Can heap dump be created for analyzing memory leak without garbage collection?

Question

We have some memory leak issues on VMs in production which are running Java applications, the Old Gen heap usage grows fast every day, so I want to create heap dump to analyze it.

However, I noticed that VisualVM will perform full GC before heap dump which will clean the Old Gen, in that case, the heap dump would be useless.

I also tried to use the following command:

jmap -dump:live,format=b,file=heap.bin

It will also trigger a full GC.

May I ask if there is a way that heap dump can be created without full GC (or without GC)? Or is there any better way to analyze the memory leak?

JDK version: 1.7.0_45

Thanks.

Answer 1

To create a heap dump, there will be a Full GC. The same applies when creating Class histogram out of heap.

If you want to analyze memory leak between Full GCs, then probably memory profiling using a Java profiler (Mission Control, jProfiler, etc) is your only option.

See this Q/A for Java Mission Control Heap Profile.

Answer 2

The answer marked as correct isn't correct anymore. As Sumit says, it will cause a Full GC only when the live option is used (in both histo & dump operations).

Java 7 and Java 8 have this option

-histo[:live]

Prints a histogram of the heap. For each Java class, the number of objects, memory size in bytes, and the fully qualified class names are printed. The JVM internal class names are printed with an asterisk (*) prefix. If the live suboption is specified, then only active objects are counted.

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-dump:[live,] format=b, file=filename

Dumps the Java heap in hprof binary format to filename. The live suboption is optional, but when specified, only the active objects in the heap are dumped. To browse the heap dump, you can use the jhat(1) command to read the generated file.

You could aldo use the jcmd command with the operation GC.heap_dump and the option -all

GC.heap_dump

Generate a HPROF format dump of the Java heap.

Impact: High: Depends on Java heap size and content. Request a full GC unless the '-all' option is specified.

Permission: java.lang.management.ManagementPermission(monitor)

Syntax : GC.heap_dump [options]

Arguments:

• filename : Name of the dump file (STRING, no default value)

Options: (options must be specified using the or = syntax)

• -all : [optional] Dump all objects, including unreachable objects (BOOLEAN, false)

Example: jcmd 3181 GC.heap_dump -all dump

You could add the -XX:+PrintGCDetails flag to see if a Full GC is hapenning. For example, when I use jcmd without -all I see something like this.

200,658: [Full GC (Heap Dump Initiated GC) 200,658: [CMS: 5040K->4158K(18432K), 0,0171885 secs] 11239K->4158K(25856K), [Metaspace: 18053K->18053K(1064960K)], 0,0173941 secs] [Times: user=0,01 sys=0,00, real=0,02 secs]

Answer 3

You can trigger HeapDump using JMX bean HotSpotDiagnostic and second parameter of method set to false.

See this answer for a more detailed response: https://stackoverflow.com/a/35575793/236528

Answer 4

Just remove live from the option and you should be good. When you're supplying "live" option to jmap. you're forcing JVM to run a Full GC and capture those supposed to be "live". use jmap -dump:format=b,file=hd.hprof rather.