In my application I have a number of classes that can compute their tasks using Parallel.For statement. Each Parallel.For uses its own ParallelOption that sets a maximum degree of parallelism.
The application flow is dependent on a user setup so there are situations like: class A (which computes in parallel) calls class B (that also computes in parallel) or it calls class C (that is not using any parallel computing).
Another pattern is recursive calls where on each level of recursion a parallel computation is done.
My questions:
Is it possible to setup a global thread pool that is used by all classes to do its calculatins, and the numer of threads is controlled globally for application?
Is it possible to combine thread pool with Parallel.For so most of the existing code stays unchanged?
How much overhead puts a thread pool compared to independend parallel.for calls (I know that it all depends, ad by the end I need to do some testing, but maybe you have some good and bad expirence and good advices)
Is it possible to setup a global thread pool that is used by all
classes to do its calculatins, and the numer of threads is controlled
globally for application?
This is already the case. Currently, the application thread pool is managed by the .NET runtime. So you can be pretty sure, that all your classes (in a running application) use the same thread pool. You can access the thread pool either by using Tasks, the static ThreadPool class or the static Parallel class.
Is it possible to combine thread pool with Parallel.For so most of the
existing code stays unchanged?
Parallel.For internally uses Tasks and the TaskScheduler. So, the same thread pool will be used.
Parallel.For will use all the thread pool threads by default, and the number of thread pool threads is optimized for your cpu(s). MaxDegreeOfParallelism is only useful if you want to not use all of the thread pool threads for a given loop.
Just use Parallel.For and don't overthink it.
Later on, if your app really requires it, you can still create your own TaskScheduler with your own thread pool... but it's very improbable that you actually need to do that.
It appears that the Task class provides us the ability to use multiple processors of the system. Does the Thread class work on multiple processors as well or does it use time slicing only on a single processor? (Assuming a system with multiple cores).
My question is if threads will/could be executed on multiple cores then what is so special about Task and Parallelism ?
When you create a thread it forms kind of a logical group of work. The .NET Framework will aquire CPU-Time from the system. Most likely multiple threads will run on different cores (This is something the system handeles - not even .NET has any influence on this)
But it might be possible that the system will execute all your Threads on the same core or even moves the execution between several cores during the execution. Keep in Mind, that you are crating managed Threads, and not real System-Threads.
(Correctly spoken I should say: The System could execute your managed Threads within the same System-Thread or use multiple System-Threads for multiple managed threads.)
Maybe you want to have a look at this Blog-Post: http://www.drdobbs.com/parallel/managed-threads-are-different-from-windo/228800359 The explanation there is pretty good in terms of details.
Not a bad first question. +1
I would suggest you to read Threading in C# by Joseph Albahari. If you read through the post you will find:
How Threading Works
Multithreading is managed internally by a thread scheduler, a
function the CLR typically delegates to the operating system. A
thread scheduler ensures all active threads are allocated appropriate
execution time, and that threads that are waiting or blocked (for
instance, on an exclusive lock or on user input) do not consume CPU
time.
So multi threading is handled by operating system through a thread scheduler.
Further the post has:
On a multi-processor computer, multithreading is implemented with
a mixture of time-slicing and genuine concurrency, where different
threads run code simultaneously on different CPUs. It’s almost certain
there will still be some time-slicing, because of the operating
system’s need to service its own threads — as well as those of other
applications.
-It appears that Task class provide us the ability to use on multiple processors in the system.
-if threads will/could be executed on multiple cores then what is so special about Task Parallelism ?
The Task class is just a small, but important, part of TPL (Task Parallel Library). TPL is a high level abstraction, so you don't have to work with threads directly. It encapsulates and hides most of the churn you'd have to implement for any decent multi-threaded application.
Tasks don't introduce any new functionality that you couldn't implement on your own, per se (which is the core of your questions, I believe). They can be synchronous or asynchronous - when they are async, they're either using Thread class internally or IOCP ports.
Some of the points addressed by TPL are:
Rethrow exceptions from a child thread on the calling thread.
Asynchronous code (launch thread -> run arbitrary code while waiting for child thread -> resume when child thread is over) looks as if it were synchronous, greatly improving readability and maintainability
Simpler thread cancelation (using CancellationTokenSource)
Parallel queries/data manipulation using PLINQ or the Parallel class
Asynchronous workflows using TPL Dataflow
I have done a bunch of reading on the dynamics and implications of multi-threading in a server app (starving the clr thread pool, etc.) but let's say for sake of argument I have EXACTLY 4 async processes I need to accomplish per request of my (asp.net) page... Now let's say time is the more critical element and my site should not experience heavy traffic. In this scenario, is it preferable to spawn 4 threads using the new Thread() approach or the ThreadPool.QueueUserWorkItem method?
