What does "GC--" mean in a java garbage collection log?

Question

We've turned on verbose GC logging to keep track of a known memory leak and got the following entries in the log:

...
3607872.687: [GC 471630K->390767K(462208K), 0.0325540 secs]
3607873.213: [GC-- 458095K->462181K(462208K), 0.2757790 secs]
3607873.488: [Full GC 462181K->382186K(462208K), 1.5346420 secs]
...

I understand the first and third of those, but what does the "GC--" one mean?

Answer 1

I got these kind of lines in my gc output:

44871.602: [GC-- [PSYoungGen: 342848K->342848K(345600K)] 961401K->1041877K(1044672K), 0.1018780 secs] [Times: user=0.16 sys=0.00, real=0.11 secs]

I read Yishai's answer and it would make sense, but I wanted to see it for myself in the Java GC source code, when the JVM prints "--" in the GC log and why.

Because to my knowledge "Parallel Scavenge" of the Young Gen is a stop-the-world GC, so there couldn't be any objects created parallel to this GC. (see https://blogs.oracle.com/jonthecollector/entry/our_collectors)

You can find this in the jdk source code (see http://hg.openjdk.java.net/jdk7/jdk7) g1CollectedHeap.cpp and psScavenge.cpp

jdk7-ee67ee3bd597/hotspot/src/share$ egrep -h -A2 -B5 -r '"\-\-"' *
# G1 Collector
if (evacuation_failed()) {
  remove_self_forwarding_pointers();
  if (PrintGCDetails) {
    gclog_or_tty->print(" (to-space overflow)");
  } else if (PrintGC) {
    gclog_or_tty->print("--");
  }
}
--
# Parallel Scavenge Collector
promotion_failure_occurred = promotion_failed();
if (promotion_failure_occurred) {
  clean_up_failed_promotion();
  if (PrintGC) {
    gclog_or_tty->print("--");
  }
}

Reason for GC-- with the Parallel Scavenge Collector

The Young GC encountered a promotion failure (see http://mail.openjdk.java.net/pipermail/hotspot-gc-use/2010-March/000567.html):

A promotion failure is a scavenge that does not succeed because there is not enough space in the old gen to do all the needed promotions. The scavenge is in essence unwound and then a full STW compaction of the entire heap is done.

'Not enough space' doesn't necessarily mean that there isn't enough space in old, but that the old space is heavily fragmented (see http://blog.ragozin.info/2011/10/java-cg-hotspots-cms-and-heap.html):

[...] it is impossible to find certain amount of continuous memory to promote particular large object, even though total number of free bytes is large enough.

These two JVM options could help you analyze your heap fragmentation (see http://blog.ragozin.info/2011/10/java-cg-hotspots-cms-and-heap.html):

-XX:+PrintPromotionFailure
-XX:PrintFLSStatistics=1

Reason for GC-- with the G1 Collector

A evacuation failure with the G1 is when a Survivor Region hasn't enough space for the surviving objects from a Young Region.

I don't know if the G1 Collector responds to a evacuation failure with a Full GC or not.