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Something wrong with parallel task execution along with Thread.Sleep [duplicate]

In C# 4.0, we have Task in the System.Threading.Tasks namespace. What is the true difference between Thread and Task. I did some sample program(help taken from MSDN) for my own sake of learning with
Parallel.Invoke
Parallel.For
Parallel.ForEach
but have many doubts as the idea is not so clear.
I have initially searched in Stackoverflow for a similar type of question but may be with this question title I was not able to get the same. If anyone knows about the same type of question being posted here earlier, kindly give the reference of the link.

In computer science terms, a Task is a future or a promise. (Some people use those two terms synonymously, some use them differently, nobody can agree on a precise definition.) Basically, a Task<T> "promises" to return you a T, but not right now honey, I'm kinda busy, why don't you come back later?
A Thread is a way of fulfilling that promise. But not every Task needs a brand-new Thread. (In fact, creating a thread is often undesirable, because doing so is much more expensive than re-using an existing thread from the thread pool. More on that in a moment.) If the value you are waiting for comes from the filesystem or a database or the network, then there is no need for a thread to sit around and wait for the data when it can be servicing other requests. Instead, the Task might register a callback to receive the value(s) when they're ready.
In particular, the Task does not say why it is that it takes such a long time to return the value. It might be that it takes a long time to compute, or it might be that it takes a long time to fetch. Only in the former case would you use a Thread to run a Task. (In .NET, threads are freaking expensive, so you generally want to avoid them as much as possible and really only use them if you want to run multiple heavy computations on multiple CPUs. For example, in Windows, a thread weighs 12 KiByte (I think), in Linux, a thread weighs as little as 4 KiByte, in Erlang/BEAM even just 400 Byte. In .NET, it's 1 MiByte!)

A task is something you want done.
A thread is one of the many possible workers which performs that task.
In .NET 4.0 terms, a Task represents an asynchronous operation. Thread(s) are used to complete that operation by breaking the work up into chunks and assigning to separate threads.

Thread
The bare metal thing, you probably don't need to use it, you probably can use a LongRunning task and take the benefits from the TPL - Task Parallel Library, included in .NET Framework 4 (february, 2002) and above (also .NET Core).
Tasks
Abstraction above the Threads. It uses the thread pool (unless you specify the task as a LongRunning operation, if so, a new thread is created under the hood for you).
Thread Pool
As the name suggests: a pool of threads. This is the .NET framework handling a limited number of threads for you. Why? Because opening 100 threads to execute expensive CPU operations on a Processor with just 8 cores definitely is not a good idea. The framework will maintain this pool for you, reusing the threads (not creating/killing them at each operation), and executing some of them in parallel, in a way that your CPU will not burn.
OK, but when to use each one?
In resume: always use tasks.
Task is an abstraction, so it is a lot easier to use. I advise you to always try to use tasks and if you face some problem that makes you need to handle a thread by yourself (probably 1% of the time) then use threads.
BUT be aware that:
I/O Bound: For I/O bound operations (database calls, read/write files, APIs calls, etc) avoid using normal tasks, use LongRunning tasks (or threads if you need to). Because using tasks would lead you to a thread pool with a few threads busy and a lot of other tasks waiting for its turn to take the pool.
CPU Bound: For CPU bound operations just use the normal tasks (that internally will use the thread pool) and be happy.

In addition to above points, it would be good to know that:
A task is by default a background task. You cannot have a foreground task. On the other hand a thread can be background or foreground (Use IsBackground property to change the behavior).
Tasks created in thread pool recycle the threads which helps save resources. So in most cases tasks should be your default choice.
If the operations are quick, it is much better to use a task instead of thread. For long running operations, tasks do not provide much advantages over threads.

