zgc-jdk25-benchmarks/shenandoah-jdk28/scripts/LoadGen.java

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.time.Duration;
import java.util.Arrays;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.LockSupport;

/**
 * Load generator, run with the source launcher: java LoadGen.java open|closed URL RATE_OR_0 CONCURRENCY WARMUP_S MEASURE_S
 *
 * open   : requests are SCHEDULED at a fixed rate and latency is measured from the scheduled time, so a server
 *          that stalls is charged for the requests that queued behind the stall (no coordinated omission).
 * closed : CONCURRENCY clients send back to back; the result is the peak throughput.
 *
 * Prints one line: mode=... achieved=... p50_ms=... p99_ms=... p999_ms=... max_ms=... errors=... window_start_ms=... window_end_ms=...
 * (the window is the measured interval in epoch milliseconds, so GC pauses can be cut to exactly that interval).
 */
public class LoadGen {

    public static void main(String[] a) throws Exception {
        String mode = a[0];
        URI uri = URI.create(a[1]);
        double rate = Double.parseDouble(a[2]);
        int concurrency = Integer.parseInt(a[3]);
        int warmup = Integer.parseInt(a[4]);
        int measure = Integer.parseInt(a[5]);
        boolean open = mode.equals("open");

        HttpClient client = HttpClient.newBuilder().version(HttpClient.Version.HTTP_1_1)
                .connectTimeout(Duration.ofSeconds(5)).build();
        HttpRequest request = HttpRequest.newBuilder(uri).timeout(Duration.ofSeconds(30)).GET().build();

        long start = System.nanoTime();
        long epochStartMs = System.currentTimeMillis();
        long measureFrom = start + warmup * 1_000_000_000L;
        long end = measureFrom + measure * 1_000_000_000L;
        int capacity = (int) (open ? rate * measure * 2 + 1000 : 2_000_000);
        long[] latencies = new long[capacity];
        AtomicInteger count = new AtomicInteger();
        AtomicLong errors = new AtomicLong();
        AtomicLong next = new AtomicLong();
        long intervalNanos = open ? (long) (1e9 / rate) : 0;

        ExecutorService pool = Executors.newVirtualThreadPerTaskExecutor();
        for (int t = 0; t < concurrency; t++) {
            pool.submit(() -> {
                while (true) {
                    long intended;
                    if (open) {
                        intended = start + next.getAndIncrement() * intervalNanos;
                        if (intended >= end) return;
                        long wait = intended - System.nanoTime();
                        if (wait > 0) LockSupport.parkNanos(wait);
                    } else {
                        intended = System.nanoTime();
                        if (intended >= end) return;
                    }
                    try {
                        HttpResponse<Void> r = client.send(request, HttpResponse.BodyHandlers.discarding());
                        long done = System.nanoTime();
                        if (r.statusCode() != 200) errors.incrementAndGet();
                        if (intended >= measureFrom) {
                            int i = count.getAndIncrement();
                            if (i < latencies.length) latencies[i] = done - intended;
                        }
                    } catch (Exception e) {
                        errors.incrementAndGet();
                    }
                }
            });
        }
        pool.shutdown();
        pool.awaitTermination(measure + warmup + 120, TimeUnit.SECONDS);

        int n = Math.min(count.get(), latencies.length);
        long[] sorted = Arrays.copyOf(latencies, n);
        Arrays.sort(sorted);
        System.out.printf("mode=%s target=%.0f achieved=%.1f requests=%d p50_ms=%.2f p99_ms=%.2f p999_ms=%.2f max_ms=%.2f errors=%d window_start_ms=%d window_end_ms=%d%n",
                mode, rate, n / (double) measure, n, ms(sorted, 0.50), ms(sorted, 0.99), ms(sorted, 0.999),
                n == 0 ? 0 : sorted[n - 1] / 1e6, errors.get(),
                epochStartMs + warmup * 1000L, epochStartMs + (warmup + measure) * 1000L);
    }

    static double ms(long[] sorted, double q) {
        if (sorted.length == 0) return 0;
        return sorted[Math.min(sorted.length - 1, (int) Math.ceil(q * sorted.length) - 1)] / 1e6;
    }
}