c# - 在 Parallel.ForEach 中指定默认的 MaxDegreeOfParallelism？

Question

我们希望有选择地控制并行循环中“线程”的数量，以避免网络服务不堪重负(例如)。

是否可以在 Parallel.ForEach 循环中指定自定义 MaxDegreeOfParallelism，但也可以根据需要恢复为默认值？看似零 (0) 是 MaxDegreeOfParallelism 的无效值，而我希望它可以简单地表示“忽略”。

换句话说，你能避免写这种类型的代码吗？

int numParallelOperations = GetNumParallelOperations();
if (numParallelOperations > 0)
{
 ParallelOptions options = new ParallelOptions();
 options.MaxDegreeOfParallelism = numParallelOperations;
 Parallel.ForEach(items, options, i => 
 {
   Foo(i);
 });
}
else
{ 
 Parallel.ForEach(items, i => 
 { 
   Foo(i);
 });
}

Answer 1

你是说 -1 根据 MSDN :

The MaxDegreeOfParallelism limits the number of concurrent operations run by Parallel method calls that are passed this ParallelOptions instance to the set value, if it is positive. If MaxDegreeOfParallelism is -1, then there is no limit placed on the number of concurrently running operations.

您可以像这样控制近似线程数:

// use only (ca) one kernel:
int degreeOfParallelism = 1;
// leave (ca) one kernel idle:
int degreeOfParallelism = Environment.ProcessorCount - 1;
// use (ca) half of the kernels:
int degreeOfParallelism = Environment.ProcessorCount > 1 ? 
                          Environment.ProcessorCount / 2 : 1;
// run at full speed:
int degreeOfParallelism =  - 1;

var options = new ParallelOptions();
options.MaxDegreeOfParallelism = degreeOfParallelism;

Parallel.For(0, x, options, y => 
//...