Uderstanding GC logs and delays in the web applica

Question

This is a GC logs of webapp (JBoss 7.1). How to interpret this information? Webapp sometimes has noticeable delays and I try to find the reason.

362.261: [GC [PSYoungGen: 12772K->677K(13056K)] 47780K->35813K(57600K), 0.0214030 secs] [Times: user=0.01 sys=0.00, real=0.02 secs]
375.011: [GC [PSYoungGen: 12965K->537K(13056K)] 48101K->36056K(57600K), 0.0410890 secs] [Times: user=0.01 sys=0.00, real=0.04 secs]
377.211: [GC [PSYoungGen: 12823K->764K(12800K)] 48342K->36391K(57344K), 0.0774440 secs] [Times: user=0.01 sys=0.00, real=0.08 secs]
400.197: [GC [PSYoungGen: 12796K->777K(13056K)] 48423K->36464K(57600K), 0.0204340 secs] [Times: user=0.01 sys=0.00, real=0.02 secs]
...
4222.821: [Full GC [PSYoungGen: 4096K->0K(8704K)] [ParOldGen: 55161K->40145K(55552K)] 59257K->40145K(64256K) [PSPermGen: 52107K->52107K(87552K)], 1.0148930 secs] [Times: user=0.79 sys=0.01, real=1.02 secs]

Answer 1

[Times: user=0.79 sys=0.01, real=1.02 secs] - is a summary of Stop-the-World pause. For 1.02 secs of wallclock time all application threads were suspended (thus all HTTP request being processed were delayed).

"Full GC" means major GC when both young and old space is collected. Just "GC" is minor GC, only garbage in young space is collected (thus StW pause is shorter).

You can find more information about GC algorithm available in HotSpot JVM here. If you curious about what is happening during StW pause take a look at this article.