19

によって保護されてAtomicIntegerいるよりも少なくとも1桁遅くなる可能性があるため、なぜAtomicIntegerを使用したいのでしょうか?intsynchronized

たとえば、スレッドセーフな方法で値をインクリメントすることだけが必要な場合は、常に使用intしないのはなぜですか。

synchronized(threadsafeint) {
  threadsafeint++;
}

はるかに遅いAtomicInteger.incrementAndGet()を使用する代わりに?

4

2 に答える 2

19

AtomicInteger は、ynchronized で保護された int よりも少なくとも 1 桁遅くなる可能性があるため、なぜ AtomicInteger を使用する必要があるのでしょうか?

AtomicInteger ははるかに高速です。

static final Object LOCK1 = new Object();
static final Object LOCK2 = new Object();
static int i1 = 0;
static int i2 = 0;
static final AtomicInteger ai1 = new AtomicInteger();
static final AtomicInteger ai2 = new AtomicInteger();

public static void main(String... args) throws IOException {
    for(int i=0;i<5;i++) {
        testSyncInt();
        testAtomicInt();
    }
}

private static void testSyncInt() {
    long start = System.nanoTime();
    int runs = 10000000;
    for(int i=0;i< runs;i+=2) {
        synchronized (LOCK1) {
            i1++;
        }
        synchronized (LOCK2) {
            i2++;
        }
    }
    long time = System.nanoTime() - start;
    System.out.printf("sync + incr: Each increment took an average of %.1f ns%n", (double) time/runs);
}

private static void testAtomicInt() {
    long start = System.nanoTime();
    int runs = 10000000;
    for(int i=0;i< runs;i+=2) {
        ai1.incrementAndGet();
        ai2.incrementAndGet();
    }
    long time = System.nanoTime() - start;
    System.out.printf("incrementAndGet: Each increment took an average of %.1f ns%n", (double) time/runs);
}

版画

sync + incr: Each increment took an average of 32.4 ns
incrementAndGet: Each increment took an average of 20.6 ns
sync + incr: Each increment took an average of 31.4 ns
incrementAndGet: Each increment took an average of 12.9 ns
sync + incr: Each increment took an average of 29.6 ns
incrementAndGet: Each increment took an average of 12.9 ns
sync + incr: Each increment took an average of 35.1 ns
incrementAndGet: Each increment took an average of 16.6 ns
sync + incr: Each increment took an average of 29.9 ns
incrementAndGet: Each increment took an average of 13.0 ns

@assyliasが示唆するように、いくつかの競合を追加します。実際に 1 つのスレッドのみを使用している場合、CPU がアクセスを最適化できることを示しています。

static final Object LOCK1 = new Object();
static final Object LOCK2 = new Object();
static int i1 = 0;
static int i2 = 0;
static final AtomicInteger ai1 = new AtomicInteger();
static final AtomicInteger ai2 = new AtomicInteger();

public static void main(String... args) throws  ExecutionException, InterruptedException {
    for(int i=0;i<5;i++) {
        testSyncInt();
        testAtomicInt();
    }
}

private static void testSyncInt() throws ExecutionException, InterruptedException {
    long start = System.nanoTime();
    final int runs = 1000000;
    ExecutorService es = Executors.newFixedThreadPool(2);
    List<Future<Void>> futures = new ArrayList<>();
    for(int t=0;t<8;t++) {
        futures.add(es.submit(new Callable<Void>() {
            public Void call() throws Exception {
                for (int i = 0; i < runs; i += 2) {
                    synchronized (LOCK1) {
                        i1++;
                    }
                    synchronized (LOCK2) {
                        i2++;
                    }
                }
                return null;
            }
        }));
    }
    for (Future<Void> future : futures) {
        future.get();
    }
    es.shutdown();
    long time = System.nanoTime() - start;
    System.out.printf("sync + incr: Each increment took an average of %.1f ns%n", (double) time/runs/2);
}

private static void testAtomicInt() throws ExecutionException, InterruptedException {
    long start = System.nanoTime();
    final int runs = 1000000;
    ExecutorService es = Executors.newFixedThreadPool(2);
    List<Future<Void>> futures = new ArrayList<>();
    for(int t=0;t<8;t++) {
        futures.add(es.submit(new Callable<Void>() {
            public Void call() throws Exception {
                for (int i = 0; i < runs; i += 2) {
                    ai1.incrementAndGet();
                    ai2.incrementAndGet();
                }
                return null;
            }
        }));
    }
    for (Future<Void> future : futures) {
        future.get();
    }
    es.shutdown();
    long time = System.nanoTime() - start;
    System.out.printf("incrementAndGet: Each increment took an average of %.1f ns%n", (double) time/runs/2);
}

版画

sync + incr: Each increment took an average of 478.6 ns
incrementAndGet: Each increment took an average of 191.5 ns
sync + incr: Each increment took an average of 437.5 ns
incrementAndGet: Each increment took an average of 169.8 ns
sync + incr: Each increment took an average of 408.1 ns
incrementAndGet: Each increment took an average of 180.8 ns
sync + incr: Each increment took an average of 511.5 ns
incrementAndGet: Each increment took an average of 313.4 ns
sync + incr: Each increment took an average of 441.6 ns
incrementAndGet: Each increment took an average of 219.7 ns
于 2012-07-26T13:46:32.803 に答える
2

java.util.concurrentなぜ物事が優れているのか、従来の同期アプローチと比較してどのような違いがあるのか​​ について詳しく知りたい場合は、このリンクを読んでください（およびブログ全体を一般的に読んでください）

于 2012-07-26T13:50:06.907 に答える