I have a GUI application, which listens to a network port from a second thread. I was looking at OpenMP and I was wondering if there are easy ways to create threads like this. I was searching for documentation, but the OpenMP site is not very convenient to navigate. Could someone help?
As far as I understand OpenMP is a compiler-assisted parallelizing framework/library targeted to heavy computations. You hint the compiler which parts of your code (usually loops) can run in parallel. The compiler does its magic (inserting library calls, sharing/unsharing variables, etc.) and, poof, the program can now run faster (sometimes) on several cores. It might be possible to do what you want with OpenMP, I don't know, but I think you are looking at the wrong tool. Doing things directly with pthreads is one alternative.
Related
I have asked a related question before Why OCaml's threading is considered as `not enough`?
No matter how "bad" ocaml's threading is, I notice some libraries say they can do real threading.
For example, Lwt
Lwt offers a new alternative. It provides very light-weight
cooperative threads; ``launching'' a thread is a very fast operation,
it does not require a new stack, a new process, or anything else.
Moreover context switches are very fast. In fact, it is so easy that
we will launch a thread for every system call. And composing
cooperative threads will allow us to write highly asynchronous
programs.
Also Jane Street's aync_core also provides similar things, if I am right.
But I am quite confused. Do Lwt or aync_core provide threading like Java threading?
If I use them, can I utilise multiple cpu?
In what way, can I get a "real threading" (just like in Java) in OCaml?
Edit
I am still confused.
Let me add a scenario:
I have a server (16 cpu cores) and a server application.
What the server application does are:
It listens to requests
For each request, it starts a computational task (let's say costs 2 minutes to finish)
When each task finishes, the task will either return the result back to the main or just send the result back to client directly
In Java, it is very easy. I create a thread pool, then for each request, I create a thread in that pool. that thread will run the computational task. This is mature in Java and it can utilize the 16 cpu cores. Am I right?
So my question is: can I do the same thing in OCaml?
The example of parallelized server that you cite is one of those embarassingly parallel problem that are well solved with a simple multiprocessing model, using fork. This has been doable in OCaml for decades, and yes, you will an almost linear speedup using all the cores of your machine if you need.
To do that using the simple primitives of the standard library, see this Chapter of the online book "Unix system programming in OCaml" (first released in 2003), and/or this chapter of the online book "Developing Applications with OCaml" (first released in 2000).
You may also want to use higher-level libraries such as Gerd Stolpmann's OCamlnet library mentioned by rafix, which provides a lot of stuff from direct helper for the usual client/server design, to lower-level multiprocess communication libraries; see the documentation.
The library Parmap is also interesting, but maybe for slightly different use case (it's more that you have a large array of data available all at the same time, that you want to process with the same function in parallel): a drop-in remplacement of Array.map or List.map (or fold) that parallelizes computations.
The closest thing you will find to real (preemptive) threading is the built in threading library. By that mean I mean that your programming model will be the same but with 2 important differences:
OCaml's native threads are not lightweight like Java's.
Only a single thread executes at a time, so you cannot take advantage of multiple processes.
This makes OCaml's threads a pretty bad solution to either concurrency or parallelism so in general people avoid using them. But they still do have their uses.
Lwt and Async are very similar and provide you with a different flavour of threading - a cooperative style. Cooperative threads differ from preemptive ones in the fact context switching between threads is explicit in the code and blocking calls are always apparent from the type signature. The cooperative threads provided are very cheap so very well suited for concurrency but again will not help you with parallelilsm (due to the limitations of OCaml's runtime).
See this for a good introduction to cooperative threading: http://janestreet.github.io/guide-async.html
EDIT: for your particular scenario I would use Parmap, if the tasks are so computationally intensive as in your example then the overhead of starting the processes from parmap should be negligible.
For multi-thread programming, with the considerations of combinations with HPC application (MPI), which one is better, can we say, in terms of functionality, Intel TBB (thread building block) is comparable to pthread or not? I only get experience in open mp, but I heard both TBB and Pthread offers finer thread control comparing to open mp, but can TBB or TBB+OpenMP offer similiar functionality compared to pthread?
pthread is a thin wrapper above the OS infrastructure. It allows you to create a thread with a given thread main function, and some synchronization primitives (mutexes semaphores etc). Under Linux pthread is implemented on top of the clone(2) system call. The equivilant under Windows is called CreateThread. All the other threading stuff is built on top of this base.
Intel TBB is higher level, it gives parallel_for and parallel_reduce and similiar higher level constructs similar to OpenMP but implemented as a library not a language extension.
OpenMPI is even higher level still with multi-machine distributed infrastructure, but it is very old fashioned and a little clunky.
My advice would be to learn the pthread library first until you completely understand it, and then look at higher level libraries afterward.
TBB allows you to write portable code on top of the native threading functionality, so it makes the code more portable over different OS architectures. I don't think it's "more efficient" than pthread.
