I am looking for information on how to debug a problem. Any pointers to
documentation, suggestions of approaches etc. would be much appreciated.
I am having a problem with my tomcat server periodically going into a
state where it is using up all the available cpu and providing very slow
response. None of the jsp or servlets on the site should take up a lot
of cpu, they all just do relatively simple database queries and produce
simple html pages from the results. During the times when the cpu is
busy the database will generally not be under any significant load.
Top shows process 324 (0x144) as the busy process, and ps shows it
having consumed a lot of cpu:
PID TTY STAT TIME COMMAND
324 ? S 837:41 /usr/java/jdk1.5.0_10/bin/java -Djava...
Expecting to find that I had some error in my webapp I did a few thread
dumps, but assuming that nid in the thread dump corresponds to pid from
ps I found that the process that was spinning was "VM Thread":
"VM Thread" prio=1 tid=0x08098a48 nid=0x144 runnable
The thread dump does not give any additional information about this
thread. I presume it is the Virtual Machine itself.
If I look at the system status using the Tomcat manager webapp there are
often requests listed with ridiculously large values in the Time column,
several hundred seconds for jsp pages that only take a fraction of a
second to generate, and I can cut an paste the request into my browser
and the request will finish in a fraction of a second.
Sometimes it seems that reloading some of the webapps can "fix" the
problem, at least for a while - I don't know for sure, it could be
coincidence, but if I don't do anything the problem can persist for long
periods of time and then go away after a few webapp reloads. I have
tried to detect a pattern about which app reload solves the problem but
it seems to be different different times. vmstat doesn't show the
machine as thrashing or using much IO (everything basically fits into
physical memory). The only thing I can think of is that the VM is going
into a marathon garbage collection phase, but I don't know how to test
that hypothesis, and it seems odd that even that could go on for lengthy
periods of time.
Help?!?
JVM: jdk1.5.0_10
Tomcat: apache-tomcat-6.0.13
uname -a: Linux [hostname here] 2.4.34-grsec #27 SMP Mon Jan 22 21:19:29
CST 2007 i686 unknown
The system is a two cpu system with 4GB RAM, and when not in this
spinning state it handles its load with 10-15% cpu load.
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