Minor GC

First of the GC events in log denotes a minor GC cleaning the Young space. Let’s analyze how this collector combination behaves in this regard:

2015-05-26T16:23:07.219-02001: 64.3222:[GC3(Allocation Failure4) 64.322: [ParNew5: 613404K->68068K6(613440K) 7, 0.1020465 secs8] 10885349K->10880154K 9(12514816K)10, 0.1021309 secs11][Times: user=0.78 sys=0.01, real=0.11 secs]12

  1. 2015-05-26T16:23:07.219-0200 – Time when the GC event started.
  2. 64.322 – Time when the GC event started, relative to the JVM startup time. Measured in seconds.
  3. GC – Flag to distinguish between Minor & Full GC. This time it is indicating that this was a Minor GC.
  4. Allocation Failure – Cause of the collection. In this case, the GC is triggered due to a requested allocation not fitting into any region in Young Generation.
  5. ParNew – Name of the collector used, this time it indicates a parallel mark-copy stop-the-world garbage collector used in the Young Generation, designed to work in conjunction with Concurrent Mark & Sweep garbage collector in the Old Generation.
  6. 613404K->68068K – Usage of the Young Generation before and after collection.
  7. (613440K) – Total size of the Young Generation.
  8. 0.1020465 secs – Duration for the collection w/o final cleanup.
  9. 10885349K->10880154K – Total used heap before and after collection.
  10. (12514816K) – Total available heap.
  11. 0.1021309 secs – The time it took for the garbage collector to mark and copy live objects in the Young Generation. This includes communication overhead with ConcurrentMarkSweep collector, promotion of objects that are old enough to the Old Generation and some final cleanup at the end of the garbage collection cycle.
  12. [Times: user=0.78 sys=0.01, real=0.11 secs] – Duration of the GC event, measured in different categories:
    • user – Total CPU time that was consumed by Garbage Collector threads during this collection
    • sys – Time spent in OS calls or waiting for system event
    • real – Clock time for which your application was stopped. With Parallel GC this number should be close to (user time + system time) divided by the number of threads used by the Garbage Collector. In this particular case 8 threads were used. Note that due to some activities not being parallelizable, it always exceeds the ratio by a certain amount.

From the above we can thus see that before the collection the total used heap was 10,885,349K and the used Young Generation share was 613,404K. This means that the Old Generation share was 10,271,945K. After the collection, Young Generation usage decreased by 545,336K but total heap usage decreased only by 5,195K. This means that 540,141K was promoted from the Young Generation to Old.

Java ParalleGC