write the parameters correctly, I forgot the "+" and my JBoss would not start up, without any error message!

User · Answer

Most of it is explained in the GC Tuning Guide  which you would do well to read anyway       The command line option -verbose gc causes information about the heap and garbage collection to be printed at each collection  For example  here is output from a large server application     GC 325407K- gt 83000K 776768K   0 2300771 secs   GC 325816K- gt 83372K 776768K   0 2454258 secs   Full GC 267628K- gt 83769K 776768K   1 8479984 secs        Here we see two minor collections followed by one major collection  The numbers before and after the arrow  e g   325407K- gt 83000K from the first line  indicate the combined size of live objects before and after garbage collection  respectively  After minor collections the size includes some objects that are garbage  no longer alive  but that cannot be reclaimed  These objects are either contained in the tenured generation  or referenced from the tenured or permanent generations        The next number in parentheses  e g    776768K  again from the first line  is the committed size of the heap  the amount of space usable for java objects without requesting more memory from the operating system  Note that this number does not include one of the survivor spaces  since only one can be used at any given time  and also does not include the permanent generation  which holds metadata used by the virtual machine        The last item on the line  e g   0 2300771 secs  indicates the time taken to perform the collection  in this case approximately a quarter of a second        The format for the major collection in the third line is similar        The format of the output produced by -verbose gc is subject to change in future releases      I m not certain why there s a PSYoungGen in yours  did you change the garbage collector

User · Answer

PSYoungGen refers to the garbage collector in use for the minor collection   PS stands for Parallel Scavenge  The first set of numbers are the before after sizes of the young generation and the second set are for the entire heap   Diagnosing a Garbage Collection problem details the format  The name indicates the generation and collector in question  the second set are for the entire heap    An example of an associated full GC also shows the collectors used for the old and permanent generations   3 757   Full GC  PSYoungGen  2672K- gt 0K 35584K                 ParOldGen  3225K- gt 5735K 43712K   5898K- gt 5735K 79296K                PSPermGen  13533K- gt 13516K 27584K    0 0860402 secs    Finally  breaking down one line of your example log output   8109 128   GC  PSYoungGen  109884K- gt 14201K 139904K   691015K- gt 595332K 1119040K   0 0454530 secs     107Mb used before GC  14Mb used after GC  max young generation size 137Mb 675Mb heap used before GC  581Mb heap used after GC  1Gb max heap size minor GC occurred  8109 128 seconds since the start of the JVM and took 0 04 seconds

[java] Java Garbage Collection Log messages

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