Deceive the JVM about the number of available cores (on linux)

Question

In some purpose it is needed to make JVM think about it runs on machine with Ncores on board instead of real number of cores (e.g. 4 cores instead of 16).

JVM runs under some Linux build, based on Mandriva/Red Hat Linux core.

This question is borderline case because I expect various solutions of this problem. This is not pure linux-administration question, and it isn't pure programmer's question.

So... any ideas?

Answer 1

In order to make Runtime.getRuntime().availableProcessors() return whatever you want, you can override JVM_ActiveProcessorCount function using LD_PRELOAD trick. Here is a tiny program to do this:

#include <stdlib.h>
#include <unistd.h>

int JVM_ActiveProcessorCount(void) {
    char* val = getenv("_NUM_CPUS");
    return val != NULL ? atoi(val) : sysconf(_SC_NPROCESSORS_ONLN);
}

First, make a shared library of this:

gcc -O3 -fPIC -shared -Wl,-soname,libnumcpus.so -o libnumcpus.so numcpus.c

Then run Java as follows:

$ LD_PRELOAD=/path/to/libnumcpus.so _NUM_CPUS=2 java AvailableProcessors