The Java GC logs are one of the places where we can start debugging an application in the event of a performance issue. The GC logs provide important information, such as:

• The last time the GC ran
• The number of GC cycles run
• The interval at which the GC ran
• The amount of memory freed up after the GC ran
• The time the GC took to run
• The amount of time for which the JVM paused when the garbage collector ran
• The amount of memory allocated to each generation

The following is the sample GC logs:

2018-05-09T14:02:17.676+0530: 0.315: Total time for which application threads were stopped: 0.0001783 seconds, Stopping threads took: 0.0000239 seconds
2018-05-09T14:02:17.964+0530: 0.603: Application time: 0.2881052 seconds
.....
2018-05-09T14:02:18.940+0530: 1.579: Total time for which application threads were stopped: 0.0003113 seconds, Stopping threads took: 0.0000517 seconds
2018-05-09T14:02:19.028+0530: 1.667: Application time: 0.0877361 seconds
2018-05-09T14:02:19.028+0530: 1.667: [GC (Allocation Failure) [PSYoungGen: 65536K->10723K(76288K)] 65536K->13509K(251392K), 0.0176650 secs] [Times: user=0.05 sys=0.00, real=0.02 secs] 
2018-05-09T14:02:19.045+0530: 1.685: Total time for which application threads were stopped: 0.0179326 seconds, Stopping threads took: 0.0000525 seconds
2018-05-09T14:02:20.045+0530: 2.684: Application time: 0.9992739 seconds
.....
2018-05-09T14:03:54.109+0530: 96.748: Total time for which application threads were stopped: 0.0000498 seconds, Stopping threads took: 0.0000171 seconds
Heap
 PSYoungGen total 76288K, used 39291K [0x000000076b200000, 0x0000000774700000, 0x00000007c0000000)
  eden space 65536K, 43% used [0x000000076b200000,0x000000076cde5e30,0x000000076f200000)
  from space 10752K, 99% used [0x000000076f200000,0x000000076fc78e28,0x000000076fc80000)
  to space 10752K, 0% used [0x0000000773c80000,0x0000000773c80000,0x0000000774700000)
 ParOldGen total 175104K, used 2785K [0x00000006c1600000, 0x00000006cc100000, 0x000000076b200000)
  object space 175104K, 1% used [0x00000006c1600000,0x00000006c18b86c8,0x00000006cc100000)
 Metaspace used 18365K, capacity 19154K, committed 19456K, reserved 1067008K
  class space used 2516K, capacity 2690K, committed 2816K, reserved 1048576K
2018-05-09T14:03:54.123+0530: 96.761: Application time: 0.0131957 seconds

These logs are very difficult to interpret quickly. If we have a tool that can render these logs in a visual interface, it would be easy and quick to understand what is going on with the GC. We will take a look at one such tool to interpret the GC logs in the next section.