java - 使用 Java 7 进行逃逸分析/堆栈分配的资格

Question

我正在使用 Java 7 中的 escape analysis 进行一些测试，以便更好地了解哪些对象有资格进行堆栈分配。

这是我为测试堆栈分配而编写的代码:

import java.util.ArrayList;
import java.util.Iterator;


public class EscapeAnalysis {

    private static final long TIME_TO_TEST = 10L * 1000L; // 10s

    static class Timestamp {
        private long millis;
        public Timestamp(long millis) {
            this.millis = millis;
        }
        public long getTime() {
            return millis;
        }
        public void setTime(long time) {
            millis = time;
        }
    }

    public static void main(String[] args) {
        long r = 0;
        System.out.println("test1");
        r += test1();
        System.out.println("test2");
        r += test2();
        System.out.println("test3");
        r += test3();
        System.out.println("test4");
        r += test4();
        System.out.println("test5");
        r += test5();
        System.out.println("test6");
        r += test6();
        System.out.println(r);
    }

    public static long test1() {
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            r += new Timestamp(System.currentTimeMillis()).getTime();
        }
        return r;
    }

    public static long test2() {
        ArrayList<Integer> l = new ArrayList<Integer>(1000);
        for (int i = 0; i < 1000; ++i) {
            l.add(i);
        }
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            for (Iterator<Integer> it = l.iterator(); it.hasNext(); ) {
                r += it.next().longValue();
            }
        }
        return r;
    }

    public static long test3() {
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            Timestamp ts = new Timestamp(System.currentTimeMillis());
            ts.setTime(42);
            r += ts.getTime();
        }
        return r;
    }

    public static long test4() {
        ArrayList<Integer> l = new ArrayList<Integer>(1000);
        for (int i = 0; i < 1000; ++i) {
            l.add(i);
        }
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            Iterator<Integer> it = l.iterator();
            r += it.next().longValue();
            r += it.next().longValue();
            r += it.next().longValue();
            r += it.next().longValue();
        }
        return r;
    }

    public static long test5() {
        ArrayList<Integer> l = new ArrayList<Integer>(1000);
        for (int i = 0; i < 1000; ++i) {
            l.add(i);
        }
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            Iterator<Integer> it = l.iterator();
            for (int i = 0; i < l.size(); ++i) {
                r += it.next().longValue();
            }
        }
        return r;
    }

    public static long test6() {
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            for (Timestamp ts = new Timestamp(System.currentTimeMillis());
                    ts.getTime() > 0;
                    ts.setTime(ts.getTime() + System.currentTimeMillis())) {
                r += ts.getTime();
            }
        }
        return r;
    }

}

这是它在 Linux 上使用 Java 7 输出的内容

java -server -version                                                 
java version "1.7.0_02"
Java(TM) SE Runtime Environment (build 1.7.0_02-b13)
Java HotSpot(TM) 64-Bit Server VM (build 22.0-b10, mixed mode)

java -server -verbose:gc -XX:CompileThreshold=1 -cp bin EscapeAnalysis
test1
test2
[GC 15616K->352K(59776K), 0,0014270 secs]
[GC 15968K->288K(59776K), 0,0011790 secs]
[GC 15904K->288K(59776K), 0,0018170 secs]
[GC 15904K->288K(59776K), 0,0011100 secs]
[GC 15904K->288K(57152K), 0,0019790 secs]
[GC 15520K->320K(56896K), 0,0011670 secs]
[GC 15232K->284K(56256K), 0,0011440 secs]
test3
test4
test5
[GC 14876K->348K(55936K), 0,0005340 secs]
[GC 14620K->348K(56000K), 0,0004560 secs]
[GC 14300K->316K(55296K), 0,0004680 secs]
[GC 13948K->316K(55488K), 0,0003590 secs]
[GC 13692K->316K(54784K), 0,0004580 secs]
[GC 13436K->316K(54976K), 0,0005430 secs]
[GC 13180K->316K(54272K), 0,0004500 secs]
[GC 12924K->316K(54464K), 0,0005090 secs]
[GC 12668K->316K(53760K), 0,0004490 secs]
[GC 12412K->316K(53888K), 0,0004350 secs]
[GC 12156K->316K(53312K), 0,0005060 secs]
test6
6737499643744733086

我正在使用 GC 日志来了解是否在堆栈上分配了对象(来自 Escape analysis in Java 的想法)，这可能不是 100% 可靠，但似乎提供了很好的提示。

根据输出，堆栈分配适用于 test1、test3、test4 和 test6，不适用于 test2 和 test5。我不明白为什么这对 for 循环中的迭代器不起作用，尽管它可以工作

