cpuhack.sh

#!/bin/bash
set -e

libhack=$(mktemp --tmpdir libhack_XXXXX.so)
trap "rm -f ${libhack}" EXIT

gcc -O3 -fPIC -shared -Wl,-soname,libnumcpus.so -o ${libhack} -x c - <<EOF
#include <stdlib.h>
#include <unistd.h>

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

LD_PRELOAD=${libhack} _NUM_CPUS=2 "$@"