I haven't used open MP personally, but in the past I've worked with developers using open MP (as a technical specialist on the processors they were using), and it seems to work reasonably well for certain things, but others are harder to use in open mp than writing your own code. It all depends on what exactly you are doing. One of the benefits with openmp of course is that you can always recompile the code without the openmp option, and the code just works directly as you expect it to [but not spread out, of course].
With a programmes threading approach, you can have much more control over exactly what happens on what thread, yes. But it also means a lot more work...
I am using a simple Concurrency Runtime task_group in Visual Studio 2010 to run a single working thread to separate the work from the GUI thread.
However one of my colleagues told me that I'm using CR wrong: it was designed for parallelizing lightweight tasks with small context and not for separating bulky and I/O-dependent threads from the GUI. He said that he'd taken this from the documentation, but failed to provide any specific links.
So, what are the limitations of Microsoft Concurrency Runtime and to solve what problems I should NOT use it?
Of course CR is not portable, but let's leave it out: I'm talking about situations, when you code compiles, but you get problems nevertheless.
The concurrency runtime is a cooperative scheduling infrastructure. If you're not going to take advantage of cooperative scheduling, then you're better off creating threads when you need to, and letting the OS take care of scheduling.
If you are into cooperative scheduling, then there's really no point to wait for an IO operation to complete, because you're blocking a thread which could have otherwise been used for running other tasks, which do not depend on this IO operation to complete. If other tasks depend on the IO task to complete, you can simply make them continuations, and the ConcRT scheduler will make sure to run them when their time comes.
So it's really not about limitations here. It's simply about knowing what you're trying to achieve, and picking the right tool for the job.
As Yam mentioned, concurrency runtime does not provide the parallel execution guarantee, it just makes a potential possibility, and that is the difference between notions of tasks and threads. If you get your tasks right (not too granular to spend much time on switching between tasks, and not too coarse to always have some work for all the cores - in your case - just one), then the overhead will not be significant, and your program will be ready for running on a multi-core or a multi-processor platform, "future proof" as MSFT people like to say.
I have a C++ program that performs some lengthy computation in parallel using OpenMP. Now that program also has to respond to user input and update some graphics. So far I've been starting my computations from the main / GUI thread, carefully balancing the workload so that it is neither to short to mask the OpenMP threading overhead nor to long so the GUI becomes unresponsive.
Clearly I'd like to fix that by running everything concurrently. As far as I can tell, OpenMP 2.5 doesn't provide a good mechanism for doing this. I assume it wasn't intended for this type of problem. I also wouldn't want to dedicate an entire core to the GUI thread, it just needs <10% of one for its work.
I thought maybe separating the computation into a separate pthread which launches the parallel constructs would be a good way of solving this. I coded this up but had OpenMP crash when invoked from the pthread, similar to this bug: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36242 . Note that I was not trying to launch parallel constructs from more than one thread at a time, OpenMP was only used in one pthread throughout the program.
It seems I can neither use OpenMP to schedule my GUI work concurrently nor use pthreads to have the parallel constructs run concurrently. I was thinking of just handling my GUI work in a separate thread, but that happens to be rather ugly in my case and might actually not work due to various libraries I use.
What's the textbook solution here? I'm sure others have used OpenMP in a program that needs to concurrently deal with a GUI / networking etc., but I haven't been able to find any information using Google or the OpenMP forum.
Thanks!
There is no textbook solution. The textbook application for OpenMP is non-interactive programs that read input files, do heavy computation, and write output files, all using the same thread pool of size ~ #CPUs in your supercomputer. It was not designed for concurrent execution of interactive and computation code and I don't think interop with any threads library is guaranteed by the spec.
Leaving theory aside, you seem to have encountered a bug in the GCC implementation of OpenMP. Please file a bug report with the GCC maintainers and for the time being, either look for a different compiler or run your GUI code in a separate process, communicating with the OpenMP program over some IPC mechanism. (E.g., async I/O over sockets.)
I have never come across multithreading but I hear about it everywhere. What should I know about it and when should I use it? I code mainly in c++.
Mostly, you will need to learn about MT libraries on OS on which your application needs to run. Until and unless C++0x becomes a reality (which is a long way as it looks now), there is no support from the language proper or the standard library for threads. I suggest you take a look at the POSIX standard pthreads library for *nix and Windows threads to get started.
This is my opinion, but the biggest issue with multithreading is that it is difficult. I don't mean that from an experienced programmer point of view, I mean it conceptually. There really are a lot of difficult concurrency problems that appear once you dive into parallel programming. This is well known, and there are many approaches taken to make concurrency easier for the application developer. Functional languages have become a lot more popular because of their lack of side effects and idempotency. Some vendors choose to hide the concurrency behind API's (like Apple's Core Animation).
