In this article, we will discuss the following topics:
- Heap histogram vs. heap dump[1]
- How to generate heap histogram or heap dump in HotSpot
Heap Histogram vs. Heap Dump
Without much ado, read this companion article for the comparison. For heap analysis, you can use either jmap or jcmd to do the job[5]. Here we focus only on using jmap.
$ jdk-hs/bin/jmap -help
Usage:
jmap [option]
(to connect to running process)
jmap [option]
(to connect to a core file)
jmap [option] [server_id@]
(to connect to remote debug server)
where <option> is one of:
<none> to print same info as Solaris pmap
-heap to print java heap summary
-histo[:live] to print histogram of java object heap; if the "live"
suboption is specified, only count live objects
-permstat to print permanent generation statistics
-finalizerinfo to print information on objects awaiting finalization
-dump:<dump-options> to dump java heap in hprof binary format
dump-options:
live dump only live objects; if not specified,
all objects in the heap are dumped.
format=b binary format
file= dump heap to
Example: jmap -dump:live,format=b,file=heap.bin <pid>
-F force. Use with -dump:<dump-options> <pid> or -histo
to force a heap dump or histogram when <pid> does not
respond. The "live" suboption is not supported
in this mode.
-h | -help to print this help message
-J<flag> to pass <flag> directly to the runtime system
Generating Heap Histogram
num #instances #bytes class name ---------------------------------------------- 1: 2099805 195645632 [C 2: 347553 49534472 <constMethodKlass> 3: 2055692 49336608 java.lang.String 4: 347553 44501600 <methodKlass> 5: 30089 36612792 <constantPoolKlass> 6: 1044560 33425920 java.util.HashMap$Entry 7: 90868 24909264 [B 8: 30089 23289072 <instanceKlassKlass> 9: 22323 18194144 <constantPoolCacheKlass> 10: 177458 15661816 [Ljava.util.HashMap$Entry; 11: 642260 15414240 javax.management.ObjectName$Property 12: 159785 15405144 [Ljava.lang.Object;
In the output, it shows the total size and instance count for each class type in the heap. For example, there are 2099805 instances of character arrays (i..e, [C), which has a total size of 195645632 bytes. Because the suboption live was specified, only live objects were counted (i.e., a full GC was forced before histogram was collected).
Generating Heap Dump
$ ~/JVMs/jdk-hs/bin/jmap -dump:live,file=/tmp/hs_jmap_dump.hprof 7891
Dumping heap to /tmp/hs_jmap_dump.hprof ...
Heap dump file created
Including the live option in jmap will force a full GC to occur before the heap is dumped so that it contains only live objects. We recommend taking multiple heap dumps. For example, 30 minutes and 1 hour into the run. Then use Eclipse MAT[2,3] to examine the heap dumps.
Finally, there are other ways to generate a java heap dump:
Finally, there are other ways to generate a java heap dump:
- Use jconsole option to obtain a heap dump via HotSpotDiagnosticMXBean at runtime
- Heap dump will be generated when OutOfMemoryError is thrown by specifying
- -XX:+HeapDumpOnOutOfMemoryError VM option
- Use hprof[7]
References
- Diagnosing OutOfMemoryError or Memory Leaks in JRockit (XML and More)
- Memory Analyzer (MAT)
- Diagnosing Java.lang.OutOfMemoryError (XML and More)
- MAT Documentation
- JRockit JDK and HotSpot JDK: Tool Equivalents (XML and More)
- A Case Study of java.lang.OutOfMemoryError: GC overhead limit exceeded (XML and More)
- hprof - Heap and CPU profiling tool
- What do those strange class names in a java heap dump mean?
- StackOverflowError when serializing an object in Java
- A good place to check when you run into OOM: object serialization which can grow memory.
- I Bet You Have a Memory Leak in Your Application by Nikita Salnikov-Tarnovski
- Classloader leak is the most common leak in web applications
- Diagnose Leaks in Java Language Code (Java SE 8)
