Fool JVM about the number of available processors according to /sys/devices/system/cpu/online

proccount.c

// Compile: gcc -O2 -fPIC -shared -o libproccount.so proccount.c
// Run:     LD_PRELOAD=/path/to/libproccount.so java args

#define _GNU_SOURCE
#include <sched.h>
#include <unistd.h>

static int online_cpus;

__attribute__((constructor))
static void init_online_cpus() {
    online_cpus = (int)sysconf(_SC_NPROCESSORS_ONLN);
}

int sched_getaffinity(pid_t pid, size_t cpusetsize, cpu_set_t *mask) {
    CPU_ZERO_S(cpusetsize, mask);
    int i;
    for (i = 0; i < online_cpus; i++) {
        CPU_SET_S(i, cpusetsize, mask);
    }
    return 0;
}