Multitheaded programs can see some huge gains in performance (both in user perception and actual amount of work done), but you do have to spend time to understand the interactions that your code and data structures make.
MSDN Multithreading for Rookies article is probably worth reading. Being from Microsoft, it's written in terms of what Microsoft OSes support(ed in 1993), but most of the basic ideas apply equally to other systems, with suitable renaming of functions and such.
That is a huge subject.
A few points...
With multi-core, the importance of multi-threading is now huge. If you aren't multithreading, you aren't getting the full performance capability of the machine.
Multi-threading is hard. Communicating and synchronization between threads is tricky to get right. Problems are often intermittent, hard to diagnose, and if the design isn't right for multi-threading, hard to fix.
Multi-threading is currently mostly non-portable and platform specific.
There are portable libraries with wrappers around threading APIs. Boost is one. wxWidgets (mainly a GUI library) is another. It can be done reasonably portably, but you won't have all the options you get from platform-specific APIs.
I've got an introduction to multithreading that you might find useful.
In this article there isn't a single
line of code and it's not aimed at
teaching the intricacies of
multithreaded programming in any given
programming language but to give a
short introduction, focusing primarily
on how and especially why and when
multithreaded programming would be
useful.
Here's a link to a good tutorial on POSIX threads programming (with diagrams) to get you started. While this tutorial is pthread specific, many of the concepts transfer to other systems.
To understand more about when to use threads, it helps to have a basic understanding of parallel programming. Here's a link to a tutorial on the very basics of parallel computing intended for those who are just becoming acquainted with the subject.
The other replies covered the how part, I'll briefly mention when to use multithreading.
The main alternative to multithreading is using a timer. Consider for example that you need to update a little label on your form with the existence of a file. If the file exists, you need to draw a special icon or something. Now if you use a timer with a low timeout, you can achieve basically the same thing, a function that polls if the file exists very frequently and updates your ui. No extra hassle.
But your function is doing a lot of unnecessary work, isn't it. The OS provides a "hey this file has been created" primitive that puts your thread to sleep until your file is ready. Obviously you can't use this from the ui thread or your entire application would freeze, so instead you spawn a new thread and set it to wait on the file creation event.
Now your application is using as little cpu as possible because of the fact that threads can wait on events (be it with mutexes or events). Say your file is ready however. You can't update your ui from different threads because all hell would break loose if 2 threads try to change the same bit of memory at the same time. In fact this is so bad that windows flat out rejects your attempts to do it at all.
So now you need either a synchronization mechanism of sorts to communicate with the ui one after the other (serially) so you don't step on eachother's toes, but you can't code the main thread part because the ui loop is hidden deep inside windows.
The other alternative is to use another way to communicate between threads. In this case, you might use PostMessage to post a message to the main ui loop that the file has been found and to do its job.
Now if your work can't be waited upon and can't be split nicely into little bits (for use in a short-timeout timer), all you have left is another thread and all the synchronization issues that arise from it.
It might be worth it. Or it might bite you in the ass after days and days, potentially weeks, of debugging the odd race condition you missed. It might pay off to spend a long time first to try to split it up into little bits for use with a timer. Even if you can't, the few cases where you can will outweigh the time cost.
You should know that it's hard. Some people think it's impossibly hard, that there's no practical way to verify that a program is thread safe. Dr. Hipp, author of sqlite, states that thread are evil. This article covers the problems with threads in detail.
The Chrome browser uses processes instead of threads, and tools like Stackless Python avoid hardware-supported threads in favor of interpreter-supported "micro-threads". Even things like web servers, where you'd think threading would be a perfect fit, and moving towards event driven architectures.
I myself wouldn't say it's impossible: many people have tried and succeeded. But there's no doubt writting production quality multi-threaded code is really hard. Successful multi-threaded applications tend to use only a few, predetermined threads with just a few carefully analyzed points of communication. For example a game with just two threads, physics and rendering, or a GUI app with a UI thread and background thread, and nothing else. A program that's spawning and joining threads throughout the code base will certainly have many impossible-to-find intermittent bugs.
It's particularly hard in C++, for two reasons:
the current version of the standard doesn't mention threads at all. All threading libraries and platform and implementation specific.
The scope of what's considered an atomic operation is rather narrow compared to a language like Java.
cross-platform libraries like boost Threads mitigate this somewhat. The future C++0x will introduce some threading support. But boost also has good interprocess communication support you could use to avoid threads altogether.
If you know nothing else about threading than that it's hard and should be treated with respect, than you know more than 99% of programmers.
If after all that, you're still interested in starting down the long hard road towards being able to write a multi-threaded C++ program that won't segfault at random, then I recommend starting with Boost threads. They're well documented, high level, and work cross platform. The concepts (mutexes, locks, futures) are the same few key concepts present in all threading libraries.