Garbage Collection - timings and suspension

[Rohit Mohta]

Hi All,

   Can someone please help me understand the below snippet of Garbage Collection logs?


(a)    What's the importance of values user, sys and real time values?  Should I care about those? For instance in the below snippet, 'user' has a value of 0.79second.

My understanding is,

*        'real' represents the actual time taken by garbage collector process to actually free up memory i.e. create object graph, and the whole mark-sweep process.

*        'sys' represents the time spent   in System Space (OS/Kernel related???)

*        'user' represents the time spent in User space i.e. application.


(b)    I can see "Stopping threads took: 0.0001230 seconds" statement in numerous occasions. Even when, GC was not taking place.

*        What does it mean by 'stopping threads took'? Does it mean, garbage collector process was trying to 'park' all the threads, without interrupting ongoing instructions. So it had to wait for ongoing processing to complete, before it could put the thread in 'park' state?

*        Why does JVM stop application threads even when we don't see any Full GC events? In my system, full gc event is fired every 60 mins - but I can see the above statement numerous times. Why does that happen?

*        When we talk about GC Suspension time, our definition is - "time for which application will not process anything. Application kinda hangs". Should we consider these "Total time for which application threads were stopped" also as Suspension time?

*        Any best practices to reduce the suspension time (apart from GC friendly code)?

---------------------------------------------------------------------
2016-07-22T11:17:35.193+0000: 130991.925: Total time for which application threads were stopped: 0.0014892 seconds, Stopping threads took: 0.0001230 seconds
2016-07-22T11:18:05.192+0000: 131021.924: Application time: 29.9988249 seconds
2016-07-22T11:18:05.193+0000: 131021.925: Total time for which application threads were stopped: 0.0010031 seconds, Stopping threads took: 0.0001281 seconds
2016-07-22T11:18:12.845+0000: 131029.577: Application time: 7.6521209 seconds

2016-07-22T13:01:08.837+0000: 137205.569: [Full GC (System.gc()) [PSYoungGen: 9824K->0K(319488K)] [ParOldGen: 321122K->81024K(589824K)] 33
0946K->81024K(909312K), [Metaspace: 37949K->37949K(1083392K)], 0.1677301 secs] [Times: user=0.79 sys=0.00, real=0.17 secs]
Heap after GC invocations=23415 (full 41):
PSYoungGen      total 319488K, used 0K [0x00000000e7000000, 0x00000000fbc00000, 0x0000000100000000)
  eden space 299008K, 0% used [0x00000000e7000000,0x00000000e7000000,0x00000000f9400000)
  from space 20480K, 0% used [0x00000000fa800000,0x00000000fa800000,0x00000000fbc00000)
  to   space 20480K, 0% used [0x00000000f9400000,0x00000000f9400000,0x00000000fa800000)
ParOldGen       total 589824K, used 81024K [0x00000000b5000000, 0x00000000d9000000, 0x00000000e7000000)
  object space 589824K, 13% used [0x00000000b5000000,0x00000000b9f20000,0x00000000d9000000)
Metaspace       used 37949K, capacity 38526K, committed 38952K, reserved 1083392K
  class space    used 3894K, capacity 4047K, committed 4144K, reserved 1048576K
}
2016-07-22T13:01:09.005+0000: 137205.737: Total time for which application threads were stopped: 0.1749291 seconds, Stopping threads took: 0.0001236 seconds
---------------------------------------------------------

Thank You
Rohit

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