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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.
Related
Okay, So I wanted to know what happens when I use TaskCreationOptions.LongRunning. By this answer, I came to know that for long running tasks, I should use this options because it creates a thread outside of threadpool.
Cool. But what advantage would I get when I create a thread outside threadpool? And when to do it and avoid it?
what advantage would I get when I create a thread outside threadpool?
The threadpool, as it name states, is a pool of threads which are allocated once and re-used throughout, in order to save the time and resources necessary to allocate a thread. The pool itself re-sizes on demand. If you queue more work than actual workers exist in the pool, it will allocate more threads in 500ms intervals, one at a time (this exists to avoid allocation of multiple threads simultaneously where existing threads may already finish executing and can serve requests). If many long running operations are performed on the thread-pool, it causes "thread starvation", meaning delegates will start getting queued and ran only once a thread frees up. That's why you'd want to avoid a large amount of threads doing lengthy work with thread-pool threads.
The Managed Thread-Pool docs also have a section on this question:
There are several scenarios in which it is appropriate to create and
manage your own threads instead of using thread pool threads:
You require a foreground thread.
You require a thread to have a particular priority.
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.
You need to place threads into a single-threaded apartment. All ThreadPool threads are in the multithreaded apartment.
You need to have a stable identity associated with the thread, or to dedicate a thread to a task.
For more, see:
Thread vs ThreadPool
When should I not use the ThreadPool in .Net?
Dedicated thread or thread-pool thread?
"Long running" can be quantified pretty well, a thread that takes more than half a second is running long. That's a mountain of processor instructions on a modern machine, you'd have to burn a fat five billion of them per second. Pretty hard to do in a constructive way unless you are calculating the value of Pi to thousands of decimals in the fraction.
Practical threads can only take that long when they are not burning core but are waiting a lot. Invariably on an I/O completion, like reading data from a disk, a network, a dbase server. And often the reason you'd start considering using a thread in the first place.
The threadpool has a "manager". It determines when a threadpool thread is allowed to start. It doesn't happen immediately when you start it in your program. The manager tries to limit the number of running threads to the number of CPU cores you have. It is much more efficient that way, context switching between too many active threads is expensive. And a good throttle, preventing your program from consuming too many resources in a burst.
But the threadpool manager has the very common problem with managers, it doesn't know enough about what is going on. Just like my manager doesn't know that I'm goofing off at Stackoverflow.com, the tp manager doesn't know that a thread is waiting for something and not actually performing useful work. Without that knowledge it cannot make good decisions. A thread that does a lot of waiting should be ignored and another one should be allowed to run in its place. Actually doing real work.
Just like you tell your manager that you go on vacation, so he can expect no work to get done, you tell the threadpool manager the same thing with LongRunning.
Do note that it isn't quite a bad as it, perhaps, sounds in this answer. Particularly .NET 4.0 hired a new manager that's a lot smarter at figuring out the optimum number of running threads. It does so with a feedback loop, collecting data to discover if active threads actually get work done. And adjusts the optimum accordingly. Only problem with this approach is the common one when you close a feedback loop, you have to make it slow so the loop cannot become unstable. In other words, it isn't particularly quick at driving up the number of active threads.
If you know ahead of time that the thread is pretty abysmal, running for many seconds with no real cpu load then always pick LongRunning. Otherwise it is a tuning job, observing the program when it is done and tinkering with it to make it more optimal.
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.
We have a situation where our application needs to process a series of files and rather than perform this function synchronously, we would like to employ multi-threading to have the workload split amongst different threads.
Each item of work is:
1. Open a file for read only
2. Process the data in the file
3. Write the processed data to a Dictionary
We would like to perform each file's work on a new thread?
Is this possible and should be we better to use the ThreadPool or spawn new threads keeping in mind that each item of "work" only takes 30ms however its possible that hundreds of files will need to be processed.
Any ideas to make this more efficient is appreciated.
EDIT: At the moment we are making use of the ThreadPool to handle this. If we have 500 files to process we cycle through the files and allocate each "unit of processing work" to the threadpool using QueueUserWorkItem.
Is it suitable to make use of the threadpool for this?
I would suggest you to use ThreadPool.QueueUserWorkItem(...), in this, threads are managed by the system and the .net framework. The chances of you meshing up with your own threadpool is much higher. So I would recommend you to use Threadpool provided by .net .