A Task can be seen as a convenient and easy way to execute something asynchronously and in parallel.
Normally a Task is all you need, I cannot remember if I have ever used a thread for anything other than experimentation.
You can accomplish the same thing, with a thread (with lots of effort) as you can with a task.
Thread
int result = 0;
Thread thread = new System.Threading.Thread(() => {
result = 1;
});
thread.Start();
thread.Join();
Console.WriteLine(result); //is 1
Task
int result = await Task.Run(() => {
return 1;
});
Console.WriteLine(result); //is 1
A task will by default use the Threadpool, which saves resources as creating threads can be expensive. You can see a Task as a higher level abstraction upon threads.
As this article points out, Task provides the following powerful features over Thread.
Tasks are tuned for leveraging multicore processors.
If the system has multiple Tasks then it makes use of the CLR thread pool
internally, and so does not have the overhead associated with creating
a dedicated thread using the Thread. Also reduces the context
switching time among multiple threads.
Task can return a result. There is no direct mechanism to return the result from thread.
Wait on a set of Tasks, without a signaling construct.
We can chain Tasks together to execute one after the other.
Establish a parent/child relationship when one task is started from
another task.
A child Task Exception can propagate to parent task.
Tasks support cancellation through the use of cancellation tokens.
Asynchronous implementation is easy in Task, using async and
await keywords.

I usually use Task to interact with Winforms and simple background worker to make it not freeze the UI. Here is an example of when I prefer using Task.
private async void buttonDownload_Click(object sender, EventArgs e)
{
buttonDownload.Enabled = false;
await Task.Run(() => {
using (var client = new WebClient())
{
client.DownloadFile("http://example.com/file.mpeg", "file.mpeg");
}
})
buttonDownload.Enabled = true;
}
VS
private void buttonDownload_Click(object sender, EventArgs e)
{
buttonDownload.Enabled = false;
Thread t = new Thread(() =>
{
using (var client = new WebClient())
{
client.DownloadFile("http://example.com/file.mpeg", "file.mpeg");
}
this.Invoke((MethodInvoker)delegate()
{
buttonDownload.Enabled = true;
});
});
t.IsBackground = true;
t.Start();
}
the difference is you don't need to use MethodInvoker and shorter code.

You can use Task to specify what you want to do then attach that Task with a Thread. so that Task would be executed in that newly made Thread rather than on the GUI thread.
Use Task with the TaskFactory.StartNew(Action action). In here you execute a delegate so if you didn't use any thread it would be executed in the same thread (GUI thread). If you mention a thread you can execute this Task in a different thread. This is an unnecessary work cause you can directly execute the delegate or attach that delegate to a thread and execute that delegate in that thread. So don't use it. it's just unnecessary. If you intend to optimize your software this is a good candidate to be removed.
**Please note that the Action is a delegate.

Task is like an operation that you want to perform. Thread helps to manage those operation through multiple process nodes. Task is a lightweight option as Threading can lead to complex code management.
I suggest you read from MSDN (best in world) always Task
Thread

Are TAP and async/await meant to be used for dedicated threads?

I have some number of threads dedicated to one task (receive and handle requests) and some number of threads dedicated to other tasks (long running/CPU intensive tasks).
It's a console application.
And I want to use async/await in all of the threads but I learned that async/await uses the managed thread pool to run continuation that means requests handling can be delayed because of a bunch of CPU intensive tasks shown up (yes, so many, it's why i have limited threads count to do this job)?
If i want to limit concurrency, then i must not use TAP?
If i want to do some job in background for aplication's lifetime, then i must not use TAP either?
Should I write a ThradPool implementation with it's own SyncronizationContext to do so?
Is there an other way to run continuation in some set of threads?

And I want to use async/await in all of the threads but I learned that async/await uses the managed thread pool to run continuation that means requests handling can be delayed because of a bunch of CPU intensive tasks shown up (yes, so many, it's why i have limited threads count to do this job)?
This is a close-enough analysis. Technically, it's not async/await that needs the thread pool for continuations, but internal TAP method implementations. That said, it is a common pattern in the BCL, and because of this, exhausting the thread pool can cause delays for asynchronous completion.
If i want to limit concurrency, then i must not use TAP?
As long as you don't overwhelm the thread pool, you can use TAP. Limiting concurrency would free up the thread pool, which will enable TAP.
If i want to do some job in background for aplication's lifetime, then i must not use TAP either?
I would think of it more as "don't drown the thread pool in work". An exhausted thread pool isn't great for any kind of work.
Should I write a ThradPool implementation with it's own SyncronizationContext to do so?
Is there an other way to run continuation in some set of threads?
You can use SynchronizationContext to control where await resumes executing, but there are still BCL-internal completions that need to queue to the thread pool. There's no controlling that; the only real option is to keep the thread pool free.

