CODEWITHSUNDEEP
javamemoryjvm
ŁukaszBachman
ŁukaszBachman

Reputation: 33745

How to detect real value of -Xmx param? Runtime.getRuntime().maxMemory() is not accurate

In my case this value is usually different than -Xmx that I'm specifying.

For -Xmx810M it returned 821035008 which is 783MB. According to various (not truly reliable) sources it should return exactly the value specified by -Xmx parameter.

I look for:

  1. Some reliable way of finding exact value of -Xmx
  2. Some reliable source that will prove that maxMemory() is not accurate

Upvotes: 3

Views: 1066

Answers (2)

Peter Lawrey
Peter Lawrey

Reputation: 533880

Runtime.maxMemory() is accurate. It is the -Xmx option which is not. How inaccurate it is depends on the version and I not sure what it is more of a hint than a precise setting.

When the totalMemory() matches your maxMemory() you are out of memory.

Upvotes: 1

pcjjman
pcjjman

Reputation: 116

According to https://bugs.java.com/bugdatabase/view_bug?bug_id=4391499 (which is from 2001, so it may have changed in newer versions of the JVM):

"The interpretation of the -Xmx flag is VM-dependent. Some VMs, including HotSpot, enforce a lower bound on the effective value of this option."

So as the previous answer stated, MaxMemory is accurate, and the Xmx flag is simply a suggestion.

The link above was found via an answer to an earlier similar question on SO.

And if you want to find the user specified value of Xmx, you could simply do roughly the following if using a Sun (ahem, Oracle) JVM/:

String commandLineInput = System.getProperty("sun.java.command");
Pattern pattern = Pattern.compile("-Xmx(\\w*)");
Matcher matcher = pattern.matcher(commandLineInput);
while(matcher.find()) {
  System.out.println(matcher.group(1));
}

Upvotes: 6

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