My concern (and my point) here is that using the ThreadPool method, it may create a thread pool that is too large than what I really want? When I only need 4 threads, can't I just spawn them myself to keep the number of allocated app domain, clr threads minimal?
Spawning a thread is a very costly and therefore high-latency operation. If you want to manage threads yourself, which would be reasonable but not required, you have to build a custom pool.
Using thread pool work items is not without danger because it does not guarantee you a concurrency level of 4. If you happen to get 2 or 3 you will have much more latency in your HTTP request.
I'd use the thread-pool and use SetMinThreads to ensure that threads are started without delay and that there are always enough.
I would definitely go for the ThreadPool approach. It's designed for exactly this kind of scenario. The thread pool will internally manage the number of threads required, making sure not to overburden the system. Quoting from MSDN:
The thread pool provides new worker threads or I/O completion threads
on demand until it reaches the minimum for each category. When a
minimum is reached, the thread pool can create additional threads in
that category or wait until some tasks complete. Beginning with the
.NET Framework 4, the thread pool creates and destroys worker threads
in order to optimize throughput, which is defined as the number of
tasks that complete per unit of time. Too few threads might not make
optimal use of available resources, whereas too many threads could
increase resource contention.
If you're really paranoid you can limit it manually with SetMaxThreads. Going for the manual threading management will only introduce potential bugs.
If you have access to .net 4.0 you can use the TPL Task class (it also uses the ThreadPool under the hood), as it has even more appealing features.
I have a need to create multiple processing threads in a new application. Each thread has the possibility of being "long running". Can someone comment on the viability of the built in .net threadpool or some existing custom threadpool for use in my application?
Requirements :
Works well within a windows service. (queued work can be removed from the queue, currently running threads can be told to halt)
Ability to spin up multiple threads.
Work needs to be started in sequential order, but multiple threads can be processing in parallel.
Hung threads can be detected and killed.
EDIT:
Comments seem to be leading towards manual threading. Unfortunately I am held to 3.5 version of the framework. Threadpool was appealing because it would allow me to queue work up and threads created for me when resources were available. Is there a good 3.5 compatable pattern (producer/consumer perhaps) that would give me this aspect of threadpool without actually using the threadpool?
Your requirements essentially rule out the use of the .NET ThreadPool;
It generally should not be used for long-running threads, due to the danger of exhausting the pool.
It does work well in Windows services, though, and you can spin up multiple threads - limited automatically by the pool's limits.
You can not guarantee thread starting times with the thread pool; it may queue threads for execution when it has enough free ones, and it does not even guarantee they will be started in the sequence you submit them.
There are no easy ways to detect and kill running threads in the ThreadPool
So essentially, you will want to look outside the ThreadPool; I might recommend that perhaps you might need 'full' System.Threading.Thread instances just due to all of your requirements. As long as you handle concurrency issues (as you must with any threading mechanism), I don't find the Thread class to be all that difficult to manage myself, really.
Simple answer, but the Task class (Fx4) meets most of your requirements.
Cancellation is cooperative, ie your Task code has to check for it.
But detecting hung threads is difficult, that is a very high requirement anyway.
But I can also read your requirements as for a JobQueue, where the 'work' consists of mostly similar jobs. You could roll your own system that Consumes that queue and monitors execution on a few Threads.
I've done essentially the same thing with .Net 3.5 by creating my own thread manager:
Instantiate worker classes that know how long they've been running.
Create threads that run a worker method and add them to a Queue<Thread>.
A supervisor thread reads threads from the Queue and adds them to a Dictionary<int, Worker> as it launches them until it hits its maximum running threads. Add the thread as a property of the Worker instance.
As each worker finishes it invokes a callback method from the supervisor that passes back its ManagedThreadId.
The supervisor removes the thread from the Dictionary and launches another waiting thread.
Poll the Dictionary of running workers to see if any have timed out, or put timers in the workers that invoke a callback if they take too long.
Signal a long-running worker to quit, or abort its thread.
The supervisor invokes callbacks to your main thread to inform of progress, etc.
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I have been trying to learn multi-threaded programming in C# and I am confused about when it is best to use a thread pool vs. create my own threads. One book recommends using a thread pool for small tasks only (whatever that means), but I can't seem to find any real guidelines.
What are some pros and cons of thread pools vs creating my own threads? And what are some example use cases for each?