UWP handling many threads

So I am developing a UWP application that has a large number of threads. Previously I would start all the threads with System.Threading.Tasks.Task.Run(), save their thread handles to an array, then Task.WaitAll() for completion and get the results. This currently is taking too much memory though.
I have changed my code to only wait for a smaller amount of threads and copy their results out before continuing on to more of the threads. Since UWP the UWP implementation of Task does not implement IDisposable, what is the proper way to signal the framework that I am done with a task so it can be garbage collected? I would like to read out the results of the treads after a certain number of them come in and dispose of the threads resources to make space for the next threads.
Thanks so much!

Just to point out an issue which might be degrading the performance of your application: You are deliberately blocking the thread until all Tasks complete rather than actually await for them. That would make sense, if you are not performing Asynchronous work inside them, but if you are, you should definitely switch to:
Task.WhenAll rather than Task.WaitAll , such as this:
List<Tasks> tasks = new List<Tasks> { Method1(), Method2(), ... };
Task result = await Task.WhenAll(tasks);
This way, you are actually leveraging the asynchrony of your app, and you will not block the current thread until all the tasks are completed, like Task.WaitAll() does.
Since you are utilizing the Task.Run() method, instead of the Task.Factory.StartNew(), the TaskScheduler used is the default, and utilizes Threads from the Thread Pool. So you will not actually end up blocking the UI thread, but blocking many Thread Pool threads, is also not good.
Taking from Microsoft documentation, for one of the cases where Thread Pools should not be used:
You have tasks that cause the thread to block for long periods of
time. The thread pool has a maximum number of threads, so a large
number of blocked thread pool threads might prevent tasks from
starting.
Edit:
I do not need anything else but I will look in to that! Thanks! So is
there any way I can get it to run the Tasks like a FIFO with just the
API's available with the default thread pool?
You should take a look, into Continuations
A continuation is nothing else other than a task which is activated whenever it's antecedent task/tasks have completed. If you have specific tasks which you only want to execute after another task has completed you should take a look into Continuations, since they are extremely flexible, and you can actually create really complex flow of Tasks to better suit your needs.

Garbage collection on a .Net application always works the same, when a variable is not needed anymore (out of scope) it is collected.
Why do you think the threads are consuming the memory? It is much likely than the process inside the threads is the one consuming the memory.

What is the difference between task and thread?

In C# 4.0, we have Task in the System.Threading.Tasks namespace. What is the true difference between Thread and Task. I did some sample program(help taken from MSDN) for my own sake of learning with
Parallel.Invoke
Parallel.For
Parallel.ForEach
but have many doubts as the idea is not so clear.
I have initially searched in Stackoverflow for a similar type of question but may be with this question title I was not able to get the same. If anyone knows about the same type of question being posted here earlier, kindly give the reference of the link.

In computer science terms, a Task is a future or a promise. (Some people use those two terms synonymously, some use them differently, nobody can agree on a precise definition.) Basically, a Task<T> "promises" to return you a T, but not right now honey, I'm kinda busy, why don't you come back later?
A Thread is a way of fulfilling that promise. But not every Task needs a brand-new Thread. (In fact, creating a thread is often undesirable, because doing so is much more expensive than re-using an existing thread from the thread pool. More on that in a moment.) If the value you are waiting for comes from the filesystem or a database or the network, then there is no need for a thread to sit around and wait for the data when it can be servicing other requests. Instead, the Task might register a callback to receive the value(s) when they're ready.
In particular, the Task does not say why it is that it takes such a long time to return the value. It might be that it takes a long time to compute, or it might be that it takes a long time to fetch. Only in the former case would you use a Thread to run a Task. (In .NET, threads are freaking expensive, so you generally want to avoid them as much as possible and really only use them if you want to run multiple heavy computations on multiple CPUs. For example, in Windows, a thread weighs 12 KiByte (I think), in Linux, a thread weighs as little as 4 KiByte, in Erlang/BEAM even just 400 Byte. In .NET, it's 1 MiByte!)

A task is something you want done.
A thread is one of the many possible workers which performs that task.
In .NET 4.0 terms, a Task represents an asynchronous operation. Thread(s) are used to complete that operation by breaking the work up into chunks and assigning to separate threads.

