I've been reading this article on MSDN about C# Concurrent Collections. It talks about the optimum threading to use for particular scenarios to get the most benefit out of the collections e.g:
ConcurrentQueue performs best when one dedicated thread is queuing and one dedicated thread is de-queuing. If you do not enforce this rule, then Queue might even perform slightly faster than ConcurrentQueue on computers that have multiple cores.
Is this advice still valid when one is using Tasks instead of raw Threads? From my (limited) understanding of C# Tasks, there is no guarantee that a particular Task will always run on the same thread between context switches, or does maintaining the stack frame mean that the same rules apply in terms of best usage?
Thanks.
One task always runs on the same thread. TPL is a user-mode library. User mode has no (practical) way of migrating executing code from thread to thread. Also there would be no point to doing that.
This advice applies exactly to tasks as it does to threads.
What that piece of advice means to say is that at the same time there should be one producer and one consumer only. You can have 100 threads enqueuing from time to time as long as they do not contend.
(I'm not questioning that advice here since that is out of scope for this question. But that is what's meant here.)
In C#,I want to do multi threading say I have Thread T1 and Thread T2. I do the below:
Thread T1 = new Thead(some_func).Start()
Thread T2 = new Thread(some_other_func).Start().
By doing this am I spawning threads across the cores or in the single core. If it is in single core, how can I do it across cores. Is there any built in method? or i have to do it explicitly?. some people suggest to use ThreadPool. What actually happens when we use thread pool and how is it actually used?
Some quick answers:
By doing this am I spawning threads across the cores or in the single core?
You are creating multiple threads. Whether they execute on different cores is entirely up to the framework and (because .NET typically delegates the job to the OS) the operating system. However, if cores are available then typically they will be used.
If it is in single core, how can I do it across cores.
Is there any built in method? or i have to do it explicitly?.
See above. You have no real control over this. Windows and .NET provide mechanisms to restrict which cores a thread runs on (i.e. to set "thread affinity" to a specific core or group of cores), but you can't force your collection of threads to use more cores than .NET would otherwise provide.
Fortunately, as I mention above, .NET will generally use as many cores as are available.
some people suggest to use ThreadPool. What actually happens when we use thread pool and how is it actually used?
A thread pool is a construct that abstracts a collection of threads, each of which can be reused as needed. This is useful because there is some overhead (especially on Windows…not so much on *nix operating systems) when creating threads.
The .NET thread pool starts out empty (or with a very small number of threads). When you queue something to it, it first looks for an available idle thread. If there is one, it signals that thread to execute the delegate you queued to the thread pool. Otherwise, it creates a new thread (possibly waiting a brief moment first in case some thread pool thread becomes idle and can be reused), and uses that thread to execute your delegate.
The .NET ThreadPool class encapsulates this behavior. Note that the thread pool is used by a number of mechanisms, including direct asynchronous invocation of delegates (i.e. the BeginInvoke() method each delegate type has), various asynchronous APIs in the framework, and certain kinds of asynchronous operations started using the Task class.
It is not used when you create a Thread object explicitly.
For a more detailed understanding of threading and the .NET thread pool, see the various MSDN documentation pages on the subject. IMHO any more detail than the above would be too broad for a StackOverflow question.
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'm currently trying to figure what is the best way to minimize the amount of threads I use in a TCP master server, in order to maximize performance.
As I've been reading a lot recently with the new async features of C# 5.0, asynchronous does not necessarily mean multithreaded. It could mean separated in smaller chunks of finite state objects, then processed alongside other operations, by alternating. However, I don't see how this could be done in networking, since I'm basically "waiting" for input (from the client).
Therefore, I wouldn't use ReceiveAsync() for all my sockets, it would just be creating and ending threads continuously (assuming it does create threads).
Consequently, my question is more or less: what architecture can a master server take without having one "thread" per connection?
Side question for bonus coolness points: Why is having multiple threads bad, considering that having an amount of threads that is over your amount of processing cores simply makes the machine "fake" multithreading, just like any other asynchronous method would?
No, you would not necessarily be creating threads. There are two possible ways you can do async without setting up and tearing down threads all the time:
You can have a "small" number of long-lived threads, and have them sleep when there's no work to do (this means that the OS will never schedule them for execution, so the resource drain is minimal). Then, when work arrives (i.e. Async method called), wake one of them up and tell it what needs to be done. Pleased to meet you, managed thread pool.
In Windows, the most efficient mechanism for async is I/O completion ports which synchronizes access to I/O operations and allows a small number of threads to manage massive workloads.
Regarding multiple threads:
Having multiple threads is not bad for performance, if
the number of threads is not excessive
the threads do not oversaturate the CPU
If the number of threads is excessive then obviously we are taxing the OS with having to keep track of and schedule all these threads, which uses up global resources and slows it down.
If the threads are CPU-bound, then the OS will need to perform much more frequent context switches in order to maintain fairness, and context switches kill performance. In fact, with user-mode threads (which all highly scalable systems use -- think RDBMS) we make our lives harder just so we can avoid context switches.
Update:
I just found this question, which lends support to the position that you can't say how many threads are too much beforehand -- there are just too many unknown variables.
Seems like the *Async methods use IOCP (by looking at the code with Reflector).
Jon's answer is great. As for the 'side question'... See http://en.wikipedia.org/wiki/Amdahl%27s_law. Amdel's law says that serial code quickly diminishes the gains to be had from parallel code. We also know that thread coordination (scheduling, context switching, etc) is serial - so at some point more threads means there are so many serial steps that parallelization benefits are lost and you have a net negative performance. This is tricky stuff. That's why there is so much effort going into letting .NET manage threads while we define 'tasks' for the framework to decide what thread to run on. The framework can switch between tasks much more efficiently than the OS can switch between threads because the OS has a lot of extra things it needs to worry about when doing so.
Asynchronous work can be done without one-thread-per-connection or a thread pool with OS support for select or poll (and Windows supports this and it is exposed via Socket.Select). I am not sure of the performance on windows, but this is a very common idiom elsewhere.
One thread is the "pump" that manages the IO connections and monitors changes to the streams and then dispatches messages to/from other threads (conceivably 0 ... n depending upon model). Approaches with 0 or 1 additional threads may fall into the "Event Machine" category like twisted (Python) or POE (Perl). With >1 threads the callers form an "implicit thread pool" (themselves) and basically just offload the blocking IO.
There are also approaches like Actors, Continuations or Fibres exposed in the underlying models of some languages which alter how the basic problem is approached -- don't wait, react.
Happy coding.
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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.