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 c

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 
#include 

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