java-topology/defects/xen/unit/Xen0001CreditBalanceLoadAlgorithm.java

package defects.xen.unit;

import java.util.*;

/**
 * xen-0001: Xen credit2 balance_load() O(V²) VCPU swap-search.
 *
 * Models the inner double-loop in balance_load() (credit2.c:2835–2861).
 * The defective version iterates all VCPUs on lrqd × orqd to find the best
 * (push, pull) swap pair.  The fixed version maintains runqueue lists sorted
 * by avgload so the best pair is found in O(V) time.
 */
public class Xen0001CreditBalanceLoadAlgorithm {

    public static class Vcpu {
        final int id;
        long avgload;
        boolean migratable;

        Vcpu(int id, long avgload, boolean migratable) {
            this.id = id;
            this.avgload = avgload;
            this.migratable = migratable;
        }
    }

    public static class SwapPair {
        final Vcpu push;  // from lrqd
        final Vcpu pull;  // from orqd (may be null for push-only)
        final long loadDelta; // abs delta after swap

        SwapPair(Vcpu push, Vcpu pull, long loadDelta) {
            this.push = push;
            this.pull = pull;
            this.loadDelta = loadDelta;
        }
    }

    // -----------------------------------------------------------------------
    // Defective: O(V²) — full cross-product as in credit2.c
    // -----------------------------------------------------------------------
    public static class DefectiveBalanceLoad {
        public int comparisons;

        public SwapPair findBestSwap(List<Vcpu> lrqdSvc, List<Vcpu> orqdSvc,
                                     long lLoad, long oLoad) {
            comparisons = 0;
            SwapPair best = null;
            long bestDelta = Math.abs(lLoad - oLoad);

            for (Vcpu push : lrqdSvc) {
                if (!push.migratable) continue;

                for (Vcpu pull : orqdSvc) {
                    comparisons++;
                    if (!pull.migratable) continue;

                    // delta after swapping push→orqd and pull→lrqd
                    long newL = lLoad - push.avgload + pull.avgload;
                    long newO = oLoad - pull.avgload + push.avgload;
                    long delta = Math.abs(newL - newO);
                    if (delta < bestDelta) {
                        bestDelta = delta;
                        best = new SwapPair(push, pull, delta);
                    }
                }

                // push-only consideration
                long newL = lLoad - push.avgload;
                long newO = oLoad + push.avgload;
                long delta = Math.abs(newL - newO);
                if (delta < bestDelta) {
                    bestDelta = delta;
                    best = new SwapPair(push, null, delta);
                }
            }
            return best;
        }
    }

    // -----------------------------------------------------------------------
    // Fixed: O(V) — pick heaviest push + lightest pull from sorted lists
    // -----------------------------------------------------------------------
    public static class FixedBalanceLoad {
        public int comparisons;

        /**
         * Both lrqdSvc and orqdSvc are assumed sorted descending by avgload
         * (maintained at VCPU assign/weight-update time).
         * Scan from highest-weight end for push, lowest-weight end for pull.
         */
        public SwapPair findBestSwap(List<Vcpu> lrqdSvcSorted,
                                     List<Vcpu> orqdSvcSorted,
                                     long lLoad, long oLoad) {
            comparisons = 0;
            long currentDelta = Math.abs(lLoad - oLoad);

            // Find heaviest migratable push candidate
            Vcpu bestPush = null;
            for (Vcpu v : lrqdSvcSorted) {
                comparisons++;
                if (v.migratable) { bestPush = v; break; }
            }
            if (bestPush == null) return null;

            // Find lightest migratable pull candidate (from end of sorted list)
            Vcpu bestPull = null;
            for (int i = orqdSvcSorted.size() - 1; i >= 0; i--) {
                comparisons++;
                Vcpu v = orqdSvcSorted.get(i);
                if (v.migratable) { bestPull = v; break; }
            }

            // Evaluate push+pull swap
            SwapPair best = null;
            if (bestPull != null) {
                long newL = lLoad - bestPush.avgload + bestPull.avgload;
                long newO = oLoad - bestPull.avgload + bestPush.avgload;
                long delta = Math.abs(newL - newO);
                if (delta < currentDelta) {
                    best = new SwapPair(bestPush, bestPull, delta);
                    currentDelta = delta;
                }
            }
            // Evaluate push-only
            long newL = lLoad - bestPush.avgload;
            long newO = oLoad + bestPush.avgload;
            long delta = Math.abs(newL - newO);
            if (delta < currentDelta) {
                best = new SwapPair(bestPush, null, delta);
            }
            return best;
        }
    }

    // -----------------------------------------------------------------------
    // Helpers
    // -----------------------------------------------------------------------

    /** Build a list of V VCPUs with loads evenly spread, all migratable. */
    public static List<Vcpu> buildSvcList(int count, long baseLoad, boolean sorted) {
        List<Vcpu> list = new ArrayList<>();
        for (int i = 0; i < count; i++) {
            list.add(new Vcpu(i, baseLoad + i * 10, true));
        }
        if (sorted) {
            list.sort((a, b) -> Long.compare(b.avgload, a.avgload));
        }
        return list;
    }

    /** Compute total load for a VCPU list. */
    public static long totalLoad(List<Vcpu> svc) {
        return svc.stream().mapToLong(v -> v.avgload).sum();
    }

    public static void main(String[] args) {
        System.out.println("xen-0001: credit2 balance_load O(V^2) vs O(V)");
        System.out.println("=".repeat(55));

        int[] sizes = {10, 50, 100, 200};
        DefectiveBalanceLoad defective = new DefectiveBalanceLoad();
        FixedBalanceLoad fixed = new FixedBalanceLoad();

        System.out.printf("%-8s  %-12s  %-12s  %-8s%n",
            "V/rqd", "Defect ops", "Fixed ops", "Ratio");

        for (int v : sizes) {
            List<Vcpu> lrqd = buildSvcList(v, 100, false);
            List<Vcpu> orqd = buildSvcList(v, 50, false);
            long lLoad = totalLoad(lrqd);
            long oLoad = totalLoad(orqd);

            defective.findBestSwap(lrqd, orqd, lLoad, oLoad);
            int defectOps = defective.comparisons;

            List<Vcpu> lrqdSorted = buildSvcList(v, 100, true);
            List<Vcpu> orqdSorted = buildSvcList(v, 50, true);
            fixed.findBestSwap(lrqdSorted, orqdSorted, lLoad, oLoad);
            int fixedOps = fixed.comparisons;

            double ratio = (double) defectOps / fixedOps;
            System.out.printf("%-8d  %-12d  %-12d  %.1f×%n",
                v, defectOps, fixedOps, ratio);
        }
    }
}