It's very easy to use,
ThreadPool.QueueUserWorkItem(new WaitCallback(YourMethod), ParameterToBeUsedByMethod);
YourMethod(object o){
Your Code here...
}
For more reading please follow the link http://msdn.microsoft.com/en-us/library/3dasc8as%28VS.80%29.aspx
Hope, this helps
I suggest you have a finite number of threads (say 4) and then have 4 pools of work. I.e. If you have 400 files to process have 100 files per thread split evenly. You then spawn the threads, and pass to each their work and let them run until they have finished their specific work.
You only have a certain amount of I/O bandwidth so having too many threads will not provide any benefits, also remember that creating a thread also takes a small amount of time.
Instead of having to deal with threads or manage thread pools directly I would suggest using a higher-level library like Parallel Extensions (PEX):
var filesContent = from file in enumerableOfFilesToProcess
select new
{
File=file,
Content=File.ReadAllText(file)
};
var processedContent = from content in filesContent
select new
{
content.File,
ProcessedContent = ProcessContent(content.Content)
};
var dictionary = processedContent
.AsParallel()
.ToDictionary(c => c.File);
PEX will handle thread management according to available cores and load while you get to concentrate about the business logic at hand (wow, that sounded like a commercial!)
PEX is part of the .Net Framework 4.0 but a back-port to 3.5 is also available as part of the Reactive Framework.
I suggest using the CCR (Concurrency and Coordination Runtime) it will handle the low-level threading details for you. As for your strategy, one thread per work item may not be the best approach depending on how you attempt to write to the dictionary, because you may create heavy contention since dictionaries aren't thread safe.
Here's some sample code using the CCR, an Interleave would work nicely here:
Arbiter.Activate(dispatcherQueue, Arbiter.Interleave(
new TeardownReceiverGroup(Arbiter.Receive<bool>(
false, mainPort, new Handler<bool>(Teardown))),
new ExclusiveReceiverGroup(Arbiter.Receive<object>(
true, mainPort, new Handler<object>(WriteData))),
new ConcurrentReceiverGroup(Arbiter.Receive<string>(
true, mainPort, new Handler<string>(ReadAndProcessData)))));
public void WriteData(object data)
{
// write data to the dictionary
// this code is never executed in parallel so no synchronization code needed
}
public void ReadAndProcessData(string s)
{
// this code gets scheduled to be executed in parallel
// CCR take care of the task scheduling for you
}
public void Teardown(bool b)
{
// clean up when all tasks are done
}
In the long run, I think you'll be happier if you manage your own threads. This will let you control how many are running and make it easy to report status.
Build a worker class that does the processing and give it a callback routine to return results and status.
For each file, create a worker instance and a thread to run it. Put the thread in a Queue.
Peel threads off of the queue up to the maximum you want to run simultaneously. As each thread completes go get another one. Adjust the maximum and measure throughput. I prefer to use a Dictionary to hold running threads, keyed by their ManagedThreadId.
To stop early, just clear the queue.
Use locking around your thread collections to preserve your sanity.
Use ThreadPool.QueueUserWorkItem to execute each independent task. Definitely don't create hundreds of threads. That is likely to cause major headaches.
The general rule for using the ThreadPool is if you don't want to worry about when the threads finish (or use Mutexes to track them), or worry about stopping the threads.
So do you need to worry about when the work is done? If not, the ThreadPool is the best option. If you want to track the overall progress, stop threads then your own collection of threads is best.
ThreadPool is generally more efficient if you are re-using threads. This question will give you a more detailed discussion.
Hth
Using the ThreadPool for each individual task is definitely a bad idea. From my experience this tends to hurt performance more than helping it. The first reason is that a considerable amount of overhead is required just to allocate a task for the ThreadPool to execute. By default, each application is assigned it's own ThreadPool that is initialized with ~100 thread capacity. When you are executing 400 operations in a parallel, it does not take long to fill the queue with requests and now you have ~100 threads all competing for CPU cycles. Yes the .NET framework does a great job with throttling and prioritizing the queue, however, I have found that the ThreadPool is best left for long-running operations that probably won't occur very often (loading a configuration file, or random web requests). Using the ThreadPool to fire off a few operations at random is much more efficient than using it to execute hundreds of requests at once. Given the current information, the best course of action would be something similar to this:
Create a System.Threading.Thread (or use a SINGLE ThreadPool thread) with a queue that the application can post requests to
Use the FileStream's BeginRead and BeginWrite methods to perform the IO operations. This will cause the .NET framework to use native API's to thread and execute the IO (IOCP).
