Java 进程占用的 RAM 比堆大小多得多

Question

I have a Java program that has been running for days, it processes incoming messages and forward them out.

A problem I noticed today is that, the heap size I printed via Runtime.totalMemory() shows only ~200M,but the RES column in top command shows it is occupying 1.2g RAM.

The program is not using direct byte buffer.

How can I find out why JVM is taking this much extra RAM?

Some other info:

• I am using openjdk-1.8.0
• I did not set any JVM options to limit the heap size, the startup command is simply: java -jar my.jar
• I tried heap dump using jcmd , the dump file size is only about 15M.
• I tried pmap , but there seemed to be too much info printed and I don't know which of them is useful.

Answer 1

You can use VisualVM to monitor what is happening inside your JVM. You can also use JConsole for a primary overview. It comes with JDK itself. If your JDK is setup with an environment variable, then start it from teriminal with jconsole . Then select your application and start monitoring.

Answer 2

One thing is that if your heap is not taking much memory, then check from a profiler tool how much has it taken for your non-heap memory. If that amount is high and even after a GC cycle, if its not coming down, then probably you should be looking for a memory leak ( non-heap ).

If the non-heap memory is not taking much and everything looks good when you look into the memory using profiling tools, then I guess its the JVM which holds the memory rather releasing them. So you better check if your GC hasn't work at all or if GC is being forcefully executed using a profiling tool, whether the memory comes down do does it expands or what is happening. JVM memory and Heap memory are having 2 different behaviors and JVM could assume that it should expand after a GC cycle based on

-XX:MinHeapFreeRatio=
-XX:MaxHeapFreeRatio=

above parameters. So the basic concept behind this is that after a GC cycle, the JVM starts to get measures of free memory and used memory and starts to expand itself or shrink down based on the values for above JVM flags. By default they are set to 40 and 70, which you may interested in tuning up. This is critical specially in containerized environment.

Answer 3

The Java Native Memory Tracking tool is very helpful in situations like this. You enable it by starting the JVM with the flag -XX:NativeMemoryTracking=summary .

Then when your process is running you can get the stats by executing the following command:

jcmd [pid] VM.native_memory

This will produce a detailed output listing eg the heap size, metaspace size as well as memory allocated directly on the heap.

You can also use this tool to create a baseline to monitor allocations over time.

As you will be able to see using this tool, the JVM reserves by default about 1GB for the metaspace, even though just a fraction may be used. But this may account for the RSS usage you are seeing.