Thread
The bare metal thing, you probably don't need to use it, you probably can use a LongRunning task and take the benefits from the TPL - Task Parallel Library, included in .NET Framework 4 (february, 2002) and above (also .NET Core).
Tasks
Abstraction above the Threads. It uses the thread pool (unless you specify the task as a LongRunning operation, if so, a new thread is created under the hood for you).
Thread Pool
As the name suggests: a pool of threads. This is the .NET framework handling a limited number of threads for you. Why? Because opening 100 threads to execute expensive CPU operations on a Processor with just 8 cores definitely is not a good idea. The framework will maintain this pool for you, reusing the threads (not creating/killing them at each operation), and executing some of them in parallel, in a way that your CPU will not burn.
OK, but when to use each one?
In resume: always use tasks.
Task is an abstraction, so it is a lot easier to use. I advise you to always try to use tasks and if you face some problem that makes you need to handle a thread by yourself (probably 1% of the time) then use threads.
BUT be aware that:
I/O Bound: For I/O bound operations (database calls, read/write files, APIs calls, etc) avoid using normal tasks, use LongRunning tasks (or threads if you need to). Because using tasks would lead you to a thread pool with a few threads busy and a lot of other tasks waiting for its turn to take the pool.
CPU Bound: For CPU bound operations just use the normal tasks (that internally will use the thread pool) and be happy.

In addition to above points, it would be good to know that:
A task is by default a background task. You cannot have a foreground task. On the other hand a thread can be background or foreground (Use IsBackground property to change the behavior).
Tasks created in thread pool recycle the threads which helps save resources. So in most cases tasks should be your default choice.
If the operations are quick, it is much better to use a task instead of thread. For long running operations, tasks do not provide much advantages over threads.

A Task can be seen as a convenient and easy way to execute something asynchronously and in parallel.
Normally a Task is all you need, I cannot remember if I have ever used a thread for anything other than experimentation.
You can accomplish the same thing, with a thread (with lots of effort) as you can with a task.
Thread
int result = 0;
Thread thread = new System.Threading.Thread(() => {
result = 1;
});
thread.Start();
thread.Join();
Console.WriteLine(result); //is 1
Task
int result = await Task.Run(() => {
return 1;
});
Console.WriteLine(result); //is 1
A task will by default use the Threadpool, which saves resources as creating threads can be expensive. You can see a Task as a higher level abstraction upon threads.
As this article points out, Task provides the following powerful features over Thread.
Tasks are tuned for leveraging multicore processors.
If the system has multiple Tasks then it makes use of the CLR thread pool
internally, and so does not have the overhead associated with creating
a dedicated thread using the Thread. Also reduces the context
switching time among multiple threads.
Task can return a result. There is no direct mechanism to return the result from thread.
Wait on a set of Tasks, without a signaling construct.
We can chain Tasks together to execute one after the other.
Establish a parent/child relationship when one task is started from
another task.
A child Task Exception can propagate to parent task.
Tasks support cancellation through the use of cancellation tokens.
Asynchronous implementation is easy in Task, using async and
await keywords.

I usually use Task to interact with Winforms and simple background worker to make it not freeze the UI. Here is an example of when I prefer using Task.
private async void buttonDownload_Click(object sender, EventArgs e)
{
buttonDownload.Enabled = false;
await Task.Run(() => {
using (var client = new WebClient())
{
client.DownloadFile("http://example.com/file.mpeg", "file.mpeg");
}
})
buttonDownload.Enabled = true;
}
VS
private void buttonDownload_Click(object sender, EventArgs e)
{
buttonDownload.Enabled = false;
Thread t = new Thread(() =>
{
using (var client = new WebClient())
{
client.DownloadFile("http://example.com/file.mpeg", "file.mpeg");
}
this.Invoke((MethodInvoker)delegate()
{
buttonDownload.Enabled = true;
});
});
t.IsBackground = true;
t.Start();
}
the difference is you don't need to use MethodInvoker and shorter code.