This will give you 2 leverages, one is that your requests will still get processed in parallel while allowing the operating system to manage file system access and threading. The second is that because the bottleneck of the vast majority of systems will be the HDD, you can implement a custom priority sort and throttling to your request thread to give greater control over resource usage.
Currently I have been writing a similar application and using this method is both efficient and fast... Without any threading or throttling my application was only using 10-15% CPU, which can be acceptable for some operations depending on the processing involved, however, it made my PC as slow as if an application was using 80%+ of the CPU. This was the file system access. The ThreadPool and IOCP functions do not care if they are bogging the PC down, so don't get confused, they are optimized for performance, even if that performance means your HDD is squeeling like a pig.
The only problem I have had is memory usage ran a little high (50+ mb) during the testing phaze with approximately 35 streams open at once. I am currently working on a solution similar to the MSDN recommendation for SocketAsyncEventArgs, using a pool to allow x number of requests to be operating simultaneously, which ultimately led me to this forum post.
Hope this helps somebody with their decision making in the future :)
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When should I not use the ThreadPool in .Net?
It looks like the best option is to use a ThreadPool, in which case, why is it not the only option?
What are your experiences around this?
#Eric, I'm going to have to agree with Dean. Threads are expensive. You can't assume that your program is the only one running. When everyone is greedy with resources, the problem multiplies.
I prefer to create my threads manually and control them myself. It keeps the code very easy to understand.
That's fine when it's appropriate. If you need a bunch of worker threads, though, all you've done is make your code more complicated. Now you have to write code to manage them. If you just used a thread pool, you'd get all the thread management for free. And the thread pool provided by the language is very likely to be more robust, more efficient, and less buggy than whatever you roll for yourself.
Thread t = new Thread(new ThreadStart(DoSomething));
t.Start();
t.Join();
I hope that you would normally have some additional code in between Start() and Join(). Otherwise, the extra thread is useless, and you're wasting resources for no reason.
People are way too afraid of the resources used by threads. I've never seen creating and starting a thread to take more than a millisecond. There is no hard limit on the number of threads you can create. RAM usage is minimal. Once you have a few hundred threads, CPU becomes an issue because of context switches, so at that point you might want to get fancy with your design.
A millisecond is a long time on modern hardware. That's 3 million cycles on a 3GHz machine. And again, you aren't the only one creating threads. Your threads compete for the CPU along with every other program's threads. If you use not-quite-too-many threads, and so does another program, then together you've used too many threads.
Seriously, don't make life more complex than it needs to be. Don't use the thread pool unless you need something very specific that it offers.
Indeed. Don't make life more complex. If your program needs multiple worker threads, don't reinvent the wheel. Use the thread pool. That's why it's there. Would you roll your own string class?
The only reason why I wouldn't use the ThreadPool for cheap multithreading is if I need to…
interract with the method running (e.g., to kill it)
run code on a STA thread (this happened to me)
keep the thread alive after my application has died (ThreadPool threads are background threads)
in case I need to change the priority of the Thread. We can not change priority of threads in ThreadPool which is by default Normal.
P.S.: The MSDN article "The Managed Thread Pool" contains a section titled, "When Not to Use Thread Pool Threads", with a very similar but slightly more complete list of possible reasons for not using the thread pool.
There are lots of reasons why you would need to skip the ThreadPool, but if you don't know them then the ThreadPool should be good enough for you.
Alternatively, look at the new Parallel Extensions Framework, which has some neat stuff in there that may suit your needs without having to use the ThreadPool.
To quarrelsome's answer, I would add that it's best not to use a ThreadPool thread if you need to guarantee that your thread will begin work immediately. The maximum number of running thread-pooled threads is limited per appdomain, so your piece of work may have to wait if they're all busy. It's called "queue user work item", after all.
Two caveats, of course:
You can change the maximum number of thread-pooled threads in code, at runtime, so there's nothing to stop you checking the current vs maximum number and upping the maximum if required.
Spinning up a new thread comes with its own time penalty - whether it's worthwhile for you to take the hit depends on your circumstances.
Thread pools make sense whenever you have the concept of worker threads. Any time you can easily partition processing into smaller jobs, each of which can be processed independently, worker threads (and therefore a thread pool) make sense.
