When
Generating heap dumps jmap blocks, and with large heaps this can take a very long time. In these cases
it is often much faster to get a core and then run jmap to extract a heap dump
from the core. It is typically best to create the heap dump on the same box
where the core was created to avoid environment differences.
So what is Core Dump?
When
a program terminated abnormally, the status of the program at the point of
termination should be saved some where for further analysis.This status is
recorded in the form of a Core Dump File.
The
core dump file contains details of where the abnormal termination happened,
process stack, symbol table etc.
How
can I make sure the core dump will be saved?
Each
process has a limit for how big this core can be. If the limit is exceeded no
core dump will be saved. By default this limit is 0 , that means no core will
be dumped by default.
We
need to use the “ulimit” command in linux to find the core file limitations.
The “Ulimit” command sets various limits for the current process.
Check
the core file size limitations
[root@vx111a
~]# ulimit -a | grep core
core file size (blocks, -c) 0
core file size (blocks, -c) 0
Since
“0” nothing can be saved.
[root@vx111a ~]# ulimit -c unlimited
Change
the limitation to unlimited
[root@vx111a ~]# ulimit -a | grep core
core file size (blocks, -c) unlimited
Once
we change the limitation, we generate a core dump using the “gdb” command
available in linux.
GDB
command is to allow you to see what is going on inside another program while it
executes or what another program was doing at the moment it crashed.
So I started
a Java Class called “TestOome” like
[root@vx111a ~]#
/usr/jdk1.6.0_14/bin/java -Xms1500m -Xmx1500m TestOome &
[1] 4588
Now
iam going to attach the gdb to the process 4588 like
[root@vx111a ~]# gdb --pid=4588
GNU
gdb (GDB) Red Hat Enterprise Linux (7.0.1-32.el5)
Copyright
(C) 2009 Free Software Foundation, Inc.
License
GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This
is free software: you are free to change and redistribute it.
There
is NO WARRANTY, to the extent permitted by law.
Type "show copying"
and
"show warranty" for details.
This
GDB was configured as "i386-redhat-linux-gnu".
For
bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Attaching
to process 4588
Reading
symbols from /usr/jdk1.6.0_14/bin/java...(no debugging symbols found)...done.
Reading
symbols from /lib/libpthread.so.0...(no debugging symbols found)...done.
[Thread
debugging using libthread_db enabled]
[New
Thread 0x51953b90 (LWP 4599)]
[New
Thread 0x519a4b90 (LWP 4598)]
[New
Thread 0x51a25b90 (LWP 4597)]
[New
Thread 0x51aa6b90 (LWP 4596)]
[New
Thread 0x51af7b90 (LWP 4595)]
[New
Thread 0x51d48b90 (LWP 4594)]
[New
Thread 0x51d99b90 (LWP 4593)]
[New
Thread 0x51e1ab90 (LWP 4592)]
[New
Thread 0x52064b90 (LWP 4591)]
[New
Thread 0x520e5b90 (LWP 4590)]
[New
Thread 0xb7419b90 (LWP 4589)]
Loaded
symbols for /lib/libpthread.so.0
Reading
symbols from /usr/jdk1.6.0_14/bin/../jre/lib/i386/jli/libjli.so...(no debugging
symbols found)...done.
Loaded
symbols for /usr/jdk1.6.0_14/bin/../jre/lib/i386/jli/libjli.so
Reading
symbols from /lib/libdl.so.2...(no debugging symbols found)...done.
Loaded
symbols for /lib/libdl.so.2
Reading
symbols from /lib/libc.so.6...(no debugging symbols found)...done.
Loaded
symbols for /lib/libc.so.6
Reading
symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.
Loaded
symbols for /lib/ld-linux.so.2
Reading
symbols from /usr/jdk1.6.0_14/jre/lib/i386/server/libjvm.so...(no debugging
symbols found)...done.
Loaded
symbols for /usr/jdk1.6.0_14/jre/lib/i386/server/libjvm.so
Reading
symbols from /lib/libm.so.6...(no debugging symbols found)...done.
Loaded
symbols for /lib/libm.so.6
Reading
symbols from /lib/librt.so.1...(no debugging symbols found)...done.
Loaded
symbols for /lib/librt.so.1
Reading
symbols from /usr/jdk1.6.0_14/jre/lib/i386/libverify.so...(no debugging symbols
found)...done.
Loaded
symbols for /usr/jdk1.6.0_14/jre/lib/i386/libverify.so
Reading
symbols from /usr/jdk1.6.0_14/jre/lib/i386/libjava.so...(no debugging symbols
found)...done.
Loaded
symbols for /usr/jdk1.6.0_14/jre/lib/i386/libjava.so
Reading
symbols from /lib/libnsl.so.1...(no debugging symbols found)...done.
Loaded
symbols for /lib/libnsl.so.1
Reading
symbols from /usr/jdk1.6.0_14/jre/lib/i386/native_threads/libhpi.so...(no
debugging symbols found)...done.
