C#•6mo ago

need help understanding `Task`s

if my CPU has 6 cores and 12 threads, how many Tasks can run at once?

57 Replies

Angius•6mo ago

Any number Tasks are not related to threads

cheeze2000•6mo ago

i tried running 99 tasks at once, but they seem to be limited to 12, why is that so? i also have this code

public class Program
{
    private static readonly Stopwatch stopwatch = new();
    private static readonly Mutex mutex = new();
    private static Queue<int> queue = new();

    public static void Main()
    {
        var N = 72;

        queue = new Queue<int>(Enumerable.Range(1, N));

        stopwatch.Start();
        var tasks = Enumerable.Range(1, 99).Select(_ => Task.Run(Process));
        Task.WaitAll([.. tasks]);
        Console.WriteLine($"Took {stopwatch.ElapsedMilliseconds / 1000.0:F2} s");
    }

    private static void Process()
    {
        mutex.WaitOne();

        if (queue.Count > 0)
        {
            queue.Dequeue();
            mutex.ReleaseMutex();
            Thread.Sleep(1000);
            Process();
        }
        else
        {
            mutex.ReleaseMutex();
            return;
        }
    }
}

public class Program
{
    private static readonly Stopwatch stopwatch = new();
    private static readonly Mutex mutex = new();
    private static Queue<int> queue = new();

    public static void Main()
    {
        var N = 72;

        queue = new Queue<int>(Enumerable.Range(1, N));

        stopwatch.Start();
        var tasks = Enumerable.Range(1, 99).Select(_ => Task.Run(Process));
        Task.WaitAll([.. tasks]);
        Console.WriteLine($"Took {stopwatch.ElapsedMilliseconds / 1000.0:F2} s");
    }

    private static void Process()
    {
        mutex.WaitOne();

        if (queue.Count > 0)
        {
            queue.Dequeue();
            mutex.ReleaseMutex();
            Thread.Sleep(1000);
            Process();
        }
        else
        {
            mutex.ReleaseMutex();
            return;
        }
    }
}

the time taken is always different, between 5.0s, 5.5s and 6.0s. i'm not sure why

Saryn•6mo ago

Tasks are not exactly concurrent processes. You have a limited number of threads for processing a queue of tasks. They will process as threads become available.

Angius•6mo ago

You're also using thread.Sleep() which blocks the thread Use await Task.Delay() instead, so that the thread gets freed for another task to execute

cheeze2000•6mo ago

yea, but based on my understanding, if there are 12 threads processing the tasks, why can the code above run in less than 6 seconds?

Saryn•6mo ago

Because you're telling a thread to not work for a set amount of time with thread.Sleep() as the other person said.

cheeze2000•6mo ago

but wouldn't that cause the time taken to be longer than 6 seconds? my concern is that it takes less than 6 seconds

canton7•6mo ago

I don't get why your Process is recursive

cheeze2000•6mo ago

i want them to keep taking integers from the queue until it's depleted

canton7•6mo ago

A loop is the normal way, which doesn't risk blowing the stack

cheeze2000•6mo ago

there are 72 integers in the queue, so there are essentially 72 things to do

Saryn•6mo ago

So you want 72 functions on the stack until the queue is empty?

cheeze2000•6mo ago

i want to make 99 tasks that collectively dequeue until it's empty yes and measure the time but every dequeue waits 1 second

Saryn•6mo ago

also popping from a queue recursively is not using a queue correctly.

canton7•6mo ago

Anyway, Task.Run runs ona a ThreadPool thread. The ThreadPool has a variable number of threads, and if it runs out, it will start increasing the number of threads at a rate of a couple a second (IIRC)

cheeze2000•6mo ago

this is not some sort of production code or best practices code, it's just what i randomly wrote to test out concurrency how would you explain why the code above runs in less than 6 seconds? if i run the code, it would say 5.50 s on my 6c/12t cpu, i just want to know how this works

canton7•6mo ago

The default is something like 4? So you'll start off with 4-ish tasks in parallel, then after 0.5s or so another couple will start, then 0.5s after that the first 4 finish and another 4 items start processing, and at the same time another couple of threads get added

Saryn•6mo ago

cpu thread != c# thread.

canton7•6mo ago

Start logging when each task start/stops, and what thread ID it's running on, that'll make it clearer

cheeze2000•6mo ago

so a cpu thread can be instructed to do multiple c# threads?

canton7•6mo ago

No, C# threads map 1-1 to CPU threads

cheeze2000•6mo ago

now i'm getting conflicting answers.