Thread pools do not make sense when you need thread which perform entirely dissimilar and unrelated actions, which cannot be considered "jobs"; e.g., One thread for GUI event handling, another for backend processing. Thread pools also don't make sense when processing forms a pipeline.
Basically, if you have threads which start, process a job, and quit, a thread pool is probably the way to go. Otherwise, the thread pool isn't really going to help.
I'm not speaking as someone with only
theoretical knowledge here. I write
and maintain high volume applications
that make heavy use of multithreading,
and I generally don't find the thread
pool to be the correct answer.
Ah, argument from authority - but always be on the look out for people who might be on the Windows kernel team.
Neither of us were arguing with the fact that if you have some specific requirements then the .NET ThreadPool might not be the right thing. What we're objecting to is the trivialisation of the costs to the machine of creating a thread.
The significant expense of creating a thread at the raison d'etre for the ThreadPool in the first place. I don't want my machines to be filled with code written by people who have been misinformed about the expense of creating a thread, and don't, for example, know that it causes a method to be called in every single DLL which is attached to the process (some of which will be created by 3rd parties), and which may well hot-up a load of code which need not be in RAM at all and almost certainly didn't need to be in L1.
The shape of the memory hierarchy in a modern machine means that 'distracting' a CPU is about the worst thing you can possibly do, and everybody who cares about their craft should work hard to avoid it.
When you're going to perform an operation that is going to take a long time, or perhaps a continuous background thread.
I guess you could always push the amount of threads available in the pool up but there would be little point in incurring the management costs of a thread that is never going to be given back to the pool.
Threadpool threads are appropriate for tasks that meet both of the following criteria:
The task will not have to spend any significant time waiting for something to happen
Anything that's waiting for the task to finish will likely be waiting for many tasks to finish, so its scheduling priority isn't apt to affect things much.
Using a threadpool thread instead of creating a new one will save a significant but bounded amount of time. If that time is significant compared with the time it will take to perform a task, a threadpool task is likely appropriate. The longer the time required to perform a task, however, the smaller the benefit of using the threadpool and the greater the likelihood of the task impeding threadpool efficiency.
MSDN has a list some reasons here:
http://msdn.microsoft.com/en-us/library/0ka9477y.aspx
There are several scenarios in which it is appropriate to create and
manage your own threads instead of using thread pool threads:
You require a foreground thread.
You require a thread to have a particular priority.
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.
You need to place threads into a single-threaded apartment. All ThreadPool threads are in the multithreaded apartment.
You need to have a stable identity associated with the thread, or to dedicate a thread to a task.
#Eric
#Derek, I don't exactly agree with the scenario you use as an example. If you don't know exactly what's running on your machine and exactly how many total threads, handles, CPU time, RAM, etc, that your app will use under a certain amount of load, you are in trouble.
Are you the only target customer for the programs you write? If not, you can't be certain about most of that. You generally have no idea when you write a program whether it will execute effectively solo, or if it will run on a webserver being hammered by a DDOS attack. You can't know how much CPU time you are going to have.
Assuming your program's behavior changes based on input, it's rare to even know exactly how much memory or CPU time your program will consume. Sure, you should have a pretty good idea about how your program is going to behave, but most programs are never analyzed to determine exactly how much memory, how many handles, etc. will be used, because a full analysis is expensive. If you aren't writing real-time software, the payoff isn't worth the effort.
In general, claiming to know exactly how your program will behave is far-fetched, and claiming to know everything about the machine approaches ludicrous.
And to be honest, if you don't know exactly what method you should use: manual threads, thread pool, delegates, and how to implement it to do just what your application needs, you are in trouble.
I don't fully disagree, but I don't really see how that's relevant. This site is here specifically because programmers don't always have all the answers.
If your application is complex enough to require throttling the number of threads that you use, aren't you almost always going to want more control than what the framework gives you?
No. If I need a thread pool, I will use the one that's provided, unless and until I find that it is not sufficient. I will not simply assume that the provided thread pool is insufficient for my needs without confirming that to be the case.
I'm not speaking as someone with only theoretical knowledge here. I write and maintain high volume applications that make heavy use of multithreading, and I generally don't find the thread pool to be the correct answer.
Most of my professional experience has been with multithreading and multiprocessing programs. I have often needed to roll my own solution as well. That doesn't mean that the thread pool isn't useful, or appropriate in many cases. The thread pool is built to handle worker threads. In cases where multiple worker threads are appropriate, the provided thread pool should should generally be the first approach.