gcc -shared -fPIC fake-cpu-count.c -o fake-cpu-count.so -ldl
javac ReportCpuCount.java
java -cp . ReportCpuCount
#Found 8 CPUs
#...
LD_PRELOAD=./fake-cpu-count.so java -cp . ReportCpuCount
#Found 2 CPUs
#...
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May 26, 2025 12:15
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Using LD_PRELOAD to pass a fake CPU count to Java on Linux
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| #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); | |
| } |
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| 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); | |
| } | |
| } | |
| } |
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How about
java -XX:ActiveProcessorCount=2?