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copied!<p>I got these kind of lines in my gc output:</p> <pre><code>44871.602: [GC-- [PSYoungGen: 342848K->342848K(345600K)] 961401K->1041877K(1044672K), 0.1018780 secs] [Times: user=0.16 sys=0.00, real=0.11 secs] </code></pre> <p>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.</p> <p>Because to my knowledge "Parallel Scavenge" of the Young Gen is a stop-the-world GC, so there <strong>couldn't be any objects created parallel</strong> to this GC. (see <a href="https://blogs.oracle.com/jonthecollector/entry/our_collectors" rel="noreferrer">https://blogs.oracle.com/jonthecollector/entry/our_collectors</a>)</p> <p>You can find this in the jdk source code (see <a href="http://hg.openjdk.java.net/jdk7/jdk7" rel="noreferrer">http://hg.openjdk.java.net/jdk7/jdk7</a>) g1CollectedHeap.cpp and psScavenge.cpp</p> <pre><code>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("--"); } } </code></pre> <h1>Reason for GC-- with the Parallel Scavenge Collector</h1> <p>The Young GC encountered a promotion failure (see <a href="http://mail.openjdk.java.net/pipermail/hotspot-gc-use/2010-March/000567.html" rel="noreferrer">http://mail.openjdk.java.net/pipermail/hotspot-gc-use/2010-March/000567.html</a>):</p> <blockquote> <blockquote> <p>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.</p> </blockquote> </blockquote> <p>'Not enough space' doesn't necessarily mean that there isn't enough space in old, but that the old space is heavily fragmented (see <a href="http://blog.ragozin.info/2011/10/java-cg-hotspots-cms-and-heap.html" rel="noreferrer">http://blog.ragozin.info/2011/10/java-cg-hotspots-cms-and-heap.html</a>):</p> <blockquote> <blockquote> <p>[...] 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. </p> </blockquote> </blockquote> <p>These two JVM options could help you analyze your heap fragmentation (see <a href="http://blog.ragozin.info/2011/10/java-cg-hotspots-cms-and-heap.html" rel="noreferrer">http://blog.ragozin.info/2011/10/java-cg-hotspots-cms-and-heap.html</a>):</p> <pre><code>-XX:+PrintPromotionFailure -XX:PrintFLSStatistics=1 </code></pre> <h1>Reason for GC-- with the G1 Collector</h1> <p>A evacuation failure with the G1 is when a Survivor Region hasn't enough space for the surviving objects from a Young Region.</p> <p>I don't know if the G1 Collector responds to a evacuation failure with a Full GC or not.</p>

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