You can use Task to specify what you want to do then attach that Task with a Thread. so that Task would be executed in that newly made Thread rather than on the GUI thread.
Use Task with the TaskFactory.StartNew(Action action). In here you execute a delegate so if you didn't use any thread it would be executed in the same thread (GUI thread). If you mention a thread you can execute this Task in a different thread. This is an unnecessary work cause you can directly execute the delegate or attach that delegate to a thread and execute that delegate in that thread. So don't use it. it's just unnecessary. If you intend to optimize your software this is a good candidate to be removed.
**Please note that the Action is a delegate.

Task is like an operation that you want to perform. Thread helps to manage those operation through multiple process nodes. Task is a lightweight option as Threading can lead to complex code management.
I suggest you read from MSDN (best in world) always Task
Thread

C# - ThreadPool vs Tasks

As some may have seen in .NET 4.0, they've added a new namespace System.Threading.Tasks which basically is what is means, a task. I've only been using it for a few days, from using ThreadPool.
Which one is more efficient and less resource consuming? (Or just better overall?)

The objective of the Tasks namespace is to provide a pluggable architecture to make multi-tasking applications easier to write and more flexible.
The implementation uses a TaskScheduler object to control the handling of tasks. This has virtual methods that you can override to create your own task handling. Methods include for instance
protected virtual void QueueTask(Task task)
public virtual int MaximumConcurrencyLevel
There will be a tiny overhead to using the default implementation as there's a wrapper around the .NET threads implementation, but I'd not expect it to be huge.
There is a (draft) implementation of a custom TaskScheduler that implements multiple tasks on a single thread here.

which one is more efficient and less
resource consuming?
Irrelevant, there will be very little difference.
(Or just better overall)
The Task class will be the easier-to-use as it offers a very clean interface for starting and joining threads, and transfers exceptions. It also supports a (limited) form of load balancing.

"Starting with the .NET Framework 4, the TPL is the preferred way to write multithreaded and parallel code."
http://msdn.microsoft.com/en-us/library/dd460717.aspx

Thread
The bare metal thing, you probably don't need to use it, you probably can use a LongRunning Task and benefit from its facilities.
Tasks
Abstraction above the Threads. It uses the thread pool (unless you specify the task as a LongRunning operation, if so, a new thread is created under the hood for you).
Thread Pool
As the name suggests: a pool of threads. Is the .NET framework handling a limited number of threads for you. Why? Because opening 100 threads to execute expensive CPU operations on a CPU with just 8 cores definitely is not a good idea. The framework will maintain this pool for you, reusing the threads (not creating/killing them at each operation), and executing some of they in parallel in a way that your CPU will not burn.
OK, but when to use each one?
In resume: always use tasks.
Task is an abstratcion, so it is a lot easier to use. I advise you to always try to use Tasks and if you face some problem that makes you need to handle a thread by yourself (probably 1% of the time) then use threads.
BUT be aware that:
I/O Bound: For I/O bound operations (database calls, read/write files, APIs calls, etc) never use normal tasks, use LongRunning tasks or threads if you need to, but not normal tasks. Because it would lead you to a thread pool with a few threads busy and a lot of another tasks waiting for its turn to take the pool.
CPU Bound: For CPU bound operations just use the normal tasks and be happy.

Scheduling is an important aspect of parallel tasks.
Unlike threads, new tasks don't necessarily begin executing immediately. Instead, they are placed in a work queue. Tasks run when their associated task scheduler removes them from the queue, usually as cores become available. The task scheduler attempts to optimize overall throughput by controlling the system's degree of concurrency. As long as there are enough tasks and the tasks are sufficiently free of serializing dependencies, the program's performance scales with the number of available cores. In this way, tasks embody the concept of potential parallelism
As I saw on msdn http://msdn.microsoft.com/en-us/library/ff963549.aspx

ThreadPool and Task difference is very simple.
To understand task you should know about the threadpool.
ThreadPool is basically help to manage and reuse the free threads. In
other words a threadpool is the collection of background thread.
Simple definition of task can be:
Task work asynchronously manages the the unit of work. In easy words
Task doesn’t create new threads. Instead it efficiently manages the
threads of a threadpool.Tasks are executed by TaskScheduler, which queues tasks onto threads.

Another good point to consider about task is, when you use ThreadPool, you don't have any way to abort or wait on the running threads (unless you do it manually in the method of thread), but using task it is possible. Please correct me if I'm wrong