.net - Parallel.For 多久调用一次 localInit？

Question

我一直在试验 Parallel.For。特别是支持线程本地数据的重载，例如

public static System.Threading.Tasks.ParallelLoopResult For (long fromInclusive, long toExclusive, System.Threading.Tasks.ParallelOptions parallelOptions, Func localInit, Func body, Action localFinally);

根据 documentation

The localInit delegate is invoked once for each thread that participates in the loop's execution

但是我认为我下面的例子与它相矛盾

var threads = new ConcurrentBag<int>();
ValueTuple LocalInit()
{
    threads.Add(Thread.CurrentThread.ManagedThreadId);
    return new System.ValueTuple();
}
ValueTuple Body(long i, ParallelLoopState _, ValueTuple state) 
{
    Thread.Sleep(100);
    return state;
}
void LocalFinally(ValueTuple state) { };

Parallel.For(0L, 1000L, new ParallelOptions(), LocalInit, Body, LocalFinally);

Console.WriteLine($"{threads.Count} inits between {threads.Distinct().Count()} threads");

它打印一条消息，例如

79 inits between 13 threads

这是怎么回事？

Answer 1

尝试录制任务 ID Task.CurrentId而不是线程ID。

var threads = new ConcurrentBag<int>();
var tasks = new ConcurrentBag<int>();
ValueTuple LocalInit()
{
    threads.Add(Thread.CurrentThread.ManagedThreadId);
    tasks.Add(Task.CurrentId ?? throw new Exception());
    return new System.ValueTuple();
}
ValueTuple Body(long i, ParallelLoopState _, ValueTuple state) 
{
    Thread.Sleep(100);
    return state;
}
void LocalFinally(ValueTuple state) { };

Parallel.For(0L, 1000L, new ParallelOptions(), LocalInit, Body, LocalFinally);

Console.WriteLine($"{threads.Count} inits between {threads.Distinct().Count()} threads");
Console.WriteLine($"{tasks.Count} inits between {tasks.Distinct().Count()} tasks");

这打印

87 inits between 16 threads
87 inits between 87 tasks

文档是错误的。他们应该说

The localInit delegate is invoked once for each task that participates in the loop's execution

可以有比线程更多的任务。总是线程数 ≤ 任务数 ≤ 迭代次数。