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From André Warnier (tomcat)>
Subject question on Java / Tomcat / GC
Date Thu, 14 Jul 2016 17:41:40 GMT
Hi Java GC gurus.

I am coming for a bit of expert advice, not for a problem.

At some customer site, some applications appear to react somewhat slowly sometimes, 
although these are not very heavy applications, and traffic on the site is also not very high.

Amog several other things I'm looking at, wondering if this might have something to do 
with Java/Tomcat running out of memory, and perhaps doing excessive GC's, I set up GC logging.
Below I am pasting that small Java GC log.
This is running tomcat 6 still, the java versions and tomcat JVM relevant switches are 
shown in the log.
I am starting the tomcat JVM with "-Xms1024M -Xmx1024M", and no other non-default settings

regarding memory management or GC type.

I take it that in the log below, the first number on the left is a timestamp in "seconds ,

thousands of a second".

Based on what I am seeing below, it does not appear that this Tomcat would be at any time

running short of Heap space, does it ?
The most it seems to be using below, is 153579K, when it has 1024M to play with.
Meither does it seem to be doing excessive GC either, since these logged GC's are 
separated by several thousands of seconds.

But what does puzzle me a bit, is that each time it runs a "minor GC", it seems to follow

this almost immediately (within 1-2 ms) with a "full GC".

Is this normal, expected behaviour ?
And if not, what could be the cause ?
(and, should I worry about it ?)

Should I be looking at something else than the Heap ?

-- begin log file --
Java HotSpot(TM) 64-Bit Server VM (25.51-b03) for linux-amd64 JRE (1.8.0_51-b16), built on

Jun  8 2015 19:28:07 by "java_re" with gcc 4.3.0 20080428 (Red Hat 4.3.0-8)
Memory: 4k page, physical 12198284k(871340k free), swap 8088572k(7962988k free)
CommandLine flags: -XX:InitialHeapSize=1073741824 -XX:MaxHeapSize=1073741824 -XX:+PrintGC

-XX:+PrintGCTimeStamps -XX:+UseCompressedClassPointers -XX:+UseCompressedOops 
0,465: [GC (System.gc())  26214K->3880K(1005056K), 0,0065929 secs]
0,472: [Full GC (System.gc())  3880K->3679K(1005056K), 0,0156589 secs]
3600,493: [GC (System.gc())  98060K->17186K(1005056K), 0,0158018 secs]
3600,509: [Full GC (System.gc())  17186K->14349K(1005056K), 0,0463734 secs]
7200,555: [GC (System.gc())  35320K->14389K(1005056K), 0,0026610 secs]
7200,558: [Full GC (System.gc())  14389K->8712K(1005056K), 0,0336780 secs]
10800,592: [GC (System.gc())  13957K->8744K(1005056K), 0,0013067 secs]
10800,594: [Full GC (System.gc())  8744K->8713K(1005056K), 0,0202595 secs]
14400,615: [GC (System.gc())  13956K->8745K(1005056K), 0,0011526 secs]
14400,616: [Full GC (System.gc())  8745K->8713K(1005056K), 0,0225167 secs]
18000,639: [GC (System.gc())  13956K->8777K(1005056K), 0,0011825 secs]
18000,640: [Full GC (System.gc())  8777K->8719K(1005056K), 0,0204080 secs]
21600,661: [GC (System.gc())  13962K->8751K(1005056K), 0,0014680 secs]
21600,662: [Full GC (System.gc())  8751K->8719K(1005056K), 0,0231917 secs]
25200,686: [GC (System.gc())  13962K->8751K(1005056K), 0,0013049 secs]
25200,687: [Full GC (System.gc())  8751K->8719K(1005056K), 0,0178894 secs]
28800,706: [GC (System.gc())  13962K->8751K(1005056K), 0,0009395 secs]
28800,707: [Full GC (System.gc())  8751K->8730K(1005056K), 0,0177490 secs]
32400,725: [GC (System.gc())  13973K->8762K(1005056K), 0,0012018 secs]
32400,726: [Full GC (System.gc())  8762K->8730K(1005056K), 0,0199877 secs]
36000,747: [GC (System.gc())  13973K->8762K(1005056K), 0,0045768 secs]
36000,751: [Full GC (System.gc())  8762K->8731K(1005056K), 0,0205136 secs]
39600,772: [GC (System.gc())  82213K->11405K(1005056K), 0,0035769 secs]
39600,776: [Full GC (System.gc())  11405K->11098K(1005056K), 0,0436870 secs]
43200,820: [GC (System.gc())  153579K->12647K(1005056K), 0,0053538 secs]
43200,826: [Full GC (System.gc())  12647K->11921K(1005056K), 0,0494967 secs]
46800,876: [GC (System.gc())  80022K->13513K(1005056K), 0,0029589 secs]
46800,879: [Full GC (System.gc())  13513K->12697K(1005056K), 0,0395771 secs]
50400,919: [GC (System.gc())  103251K->13825K(1005056K), 0,0030939 secs]
50400,922: [Full GC (System.gc())  13825K->12641K(1005056K), 0,0341599 secs]
54000,957: [GC (System.gc())  86842K->13305K(1005056K), 0,0018964 secs]
54000,959: [Full GC (System.gc())  13305K->12999K(1005056K), 0,0329190 secs]
57600,992: [GC (System.gc())  72900K->13583K(1005056K), 0,0024302 secs]
57600,995: [Full GC (System.gc())  13583K->13142K(1005056K), 0,0327378 secs]
-- end logfile --

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