Loaded
symbols for /usr/jdk1.6.0_14/jre/lib/i386/native_threads/libhpi.so
Reading
symbols from /lib/libnss_files.so.2...(no debugging symbols found)...done.
Loaded
symbols for /lib/libnss_files.so.2
Reading
symbols from /usr/jdk1.6.0_14/jre/lib/i386/libzip.so...(no debugging symbols
found)...done.
Loaded
symbols for /usr/jdk1.6.0_14/jre/lib/i386/libzip.so
0xb7f72402
in __kernel_vsyscall ()
(gdb)
gcore
Saved
corefile core.4588
(gdb)
detach
Detaching
from program: /usr/jdk1.6.0_14/bin/java, process 4588
(gdb)
quit
The
last 3 commands are basically important ,
Gcore
: generated the Core file
Detach
: detaches the gdb from process ID 4588
Quit
: quit
Now we
can see that the core file Is generated like
[root@vx111a ~]# file core.4588
core.4588: ELF 32-bit LSB core file Intel
80386, version 1 (SYSV), SVR4-style, from 'java'
There
is another way to generate the Core file using
[root@vx111a ~]# kill -SIGABRT 4705
Now
we need to create a heap dump from this core file usimg “jmap” like
jmap -heap:format=b JAVA_HOME/bin/java
COREFILE > heap.hprof 2>&1
Now
we can perform many other function on this core file like
To get
Thread Dumps information
[root@vx111a ~]#
/usr/jdk1.6.0_14/bin/jstack
/usr/jdk1.6.0_14/bin/java core.4588
Attaching
to core core.4588 from executable /usr/jdk1.6.0_14/bin/java, please wait...
Debugger
attached successfully.
Server
compiler detected.
JVM
version is 14.0-b16
Deadlock
Detection:
No
deadlocks found.
Thread
4595: (state = BLOCKED)
Thread
4594: (state = BLOCKED)
- java.lang.Object.wait(long) @bci=0
(Interpreted frame)
- java.lang.ref.ReferenceQueue.remove(long)
@bci=44, line=118 (Interpreted frame)
- java.lang.ref.ReferenceQueue.remove()
@bci=2, line=134 (Interpreted frame)
-
java.lang.ref.Finalizer$FinalizerThread.run() @bci=3, line=159 (Interpreted
frame)
Thread
4593: (state = BLOCKED)
- java.lang.Object.wait(long) @bci=0
(Interpreted frame)
- java.lang.Object.wait() @bci=2, line=485
(Interpreted frame)
-
java.lang.ref.Reference$ReferenceHandler.run() @bci=46, line=116 (Interpreted
frame)
Thread
4589: (state = BLOCKED)
- java.util.Arrays.copyOf(java.lang.Object[],
int, java.lang.Class) @bci=8, line=2760 (Interpreted frame)
- java.util.Arrays.copyOf(java.lang.Object[],
int) @bci=6, line=2734 (Interpreted frame)
- java.util.ArrayList.ensureCapacity(int)
@bci=51, line=167 (Compiled frame)
- java.util.ArrayList.add(java.lang.Object)
@bci=7, line=351 (Compiled frame)
- TestOome.main(java.lang.String[]) @bci=23,
line=15 (Compiled frame)
To
get the Heap Details ,jmap examnes a core file and prints out the shared object
memory maps or heap details
[root@vx111a ~]#
/usr/jdk1.6.0_14/bin/jmap
/usr/jdk1.6.0_14/bin/java core.4588
Attaching
to core core.4588 from executable /usr/jdk1.6.0_14/bin/java, please wait...
Debugger
attached successfully.
Server
compiler detected.
JVM
version is 14.0-b16
0x08048000 46K
/usr/jdk1.6.0_14/bin/java
0x00887000 134K
/lib/libpthread.so.0
0xb7f58000 37K
/usr/jdk1.6.0_14/bin/../jre/lib/i386/jli/libjli.so
0x00880000 20K
/lib/libdl.so.2
0x006fa000 1654K
/lib/libc.so.6
0x006db000 126K
/lib/ld-linux.so.2
0xb741a000 8965K
/usr/jdk1.6.0_14/jre/lib/i386/server/libjvm.so
0x00855000 211K
/lib/libm.so.6
0x008b7000 47K
/lib/librt.so.1
0xb7f65000 55K
/usr/jdk1.6.0_14/jre/lib/i386/libverify.so
0xb73a4000 184K
/usr/jdk1.6.0_14/jre/lib/i386/libjava.so
0x00924000 107K
/lib/libnsl.so.1
0xb739d000 37K
/usr/jdk1.6.0_14/jre/lib/i386/native_threads/libhpi.so
0xb7381000 49K
/lib/libnss_files.so.2
0xb7370000 74K
/usr/jdk1.6.0_14/jre/lib/i386/libzip.so
Happy
learning J
