As a
Production Support Guy ,I many times faced the out of Memory(OOM) Issues. Most
of times I take a Thread dump, Heap Dump before doing any thing to the server. I
analyze the dumps to find out the possible problems that caused the out of Memory.
So what are these Dumps?
Thread
Dump, a Thread Dump is a log containing information about currently running
threads inside the JVM
Heap Dump,
a Heap Dump is a log which contains the memory structure of the JVM with all
the classes and objects inside the JVM and their memory usage
Core
Dump, A Core Dump is recorded state of the working memory of a program at the
specific time. This is generated generally when a program has been terminated
abnormally.
Generating
these dumps is always necessary for analyzing the issues.
Heap Dump
As is said A JVM heap dump is
basically a “snapshot” of the Java heap memory at a given time. It is quite
different than a JVM thread dump which is a snapshot of the threads.
Generating a Heap Dump can be done by
using the “jmap” command available in the java software.
jmap
-dump:format=b,file=heap.hprof JAVA_PID
For Oracle Jrocket we can use
jrcmd
<JAVA_PID> hprofdump filename=abc.hprof
So Consider generating a heap Dump Files for the
Process ID 22979.
Dev:vx1379:djbs002-jas
$ jmap -dump:file=my_stack.bin 22979
Dumping
heap to /config/jboss/ews/1.0/domains/jas/my_stack.bin ...
Heap
dump file created
It’s easy to get an OutOfMememory exception when
opening the java heap. The dump file can be very memory consuming if you application
was in the moment it was taken. If you experience the problem you should give
to the JVM as much memory as you can:
Most of the times the heap dumps are
generated with the .hprof extension.
What
is HProf?
HProf is a tool built into JDK for profiling the CPU
and heap usage within a JVM. A Java process crash may produce an hprof file
containing a heap dump of the process at the time of the failure.
So Once the Heap Dump is generated we need to analyze
the file. Java provides a tool called “jhat” which helps in heap analysis.
Now in order to work with jhat to analysis the heap
file we use,
jhat
-J-mx2000m my_stack.bin
When we run this command we see,
Dev:vx1232:jbs002-jas
$ jhat -J-mx2000m my_stack.bin
Reading
from my_stack.bin...
Dump
file created Sun May 12 23:56:01 CDT 2013
Snapshot
read, resolving...
Resolving
1548901 objects...
Chasing
references, expect 309 dots
.....................................................................................................................................................................................................................................................................................................................
Eliminating
duplicate references .....................................................................................................................................................................................................................................................................................................................
Snapshot
resolved.
Started
HTTP server on port 7000
Server
is ready.
This says that the jhat Server is started. We can
access the Server using
http://vx1232:7000/
Now we can point another port as
jhat
-port 7010 -J-Xmx1G my_stack.bin
Now the Jhat web console is open, we can see “Object
Query Language” Link at the End.
A SQL-like query language to query Java heap. OQL
allows to filter/select information wanted from Java heap. OQL is based on
JavaScript expression language.
A few queries would be
select i from [Ljava.lang.Object; i where
i.length == 10
select all Strings of length 100 or more
show path value of all File objects
select file.path.toString() from
java.io.File file
show names of all ClassLoader classes
access static field 'props' of class
java.lang.System
select
heap.findClass("java.lang.System").statics.props
select
heap.findClass("java.lang.System").props
select
heap.findClass("java.lang.String").fields.length
find the object whose object id is given
select all classes that have name pattern
java.net.*
Happy Learning J
