jdk-24/test/hotspot/jtreg/gc/TestSmallHeap.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 2 along with this work; if not, write to the Free Software Foundation,
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/**
 * @test TestSmallHeap
 * @bug 8067438 8152239
 * @requires vm.gc=="null"
 * @summary Verify that starting the VM with a small heap works
 * @library /test/lib
 * @modules java.base/jdk.internal.misc
 * @build sun.hotspot.WhiteBox
 * @run driver ClassFileInstaller sun.hotspot.WhiteBox
 * @run main/othervm -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI TestSmallHeap
 */

/* Note: It would be nice to verify the minimal supported heap size here,
 * but we align the heap size based on the card table size. And the card table
 * size is aligned based on the minimal pages size provided by the os. This
 * means that on most platforms, where the minimal page size is 4k, we get a
 * minimal heap size of 2m but on Solaris/Sparc we have a page size of 8k and
 * get a minimal heap size of 4m. And on platforms where the page size is 64k
 * we get a minimal heap size of 32m. We never use large pages for the card table.
 *
 * There is also no check in the VM for verifying that the maximum heap size
 * is larger than the supported minimal heap size.
 *
 * To work around these behaviors this test uses -Xmx4m but then
 * calculates what the expected heap size should be. The calculation is a
 * simplified version of the code in the VM. We assume that the card table will
 * use one page. Each byte in the card table corresponds to 512 bytes on the heap.
 * So, the expected heap size is page_size * 512.
 *
 * There is no formal requirement for the minimal value of the maximum heap size
 * the VM should support. In most cases the VM could start with -Xmx2m.
 * But with 2m limit GC could be triggered before VM initialization completed.
 * Therefore we start the VM with 4M heap.
 */

import jdk.test.lib.Asserts;
import jdk.test.lib.process.OutputAnalyzer;
import jdk.test.lib.process.ProcessTools;

import java.util.LinkedList;

import sun.hotspot.WhiteBox;

public class TestSmallHeap {

    public static void main(String[] args) throws Exception {
        // Do all work in the VM driving the test, the VM
        // with the small heap size should do as little as
        // possible to avoid hitting an OOME.
        WhiteBox wb = WhiteBox.getWhiteBox();
        int pageSize = wb.getVMPageSize();
        int heapBytesPerCard = 512;
        long expectedMaxHeap = pageSize * heapBytesPerCard;

        verifySmallHeapSize("-XX:+UseParallelGC", expectedMaxHeap);
        verifySmallHeapSize("-XX:+UseSerialGC", expectedMaxHeap);
        verifySmallHeapSize("-XX:+UseG1GC", expectedMaxHeap);
        verifySmallHeapSize("-XX:+UseConcMarkSweepGC", expectedMaxHeap);
    }

    private static void verifySmallHeapSize(String gc, long expectedMaxHeap) throws Exception {
        long minMaxHeap = 4 * 1024 * 1024;
        LinkedList<String> vmOptions = new LinkedList<>();
        vmOptions.add(gc);
        vmOptions.add("-Xmx" + minMaxHeap);
        vmOptions.add("-XX:+PrintFlagsFinal");
        vmOptions.add(VerifyHeapSize.class.getName());

        ProcessBuilder pb = ProcessTools.createJavaProcessBuilder(vmOptions.toArray(new String[0]));
        OutputAnalyzer analyzer = new OutputAnalyzer(pb.start());
        analyzer.shouldHaveExitValue(0);

        expectedMaxHeap = Math.max(expectedMaxHeap, minMaxHeap);
        long maxHeapSize = Long.parseLong(analyzer.firstMatch("MaxHeapSize.+=\\s+(\\d+)",1));
        long actualHeapSize = Long.parseLong(analyzer.firstMatch(VerifyHeapSize.actualMsg + "(\\d+)",1));
        Asserts.assertEQ(maxHeapSize, expectedMaxHeap);
        Asserts.assertLessThanOrEqual(actualHeapSize, maxHeapSize);
    }
}

class VerifyHeapSize {
    public static final String actualMsg = "Actual heap size: ";

    public static void main(String args[]) {
        // Avoid string concatenation
        System.out.print(actualMsg);
        System.out.println(Runtime.getRuntime().maxMemory());
    }
}