I would suggest you use a thread pool in C# for the same reasons as any other language.
When you want to limit the number of threads running or don't want the overhead of creating and destroying them, use a thread pool.
By small tasks, the book you read means tasks with a short lifetime. If it takes ten seconds to create a thread which only runs for one second, that's one place where you should be using pools (ignore my actual figures, it's the ratio that counts).
Otherwise you spend the bulk of your time creating and destroying threads rather than simply doing the work they're intended to do.
If you have lots of logical tasks that require constant processing and you want that to be done in parallel use the pool+scheduler.
If you need to make your IO related tasks concurrently such as downloading stuff from remote servers or disk access, but need to do this say once every few minutes, then make your own threads and kill them once you're finished.
Edit: About some considerations, I use thread pools for database access, physics/simulation, AI(games), and for scripted tasks ran on virtual machines that process lots of user defined tasks.
Normally a pool consists of 2 threads per processor (so likely 4 nowadays), however you can set up the amount of threads you want, if you know how many you need.
Edit: The reason to make your own threads is because of context changes, (thats when threads need to swap in and out of the process, along with their memory). Having useless context changes, say when you aren't using your threads, just leaving them sit around as one might say, can easily half the performance of your program (say you have 3 sleeping threads and 2 active threads). Thus if those downloading threads are just waiting they're eating up tons of CPU and cooling down the cache for your real application
Here's a nice summary of the thread pool in .Net: http://blogs.msdn.com/pedram/archive/2007/08/05/dedicated-thread-or-a-threadpool-thread.aspx
The post also has some points on when you should not use the thread pool and start your own thread instead.
I highly recommend reading the this free e-book:
Threading in C# by Joseph Albahari
At least read the "Getting Started" section. The e-book provides a great introduction and includes a wealth of advanced threading information as well.
Knowing whether or not to use the thread pool is just the beginning. Next you will need to determine which method of entering the thread pool best suits your needs:
Task Parallel Library (.NET Framework
4.0)
ThreadPool.QueueUserWorkItem
Asynchronous Delegates
BackgroundWorker
This e-book explains these all and advises when to use them vs. create your own thread.
The thread pool is designed to reduce context switching among your threads. Consider a process that has several components running. Each of those components could be creating worker threads. The more threads in your process, the more time is wasted on context switching.
Now, if each of those components were queuing items to the thread pool, you would have a lot less context switching overhead.
The thread pool is designed to maximize the work being done across your CPUs (or CPU cores). That is why, by default, the thread pool spins up multiple threads per processor.
There are some situations where you would not want to use the thread pool. If you are waiting on I/O, or waiting on an event, etc then you tie up that thread pool thread and it can't be used by anyone else. Same idea applies to long running tasks, though what constitutes a long running task is subjective.
Pax Diablo makes a good point as well. Spinning up threads is not free. It takes time and they consume additional memory for their stack space. The thread pool will re-use threads to amortize this cost.
Note: you asked about using a thread pool thread to download data or perform disk I/O. You should not use a thread pool thread for this (for the reasons I outlined above). Instead use asynchronous I/O (aka the BeginXX and EndXX methods). For a FileStream that would be BeginRead and EndRead. For an HttpWebRequest that would be BeginGetResponse and EndGetResponse. They are more complicated to use, but they are the proper way to perform multi-threaded I/O.
Beware of the .NET thread pool for operations that may block for any significant, variable or unknown part of their processing, as it is prone to thread starvation. Consider using the .NET parallel extensions, which provide a good number of logical abstractions over threaded operations. They also include a new scheduler, which should be an improvement on ThreadPool. See here
One reason to use the thread pool for small tasks only is that there are a limited number of thread pool threads. If one is used for a long time then it stops that thread from being used by other code. If this happens many times then the thread pool can become used up.
Using up the thread pool can have subtle effects - some .NET timers use thread pool threads and will not fire, for example.
If you have a background task that will live for a long time, like for the entire lifetime of your application, then creating your own thread is a reasonable thing. If you have short jobs that need to be done in a thread, then use thread pooling.
In an application where you are creating many threads, the overhead of creating the threads becomes substantial. Using the thread pool creates the threads once and reuses them, thus avoiding the thread creation overhead.
In an application that I worked on, changing from creating threads to using the thread pool for the short lived threads really helpped the through put of the application.
For the highest performance with concurrently executing units, write your own thread pool, where a pool of Thread objects are created at start up and go to blocking (formerly suspended), waiting on a context to run (an object with a standard interface implemented by your code).
