Using LD_PRELOAD to pass a fake CPU count to Java on Linux

readme.md

Build shared library

gcc -shared -fPIC fake-cpu-count.c -o fake-cpu-count.so -ldl

Compile ReportCpuCount

javac ReportCpuCount.java

Run normally

java -cp . ReportCpuCount
#Found 8 CPUs
#...

Run with fake-cpu-count library

LD_PRELOAD=./fake-cpu-count.so java -cp . ReportCpuCount
#Found 2 CPUs
#...

fake-cpu-count.c

#define _GNU_SOURCE
#include "stdlib.h"
#include <unistd.h>
#include <dlfcn.h>

typedef long int (*orig_sysconf_f_type)(int __name);

long sysconf(int name){
   
  orig_sysconf_f_type orig_sysconf;
  orig_sysconf = (orig_sysconf_f_type)dlsym(RTLD_NEXT,"sysconf");

  if (name == _SC_NPROCESSORS_CONF){
      return 2;
  }

  return orig_sysconf(name);
}

ReportCpuCount.java

public class ReportCpuCount {
    public static void main(String[] args) throws Exception{
        while(true){
           System.out.printf("#Found %d CPUs%n", Runtime.getRuntime().availableProcessors());
           Thread.sleep(1000);
        }
    }
}

It seems to be not working on JDK21, the underling mechanism that Runtime.getRuntime().availableProcessors() uses seems to have changed in Linux

The example is almost 5 years old, so that was to expect :), but ood to know. In the current JDK there is still code that queries the sysconf property mentioned above:
https://github.com/openjdk/jdk/blob/master/src/hotspot/os/linux/os_linux.cpp#L432
However, there seems to be another abstraction wrapped around that, which is container/cgroups aware which then overrides the actual value in order to support dynamic cpu settings.
See:

• https://github.com/openjdk/jdk/blob/master/src/hotspot/os/linux/os_linux.cpp#L4744
• https://github.com/openjdk/jdk/blob/master/src/hotspot/os/linux/cgroupSubsystem_linux.cpp#L476
• https://github.com/openjdk/jdk/blob/master/src/hotspot/os/linux/os_linux.cpp#L4671

With that said, the example is just to demonstrate the mechanism, for another example one could try to replace the native logic behing System.currentTimeMillis().