import java.util.ArrayList;
import java.util.List;

/**
 * Deliberately allocates and retains heap in fixed-size chunks until it is
 * killed or finishes, printing progress as it goes. Used to demonstrate what
 * actually happens when a container's memory limit, the JVM's heap flags,
 * and real allocation pressure disagree with each other. See the post at
 * https://ankurm.com/jvm-flags-for-spring-boot-in-containers-jdk-25-27/
 * and docs/output/05-oom-*.txt / 06-oom-*.txt for the captured runs.
 */
public class MemoryHog {
    public static void main(String[] args) throws Exception {
        int chunkMb = args.length > 0 ? Integer.parseInt(args[0]) : 16;
        int sleepMs = args.length > 1 ? Integer.parseInt(args[1]) : 150;
        List<byte[]> held = new ArrayList<>();
        long totalMb = 0;
        System.out.println("MemoryHog starting: chunk=" + chunkMb + "MB sleep=" + sleepMs + "ms");
        System.out.flush();
        while (true) {
            byte[] chunk = new byte[chunkMb * 1024 * 1024];
            for (int i = 0; i < chunk.length; i += 4096) {
                chunk[i] = 1; // touch each page so it is actually resident, not just reserved
            }
            held.add(chunk);
            totalMb += chunkMb;
            Runtime rt = Runtime.getRuntime();
            System.out.printf("held=%dMB heapUsed=%dMB heapMax=%dMB freeHeap=%dMB%n",
                    totalMb,
                    (rt.totalMemory() - rt.freeMemory()) / (1024 * 1024),
                    rt.maxMemory() / (1024 * 1024),
                    rt.freeMemory() / (1024 * 1024));
            System.out.flush();
            Thread.sleep(sleepMs);
        }
    }
}