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javaamazon-web-servicesamazon-ec2out-of-memoryram

Why does Runtime.maxMemory() change its value on separate calls?

I was debugging some OutOfMemoryError issues in my code, so I built a method that printed out the RAM statistics, including the max memory.

To my surprise, multiple attempts at calling the method returned different values for the MAX ram.

I set my ram properties on my start command. Here is the snippet that is relevant.

... -Xmx=$SOME_VAR -Xms=$SOME_VAR

Because of this snippet, the output from Runtime.totalMemory() ends up being the exact same as the output from Runtime.maxMemory().

How is it that the output from Runtime.maxMemory() constantly changes? It's, admittedly, not too much in most cases. Maybe half of a gigabyte. But sometimes, those half gigs can add up to almost 3 gigabytes of difference from when we started.

So I just wanted to know -- why does the output of Runtime.maxMemory() change over time? It's not like the hardware gained or lost RAM, right?

And to give some relevant details, my system has 32 gigabytes of RAM, my SOME_VAR above evaluates to roughly 25 gigabytes, and right before I run Java, my system has roughly 29 gigabytes free.

And to be clear, when I say I get different values, I mean that the following code has the possibility to print out different values.

for (int i = 0; i < 10; i++) {
    Thread.sleep(10_000);
    System.out.println(Runtime.getRuntime().maxMemory());
}

Sometimes, I get 23.5, othertimes, I get 24, etc.

It might also help -- I am measuring this on an AWS EC2 instance. I doubt that that would help, but I am adding it in case it would.

Solution

  • Runtime.maxMemory() in Java returns the maximum amount of memory that the Java Virtual Machine (JVM) may use for heap allocation. Some JVMs (like Android’s ART or JIT-enabled JVMs) may dynamically adjust heap size based on usage patterns. Since system conditions are not exactly the same each time, the maximum memory limit can change depending on memory pressure.

    By the way, if your application or the system triggers a manual garbage collection, the JVM may adjust its memory limits dynamically.

    Since you're running this on an AWS EC2 instance, memory limits can be dynamically adjusted by AWS due to hypervisor behavior and containerization (cgroups). EC2 instances do not necessarily guarantee a static amount of RAM at all times. The JVM queries available memory from the OS, and if AWS's hypervisor reallocates memory due to other workloads, the JVM might see a different max memory value.