Java GC Log View

Java GC Type

What are the different types of GC in Java?

Parameters	Describe
Useserialgc	The virtual machine runs the default value in client mode, and when this switch parameter is turned on, Use the serial+serial old collector combination for garbage collection.
Useparnewgc	After you turn on this switch parameter, use the parnew+serial old collector combination for garbage collection.
Useconcmarksweepgc	After you turn on this switch parameter, use the parnew+cms+serial old collector combination for garbage collection. Serial old as a CMS collector appears concurrent Mode failure's backup garbage collector.
Useparallelgc	The virtual machine runs the default value in Server mode, and after you turn on this switch parameter, use the parallel scavenge+serial old collector combination for garbage collection.
Useparalleloldgc	After you turn on this switch parameter, use the Parallel scavenge+parallel old collector combination for garbage collection.

After the Java program starts, query to the currently running Java process through the JPS observation process, using the

Java code

1. Jinfo–flag USESERIALGC Process

The way you can locate the GC policy that it uses, because these parameters are Boolean constants, and if you use that kind of GC policy, the + sign will appear, otherwise-No.

Use-xx:+ the GC policy above to enable the corresponding GC policy.

GC Log View

Policies such as specifying the corresponding GC type, printing GC log information, and outputting to a file can be specified by adding parameters in the Java command type.

The GC's log is written in a replacement (>) instead of append (>>), and the previous GC content is emptied if it is written to the same file the next time.

The corresponding parameter list

Java code

1. -XX:+PRINTGC Output GC Log
2. -xx:+printgcdetails verbose log of output GC
3. -xx:+printgctimestamps timestamp of output GC (in the form of base time)
4. -xx:+printgcdatestamps the timestamp of the output GC (in the form of a date, such as 2013-05-04t21:59.234+0800)
5. -XX:+PRINTHEAPATGC to print out heaps of information before and after GC
6. -XLOGGC:.. /logs/gc.log the output path of the log file

The following parameters are used to print the log:

Java code

1. -xx:+printgcdatestamps-xx:+printgcdetails-xloggc:./gclogs

For a new generation of recycled logs, the basic content is as follows:

Java code

1. 2014-07-18t16:17.606+0800: 611.633: [GC 611.633: [Defnew:843458k->2k (948864K), 0.0059180 secs] 2186589k->1343132k (3057292K), 0.0059490 secs] [times:user=0.00 sys=0.00, Real= 0.00 secs]

The meaning of this is probably as follows:

Java code

1. 2014-07-18t16:17.606+0800 (current timestamp): 611.633 (timestamp): [GC (indicates young GC) 611.633: [ Defnew (single thread serial Young generation GC): 843458K (size before the young Generation garbage collection)->2k (size of the young generation Recycled) (948864K (total size of young generation)), 0.0059180 secs (Time of this recovery)] 2186589K (the size before the entire heap is reclaimed)->1343132k (the size of the entire heap) (3057292K (total heap size)), 0.0059490 secs (reclaim time)] [times:user= 0.00 (User Time) sys=0.00 (System time), Real=0.00 secs (actual time consuming)]

The old age recycling log is as follows:

Java code

2. 2014-07-18t16:16.794+0800: 1630.821: [GC 1630.821: [defnew:1005567k->111679k ( 1005568K), 0.9152360 secs]1631.736: [tenured:
4. 2573912k->1340650k (2574068K), 1.8511050 secs] 3122548k->1340650k (3579636K), [Perm:17882k->17882k ( 21248K)], 2.7854350 secs] [times:user=2.57 sys=0.22, real=2.79 secs]

The last of the GC logs looks like a snapshot of the system before it finishes running:

Java code

2. Heap
4. def New Generation total 1005568K, used 111158K [0x00000006fae00000, 0x000000073f110000, 0x000000075035 0000)
6. Eden Space 893888K, 12% used [0x00000006fae00000, 0x0000000701710e90, 0x00000007316f0000)
8. From Space 111680K, 3% used [0x0000000738400000, 0x000000073877c9b0, 0x000000073f110000)
10. To space 111680K, 0% used [0x00000007316f0000, 0x00000007316f0000, 0x0000000738400000)
12. Tenured generation total 2234420K, used 1347671K [0x0000000750350000, 0x00000007d895d000, 0x00000007fae00000)
14. The space 2234420K, 60% used [0x0000000750350000, 0x00000007a2765cb8, 0x00000007a2765e00, 0x00000007d895d000)
16. Compacting Perm Gen Total 21248K, used 17994K [0x00000007fae00000, 0x00000007fc2c0000, 0x0000000800000000) /c3>
18. The space 21248K, 84% used [0x00000007fae00000, 0x00000007fbf92a50, 0x00000007fbf92c00, 0x00000007fc2c0000)
20. No shared spaces configured.

Offline analysis of GC logs

You can use some offline tools to analyze GC logs, such as Sun's Gchisto (Https://java.net/projects/gchisto), Gcviewer (https://github.com/chewiebug/ Gcviewer), these are open source tools that allow users to download their source code directly from the version control tool for offline analysis

Java GC Log View