java - 增加踏板数量会导致循环持续时间更长

Question

我有一个方法，其逻辑需要约 40 毫秒

当我多次(并行)调用此方法时 - 每个循环持续时间需要更多时间(很多)。

例如:

1 次调用 - ~40 毫秒(平均)

5 次调用 - ~63 毫秒(平均)

10 次调用 - ~120 毫秒(平均)

100 次调用 - ~1200 毫秒(平均)

等等...

代码:

int numberOfExecution = 100;
ExecutorService executor = Executors.newFixedThreadPool(numberOfExecution);
for (int i = 0; i < numberOfExecution; i++) {
    executor.execute(new Runnable() {
        @Override
        public void run() {
            MyObject obj1 = readBytesFromFile(1);
            MyObject obj2 = readBytesFromFile(2);
            MyObject obj3 = readBytesFromFile(3);
            MyObject obj4 = readBytesFromFile(4);


            final Map<Integer, MyObject> objects = new TreeMap<>();
            objects.put(0, obj1);
            objects.put(1, obj2);
            objects.put(2, obj3);
            objects.put(3, obj4);

            Foo foo = new Foo(objects);

            long startTime = System.currentTimeMillis();
            Thread.sleep(40); // sleep for 40ms to demonstrate my 40ms logic
            long endTime = System.currentTimeMillis();
            float durationMs = endTime - startTime;

            System.out.println("Foo.bar() duration: " + durationMs);
        }
    });
}

executor.shutdown();
while (!executor.isTerminated()) { }

控制台:

Foo.bar() duration: 41.0
Foo.bar() duration: 40.0
Foo.bar() duration: 43.0
Foo.bar() duration: 41.0
Foo.bar() duration: 40.0
Foo.bar() duration: 41.0
Foo.bar() duration: 41.0
Foo.bar() duration: 48.0
Foo.bar() duration: 62.0
Foo.bar() duration: 48.0
Foo.bar() duration: 48.0
Foo.bar() duration: 48.0
Foo.bar() duration: 48.0
Foo.bar() duration: 49.0
Foo.bar() duration: 48.0
Foo.bar() duration: 49.0
Foo.bar() duration: 40.0
Foo.bar() duration: 40.0
Foo.bar() duration: 40.0
Foo.bar() duration: 40.0
Foo.bar() duration: 40.0
Foo.bar() duration: 40.0
Foo.bar() duration: 40.0
Foo.bar() duration: 41.0
Foo.bar() duration: 40.0
Foo.bar() duration: 41.0
Foo.bar() duration: 41.0
Foo.bar() duration: 42.0
Foo.bar() duration: 49.0
Foo.bar() duration: 40.0
Foo.bar() duration: 40.0
Foo.bar() duration: 49.0
Foo.bar() duration: 74.0
Foo.bar() duration: 40.0
Foo.bar() duration: 70.0
Foo.bar() duration: 40.0
Foo.bar() duration: 50.0
Foo.bar() duration: 43.0
Foo.bar() duration: 40.0
Foo.bar() duration: 43.0
Foo.bar() duration: 56.0
Foo.bar() duration: 63.0
Foo.bar() duration: 45.0
Foo.bar() duration: 40.0
Foo.bar() duration: 41.0
Foo.bar() duration: 43.0
Foo.bar() duration: 43.0
Foo.bar() duration: 48.0
Foo.bar() duration: 41.0
Foo.bar() duration: 58.0
Foo.bar() duration: 59.0
Foo.bar() duration: 55.0
Foo.bar() duration: 42.0
Foo.bar() duration: 40.0
Foo.bar() duration: 42.0
Foo.bar() duration: 70.0
Foo.bar() duration: 40.0
Foo.bar() duration: 41.0
Foo.bar() duration: 75.0
Foo.bar() duration: 40.0
Foo.bar() duration: 43.0
Foo.bar() duration: 40.0
Foo.bar() duration: 41.0
Foo.bar() duration: 67.0
Foo.bar() duration: 59.0
Foo.bar() duration: 40.0
Foo.bar() duration: 57.0
Foo.bar() duration: 42.0
Foo.bar() duration: 40.0
Foo.bar() duration: 41.0
Foo.bar() duration: 43.0
Foo.bar() duration: 41.0
Foo.bar() duration: 40.0
Foo.bar() duration: 41.0
Foo.bar() duration: 42.0
Foo.bar() duration: 41.0
Foo.bar() duration: 40.0
Foo.bar() duration: 42.0
Foo.bar() duration: 42.0
Foo.bar() duration: 41.0
Foo.bar() duration: 45.0
Foo.bar() duration: 44.0
Foo.bar() duration: 40.0
Foo.bar() duration: 42.0
Foo.bar() duration: 41.0
Foo.bar() duration: 40.0
Foo.bar() duration: 40.0
Foo.bar() duration: 45.0
Foo.bar() duration: 41.0
Foo.bar() duration: 40.0
Foo.bar() duration: 44.0
Foo.bar() duration: 41.0
Foo.bar() duration: 42.0
Foo.bar() duration: 40.0
Foo.bar() duration: 42.0
Foo.bar() duration: 41.0
Foo.bar() duration: 42.0
Foo.bar() duration: 42.0
Foo.bar() duration: 43.0
Foo.bar() duration: 43.0

在 MacBook Pro i9 上运行

我错过了什么？

Answer 1

一台机器的核心数量是固定的。随着我们增加需要并行执行的线程数量，线程之间将会发生上下文切换。

请查看https://www.javamex.com/tutorials/threads/context_switch.shtml .

此外，线程之间可能存在资源争用。可以并行运行的最大线程数取决于机器核心。简而言之，仅仅增加线程数并不会提高代码执行性能。