线程安全实现max

Question

我需要为Web服务器实现全局对象收集统计信息。我有Statistics单身，有方法addSample(long sample)，后来称为updateMax。这显然是线程安全的。我有这个方法来更新整个统计信息的最大值：

AtomicLong max;

private void updateMax(long sample) {
    while (true) {
        long curMax = max.get();
        if (curMax < sample) {
            boolean result = max.compareAndSet(curMax, sample);
            if (result) break;
        } else {
            break;
        }
    }
}

这个实现是否正确？我正在使用java.util.concurrent，因为我相信它会比简单的synchronized更快。是否有其他/更好的方法来实现这一点？

Answer 1

从Java 8开始，引入了LongAccumulator。建议为

当多个线程更新用于收集统计信息但不用于细粒度同步控制的目的的公共值时，此类通常优于AtomicLong。在低更新争用下，这两个类具有相似的特征。但在高争用的情况下，这一类的预期吞吐量明显更高，但代价是空间消耗更高。

您可以按如下方式使用它：

LongAccumulator maxId = new LongAccumulator(Long::max, 0); //replace 0 with desired initial value
maxId.accumulate(newValue); //from each thread

Answer 2

我认为这是正确的，但为了清晰起见，我可能会重写一点，并且肯定会添加评论：

private void updateMax(long sample) {
    while (true) {
        long curMax = max.get();
        if (curMax >= sample) {
            // Current max is higher, so whatever other threads are
            // doing, our current sample can't change max.
            break;
        }

        // Try updating the max value, but only if it's equal to the
        // one we've just seen. We don't want to overwrite a potentially
        // higher value which has been set since our "get" call.
        boolean setSuccessful = max.compareAndSet(curMax, sample);

        if (setSuccessful) {
            // We managed to update the max value; no other threads
            // got in there first. We're definitely done.
            break;
        }

        // Another thread updated the max value between our get and
        // compareAndSet calls. Our sample can still be higher than the
        // new value though - go round and try again.
    }
}

编辑：通常我至少首先尝试同步版本，只有当我发现它导致问题时才会使用这种无锁代码。

Answer 3

我相信你做的是正确的，但这是一个更简单的版本，我认为是正确的。

private void updateMax(long sample){
      //this takes care of the case where between the comparison and update steps, another thread updates the max

      //For example:
      //if the max value is set to a higher max value than the current value in between the comparison and update step
      //sample will be the higher value from the other thread
      //this means that the sample will now be higher than the current highest (as we just set it to the value passed into this function)
      //on the next iteration of the while loop, we will update max to match the true max value
      //we will then fail the while loop check, and be done with trying to update.
      while(sample > max.get()){
          sample = max.getAndSet(sample);  
      }
}

Answer 4

好像你没有选择答案，这是我的：

// while the update appears bigger than the atomic, try to update the atomic.
private void max(AtomicDouble atomicDouble, double update) {
    double expect = atomicDouble.get();
    while (update > expect) {
        atomicDouble.weakCompareAndSet(expect, update);
        expect = atomicDouble.get();
    }
}

它与接受的答案大致相同，但不使用我个人不喜欢的break或while(true)。

编辑：刚刚在java 8中发现了DoubleAccumulator。文档甚至说这是针对像你这样的摘要统计问题：

DoubleAccumulator max = new DoubleAccumulator(Double::max, Double.NEGATIVE_INFINITY);
parallelStream.forEach(max::accumulate);
max.get();

Answer 5

使用Java 8，您可以利用功能接口和简单的lambda表达式来解决这个问题，只需一行即可解决：

private void updateMax(long sample) {
    max.updateAndGet(curMax -> (sample > curMax) ? sample : curMax);
}

该解决方案使用updateAndGet(LongUnaryOperator)方法。当前值包含在curMax中，并且如果样本值大于当前最大值，则使用条件运算符执行简单测试，将当前最大值替换为样本值。