performance - Java Runtime.maxMemory incorrect?

Question

I ran the following method Runtime.getRuntime().maxMemory() and gave 85196800.

However, I then ran top from the command line and it showed

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                                                                       
 8672 root      20   0 1284m 156m 4296 S  0.3 60.9   0:33.35 java        

Doesn't that show 156M of ram used? Any ideas what is going on?

Answer 1

From the documentation,

maxMemory() - Returns the maximum amount of memory that the Java virtual machine will attempt to use.

Top only shows the amount of (virtual) memory that the system has allocated to the process - you are asking Java how much it could attempt to use in the worst case.

In general, querying the JVM and/or system for information on actual memory used is not reliable. For example, top's numbers may include memory allocated but not used or paged out. It can also include things like shared libraries, where a 10MB library may count for two processes' allocations but only have one physical copy in memory. (For example)

What are you trying to do?