Saryn•6mo ago

That wouldn't make sense with your previous comment about increasing number of threads.

cheeze2000•6mo ago

ok i will try that

canton7•6mo ago

Well, OS threads, not "CPU threads". There's no such thing as a "CPU thread" Well, unless you're getting into the realm of hyperthreading. But cpu cores/ virtual cores / etc aren't relevant here

Saryn•6mo ago

True, but just wanted to differentiate as he was wondering why a 6c/12t cpu was not being reflected given his test code

cheeze2000•6mo ago

oh wow, here are the logs

72.txt

canton7•6mo ago

Log the starts, and timestamps as well

cheeze2000•6mo ago

initially only threads 1-12, but after some time there are threads 13-16

canton7•6mo ago

Yeah, as I say, the thread pool will start adding threads after you exhaust all the threads it does have

cheeze2000•6mo ago

with timestamps

72.txt

canton7•6mo ago

Add milliseconds to that. You should see jumps as a loads of threads finish at the same time, and jumps as new threads are added

cheeze2000•6mo ago

so if 4 threads are added, meaning there are now 16 threads, how many of these threads are actually executing in parallel? 12?

canton7•6mo ago

There's no reason you won't be using all of the available threads in the threadpool at the same time

cheeze2000•6mo ago

but there's a limitation to how many tasks my cpu can take at an instant right are you saying all the threads in the threadpool execute at the same time? then my question is how is this possible

canton7•6mo ago

A thread that's sleeping won't be running on a CPU

cheeze2000•6mo ago

omg

canton7•6mo ago

Thread.Sleep just means "Start running again in X seconds"

cheeze2000•6mo ago

okay that's the answer to my question so if i have 16 threads, none of them is sleeping (doing some intensive task), how many would be executing at the same time? actually i'm gonna do that experiment and see what happens

Saryn•6mo ago

As many as your cpu allows for.

cheeze2000•6mo ago

that would be my first guess, let me try it

Saryn•6mo ago

Just mind it maybe not be 12 if you have a 6c/12t CPU... Your computer is doing other things that isn't just running your program.

cap5lut•6mo ago

thats actually handled at OS level, has to do with context switches and a lot other different complex stuff

canton7•6mo ago

Well. They'll all be "executing" at the same time, but what happens in reality is that the OS gives each thread a time-slice, then puts it to sleep and moves onto the next one. Your CPU hardware determines how many threads can actually be executing in parallel at the same time

cheeze2000•6mo ago

i'm trying to simulate an intensive task that isn't sleeping, i'm currently using for (int i = 0; i < int.MaxValue; i++) {} seems like only 12 threads are spawned at most

canton7•6mo ago

The threadpool should keep spawning new ones, IIRC

cap5lut•6mo ago

the starting size of the thread pool is at least hardware dependent and might be OS dependent as well, for windows i have seen that its usually logical processors, as in ur hardware threads

cheeze2000•6mo ago

you're right, i ran the code for longer and now threads 13, 14 start showing up i see, that makes sense from the experiments that i've done

cap5lut•6mo ago

there are some different factors that can bring the thread pool to more than that. one is for example tasks that are created TaskCreationOptions.LongRunning, for this often its own thread is created, that may or may not die later

canton7•6mo ago

TaskCreationOptions.LongRunning doesn't touch the threadpool at all It just does new Thread()

cap5lut•6mo ago

i thought i have read somewhere that that thread can later be added to the thread pool as well, might be misremembering

canton7•6mo ago

Nope, at least last time I read that code, it's just a little standalone thread https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/ThreadPoolTaskScheduler.cs,45

cap5lut•6mo ago

indeed, sorry for the misinformation.

canton7•6mo ago

Looks like ThreadPool.MinThreads defaults to the processor count btw: https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.cs,120

cap5lut•6mo ago

but in the end its finishing that early because its spawning new threads and thats why u finish in such a short time, like canton7 already said logical processor count at that, so each hardware thread counts towards this also, 1 second already counts being a long running task, iirc

cap5lut•6mo ago

if u just want to parallelize cpu bound work, u would use other means than popping a lot of tasks: https://learn.microsoft.com/en-us/dotnet/standard/parallel-programming/data-parallelism-task-parallel-library

Data Parallelism (Task Parallel Library) - .NET

Read how the Task Parallel Library (TPL) supports data parallelism to do the same operation concurrently on a source collection or array's elements in .NET.

cheeze2000•6mo ago

sure thing, i'll keep that in mind but the reason why i'm playing with concurrency is because i'm presenting about it so i thought of using c# as an example

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need help understanding `Task`s