• 在 for 循环外使用迭代器(参见测试 4)，
• 在 for 循环中使用另一个对象(参见测试 6)。

我已经阅读了 ArrayList iterator 的代码，但我不明白为什么它不符合测试 2 和 5 中堆栈分配的条件，因为它既不转义当前方法也不转义当前线程。

有什么想法吗？

Answer 1

EA 是 C2 编译器根据它生成的 IR 进行分析的东西，因此您需要它来编译方法，然后才能享受这些好处。每个测试只调用一次，因此没有机会编译。热点内部 wiki(https://wiki.openjdk.java.net/display/HotSpot/Overview+of+Ideal,+C2%27s+high+level+intermediate+representation 和 https://wiki.openjdk.java.net/display/HotSpot/EscapeAnalysis)中有关 EA 和 C2 IR 的详细信息

这是一个试图显示影响的版本

import com.sun.management.ThreadMXBean;

import java.lang.management.ManagementFactory;
import java.util.ArrayList;
import java.util.Iterator;


public class EscapeAnalysisTest {

    private static final long TIME_TO_TEST = 10L * 1000L; // 10s

    static class Timestamp {
        private long millis;

        public Timestamp(long millis) {
            this.millis = millis;
        }

        public long getTime() {
            return millis;
        }

        public void setTime(long time) {
            millis = time;
        }
    }

    public static void main(String[] args) {
        System.out.println("****");
        doIt();
        System.out.println("****");
        doIt();
        System.out.println("****");
        doIt();
        System.out.println("****");
        doIt();
        System.out.println("****");
    }

    private static void doIt() {
        final ThreadMXBean mxbean = (ThreadMXBean) ManagementFactory.getThreadMXBean();
        final long tid = Thread.currentThread().getId();
        long r = 0;
        final long allocPre = mxbean.getThreadAllocatedBytes(tid);
        r += test1();
        long alloc1 = mxbean.getThreadAllocatedBytes(tid);
        System.out.println("test1 - " + (alloc1 - allocPre));
        r += test2();
        final long alloc2 = mxbean.getThreadAllocatedBytes(tid);
        System.out.println("test2 - " + (alloc2 - alloc1));
        r += test3();
        final long alloc3 = mxbean.getThreadAllocatedBytes(tid);
        System.out.println("test3 - " + (alloc3 - alloc2));
        r += test4();
        final long alloc4 = mxbean.getThreadAllocatedBytes(tid);
        System.out.println("test4 - " + (alloc4 - alloc3));
        r += test5();
        final long alloc5 = mxbean.getThreadAllocatedBytes(tid);
        System.out.println("test5 - " + (alloc5 - alloc4));
        r += test6();
        final long alloc6 = mxbean.getThreadAllocatedBytes(tid);
        System.out.println("test6 - " + (alloc6 - alloc5));
        System.out.println(r);
    }

    public static long test1() {
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            r += new Timestamp(System.currentTimeMillis()).getTime();
        }
        return r;
    }

    public static long test2() {
        ArrayList<Integer> l = new ArrayList<Integer>(1000);
        for (int i = 0; i < 1000; ++i) {
            l.add(i);
        }
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            for (Iterator<Integer> it = l.iterator(); it.hasNext(); ) {
                r += it.next().longValue();
            }
        }
        return r;
    }

    public static long test3() {
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            Timestamp ts = new Timestamp(System.currentTimeMillis());
            ts.setTime(42);
            r += ts.getTime();
        }
        return r;
    }

    public static long test4() {
        ArrayList<Integer> l = new ArrayList<Integer>(1000);
        for (int i = 0; i < 1000; ++i) {
            l.add(i);
        }
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            Iterator<Integer> it = l.iterator();
            r += it.next().longValue();
            r += it.next().longValue();
            r += it.next().longValue();
            r += it.next().longValue();
        }
        return r;
    }

    public static long test5() {
        ArrayList<Integer> l = new ArrayList<Integer>(1000);
        for (int i = 0; i < 1000; ++i) {
            l.add(i);
        }
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            Iterator<Integer> it = l.iterator();
            for (int i = 0; i < l.size(); ++i) {
                r += it.next().longValue();
            }
        }
        return r;
    }

    public static long test6() {
        long r = 0;
        long start = System.currentTimeMillis();
        while (System.currentTimeMillis() - start < TIME_TO_TEST) {
            for (Timestamp ts = new Timestamp(System.currentTi());
                 ts.getTime() > 0;
                 ts.setTime(ts.getTime() + System.currentTimeMillis())) {
                r += ts.getTime();
            }
        }
        return r;
    }

}

当使用 -server -XX:CompileThreshold=1 运行时生成以下输出

****
test1 - 109048
test2 - 89243416
test3 - 16664
test4 - 42840
test5 - 71982168
test6 - 1400
-5351026995119026839
****
test1 - 16432
test2 - 85921464
test3 - 16664
test4 - 42840
test5 - 66777600
test6 - 1368
7844020592566674506
****
test1 - 48
test2 - 18256
test3 - 272
test4 - 18264
test5 - 18264
test6 - 272
-2137858376905291730
****
test1 - 48
test2 - 18256
test3 - 272
test4 - 18264
test5 - 18264
test6 - 272
3273987624143297143
****

这里的一个危险是这个方法的编译从根本上改变了它，我没有尝试防止这种情况所以一些使用 LogCompilation 或 PrintCompilation 可能需要检查。