So many articles about Tasks vs. Threads vs. the .NET ThreadPool fail to really give you what you need to make a decision for performance. But when you compare them, Threads win out and especially a pool of Threads. They are distributed the best across CPUs and they start up faster.
What should be discussed is the fact that the main execution unit of Windows (including Windows 10) is a thread, and OS context switching overhead is usually negligible. Simply put, I have not been able to find convincing evidence of many of these articles, whether the article claims higher performance by saving context switching or better CPU usage.
Now for a bit of realism:
Most of us won’t need our application to be deterministic, and most of us do not have a hard-knocks background with threads, which for instance often comes with developing an operating system. What I wrote above is not for a beginner.
So what may be most important is to discuss is what is easy to program.
If you create your own thread pool, you’ll have a bit of writing to do as you’ll need to be concerned with tracking execution status, how to simulate suspend and resume, and how to cancel execution – including in an application-wide shut down. You might also have to be concerned with whether you want to dynamically grow your pool and also what capacity limitation your pool will have. I can write such a framework in an hour but that is because I’ve done it so many times.
Perhaps the easiest way to write an execution unit is to use a Task. The beauty of a Task is that you can create one and kick it off in-line in your code (though caution may be warranted). You can pass a cancellation token to handle when you want to cancel the Task. Also, it uses the promise approach to chaining events, and you can have it return a specific type of value. Moreover, with async and await, more options exist and your code will be more portable.
In essence, it is important to understand the pros and cons with Tasks vs. Threads vs. the .NET ThreadPool. If I need high performance, I am going to use threads, and I prefer using my own pool.
An easy way to compare is start up 512 Threads, 512 Tasks, and 512 ThreadPool threads. You’ll find a delay in the beginning with Threads (hence, why write a thread pool), but all 512 Threads will be running in a few seconds while Tasks and .NET ThreadPool threads take up to a few minutes to all start.
Below are the results of such a test (i5 quad core with 16 GB of RAM), giving each 30 seconds to run. The code executed performs simple file I/O on an SSD drive.
Test Results
Thread pools are great when you have more tasks to process than available threads.
You can add all the tasks to a thread pool and specify the maximum number of threads that can run at a certain time.
Check out this page on MSDN:
http://msdn.microsoft.com/en-us/library/3dasc8as(VS.80).aspx
Always use a thread pool if you can, work at the highest level of abstraction possible. Thread pools hide creating and destroying threads for you, this is usually a good thing!
Most of the time you can use the pool as you avoid the expensive process of creating the thread.
However in some scenarios you may want to create a thread. For example if you are not the only one using the thread pool and the thread you create is long-lived (to avoid consuming shared resources) or for example if you want to control the stacksize of the thread.
Don't forget to investigate the Background worker.
I find for a lot of situations, it gives me just what i want without the heavy lifting.
Cheers.
I usually use the Threadpool whenever I need to just do something on another thread and don't really care when it runs or ends. Something like logging or maybe even background downloading a file (though there are better ways to do that async-style). I use my own thread when I need more control. Also what I've found is using a Threadsafe queue (hack your own) to store "command objects" is nice when I have multiple commands that I need to work on in >1 thread. So you'd may split up an Xml file and put each element in a queue and then have multiple threads working on doing some processing on these elements. I wrote such a queue way back in uni (VB.net!) that I've converted to C#. I've included it below for no particular reason (this code might contain some errors).
using System.Collections.Generic;
using System.Threading;
namespace ThreadSafeQueue {
public class ThreadSafeQueue<T> {
private Queue<T> _queue;
public ThreadSafeQueue() {
_queue = new Queue<T>();
}
public void EnqueueSafe(T item) {
lock ( this ) {
_queue.Enqueue(item);
if ( _queue.Count >= 1 )
Monitor.Pulse(this);
}
}
public T DequeueSafe() {
lock ( this ) {
while ( _queue.Count <= 0 )
Monitor.Wait(this);
return this.DeEnqueueUnblock();
}
}
private T DeEnqueueUnblock() {
return _queue.Dequeue();
}
}
}
I wanted a thread pool to distribute work across cores with as little latency as possible, and that didn't have to play well with other applications. I found that the .NET thread pool performance wasn't as good as it could be. I knew I wanted one thread per core, so I wrote my own thread pool substitute class. The code is provided as an answer to another StackOverflow question over here.
As to the original question, the thread pool is useful for breaking repetitive computations up into parts that can be executed in parallel (assuming they can be executed in parallel without changing the outcome). Manual thread management is useful for tasks like UI and IO.