在Objective C中设置for循环的并发执行的最佳方法是什么? [英] What's the best way to set up concurrent execution of for loops in Objective C?

问题描述

我遇到了一个时间密集的for循环的情况，如下所示:

I have a situation with a time-intensive for loop like this:

int sum = 0;
for (int i = 0; i < max; ++i)
   sum += dosomething(i);

do something(i)调用是独立的.这需要多线程.在两核计算机上使用NSOperationQueue，我可以这样做:

The do something(i) calls are independent. This cries out for multithreading. Using NSOperationQueue on a two-core machine, I can do this:

int __block midpoint;
int __block sum1;
int __block sum2;
midpoint = max/2;
sum1 = 0;
sum2 = 0;

NSOperationQueue *queue = [[NSOperationQueue alloc] init];

NSOperation *operation = [NSBlockOperation blockOperationWithBlock: ^{
    for (int i = 0; i < midpoint; ++i)
        sum1 += dosomething(i);
}];
[queue addOperation: operation];

operation = [NSBlockOperation blockOperationWithBlock: ^{
    for (int i = midpoint; i < lines.count; ++i)
        sum2 += dosomething(i);
}];
[queue addOperation: operation];

[queue waitUntilAllOperationsAreFinished];
int sum = sum1 + sum2;

拥有一个NSBlock并将循环的开始/停止值作为参数传递会更好，但是与NSOperation一起使用的块不能具有参数.这样更好的一个原因是，为不同数量的内核进行编码所需的体操变得非常多余.

It would be much nicer to have a single NSBlock and pass the start/stop values for the loop as parameters, but blocks used with NSOperation cannot have parameters. One reason this would be nicer is that the gymnastics needed to code for varying numbers of cores gets pretty excessive.

是否有更好的方法在iOS中进行设置，以便可以使用参数创建单个NSBlock(或其等效对象)，然后多次调用并发执行?理想情况下，该技术必须是方法的本地方法，并且具有对本地变量的读/写访问权限(对于NSBlock，可以使用__block限定符对本地变量进行访问)?

Is there a better way to set this up in iOS so a single NSBlock (or its equivalent) can be created, with parameters, and then called multiple times for concurrent execution? The technique ideally needs to be local to the method, and have read/write access to local variables (which is possible with NSBlock using the __block qualifier on local variables)?

对于后来出现的内容，这是我最终使用的从答案和几个注释中收集的代码.

For those coming along afterwards, here's the code, gleaned from both answers and several comments, that I finally used.

dispatch_queue_t coreQueue = dispatch_queue_create("coreQueue", DISPATCH_QUEUE_CONCURRENT);
int __block count = 0;
int processorCount = (int) [[NSProcessInfo processInfo] processorCount];

dispatch_apply(processorCount, coreQueue, ^(size_t i) {
    int localCount = 0;
    int imax = (i + 1)*max/processorCount;
    if (i + 1 == processorCount)
        imax = lines.count;
    int imin = i*max/processorCount;
    for (int j = imin; j < imax; ++j)
        localCount += dosomething(j);

    // The results array should only be accessed from the main queue.
    dispatch_async(dispatch_get_main_queue(), ^{
        count += localCount;
    });
});

推荐答案

使用并发调度队列.

    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
        [self doAnExpensiveOperation];
    });
    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
        [self doAnotherExpensiveOperation];
    });
    dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{

        dispatch_async(dispatch_get_main_queue(), ^{

            // called when both have finished.
            // calculate sum here
        });

    });

这篇关于在Objective C中设置for循环的并发执行的最佳方法是什么?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！