From what I understand, twisted is single threaded. Everything happens in a single event loop. Why do we require a threadpool if twisted is single-threaded?
If you know exactly why, (like the function deferToThread uses a thread), that would be great. But any pointers to articles or theory would be appreciated too! I couldn't find anything useful on the twistedmatrix website.
Related
I have a program I'm writing in vb.net that has ballooned into the most complicated thing I've ever written. Because of some complex math and image rendering that's happening constantly I've been delving into multithreading for the first time to improve overall performance. Things have honestly been running really smoothly, but we've just added more functionality that's causing me some trouble.
The new functionality comes from a pair of DLLs that are each processing a video stream from a USB camera and looking for moving objects. When I start my program I initiate the DLLs and they start viewing the cameras and processing the videos. I then periodically ping them to see if they have detected anything. This is how I start and stop them:
Declare Function StartLeftCameraDetection Lib "DetectorLibLeft.dll" Alias "StartCameraDetection" () As Integer
Declare Function StopLeftCameraDetection Lib "DetectorLibLeft.dll" Alias "StopCameraDetection" () As Integer
When I need to check if they've found any objects I use several functions like this:
Declare Function LeftDetectedObjectLeft Lib "DetectorLibLeft.dll" Alias "DetectedObjectLeft" () As Integer
All of that works really well. The problem is, I've started to notice some significant lag in my UI and I'm thinking it may be coming from the DLLs. Forgive my ignorance on this, but as I said I'm new to using multiple threads (and incorporating DLLs too if I'm honest). It seems to me that when I start a DLL it running it's background tasks on my main thread and just waiting for me to ping it for information. Is that the case? If so, is it possible to have the DLL running on a sperate thread so it doesn't affect my UI?
I've tried a few different things but I can't seem to address the lag. I moved the code that pings the DLL and processes whatever information it gets into a sperate thread, but that hasn't made any difference. I also tried calling StartLeftCameraDetection from a separate thread but that didn't seem to help either. Again, I'm guessing that's because the real culprit is the DLL itself running these constant background tasks on my main thread no what thread I actually call it's functions from.
Thanks in advance for any help you might be able to offer!
There's a lot to grok when it comes to threading, but I'll try to write a concise summary that hits the high points with enough details to cover what you need to know.
Multi-threaded synchronization is hard, so you should try to avoid it as much as possible. That doesn't mean avoiding multi-threading at all, it just means avoiding doing much more than sending a self-contained task off to a thread to run to completion and getting the results back when it's done.
Recognizing that multi-threaded synchronization is hard, it's even worse when it involves UI elements. So in .NET, the design is that any access to UI elements will only occur through one thread, typically referred to as the UI thread. If you are not explicitly writing multi-threaded code, then all of your code runs on the UI thread. And, while your code is running, the UI is blocked.
This also extends to external routines that you run through Declare Function. It's not really accurate to say that they are doing anything with "background tasks on the main thread", if they are doing anything with "background tasks" they are almost certainly implementing their own threading. More likely, they aren't doing any task breakdown at all, and all of their work is being done on whichever thread you use to call them---the UI thread if you're not doing anything else.
If the work being done in these routines is CPU-bound, then it would definitely make sense to push it off onto a worker thread. Based on your comments on what you already tried:
I moved the code that pings the DLL and processes whatever information it gets into a sperate thread, but that hasn't made any difference. I also tried calling StartLeftCameraDetection from a separate thread but that didn't seem to help either.
I think the most likely problem is that you're blocking in the UI thread waiting for a result from the background thread.
The best way to avoid this depends on exactly what the routines are doing and how they produce results. If they do some sort of extended process and return everything in function results, then I would suggest that using Await would work well. This will basically return control to the UI until the operation finishes, then resume whatever the rest of the calling routine was going to do.
Note that if you do this, the user will have full interaction with the UI, and you should react accordingly. You might need to disable some (or all) operations until it's done.
There are a lot of resources on Async and Await. I'd particularly recommend reading Stephen Cleary's blog articles to get a better understanding of how they work and potential pitfalls that you might encounter.
One of the things I was most impressed with when digging into Pharo was that the Finder could do find-by-example. I'd previously only seen this in languages like Haskell, where it's possible to know for certain that a function has no side effects. How does Pharo manage to implement this in a way that is safe, performant, and side-effect free?
Magic :)
Actually... although I've been dreaming about creating the list from the tests automatically, the reality is that we manually maintain a list of safe messages (obviously error-prone; I seriously doubt it's 100% accurate). See MethodFinder>>#initialize.
So a trick, but not exactly magic ;)
I have been experimenting with porting the underlying 'story engine' of my Objective-C iPhone adventure Scarlett and the Spark of Life to HTML5 using CoffeeScript (and I am looking into IcedCoffeeScript).
The graphical part can just use DIVs on the DOM — the requirements there are fairly simple. The problematic part is the 'command and control' story-type commands. The ideal is to be able to express high-level story commands — including conditionals — and have them executed sequentially. So, for example, in faux-CoffeeScript:
scarlett.walkTo(200,300)
scarlett.turnTo(0)
story.wait(0.8)
if interesting
scarlett.think('Looks interesting.')
else
scarlett.think('Looks boring.')
In Objective-C (this was back when scripting languages like Lua were banned on the App Store), we achieved this by having two threads. The main thread ran cocos2d-phone which handled all the OpenGL calls, animation and other cocos niceties. The 'story' thread handled the command-and-control of the story, and if necessary the thread would sleep, awaiting an NSCondition before returning from a function and proceeding to the next call.
It sounds awkward, but it allowed us to express story commands and conditionals in a sequential, natural way, just using normal-looking code. Note that in the example above, the if check for the variable interesting would be evaluated right before Scarlett says something, not at the start of the function. Also, the walkTo(), turnTo(), wait() and think() calls will not return until their associated animation, delay or text box is finished back on the main thread.
What I'm struggling with is how to achieve this expressiveness using web technologies. As I see it, my options are:
Using a Web Worker as the story 'thread'. However, as far as I'm aware, workers can't sleep, and state isn't shared so they can't even perform a busy wait.
Using a callback chain, probably utilising IcedCoffeeScript's await and defer keywords to keep the code tidier. Even with those, though, that's a lot of extra line noise.
Somehow evaluate lines from the story script one-by-one as strings. I can't help feeling that it would be highly problematic.
(Similar in some ways to 3.) Write the story commands in a specially-designed interpreted language, where the program counter could be stopped and started as needed. It seems like this is unnecessarily re-inventing the wheel.
I can't help feeling like I'm overlooking some really obvious solution, though. Am I looking at this back-to-front, somehow? Is there an acknowledged pattern for scripting sequential actions and conditionals over time using actual code, without a mountain of callbacks?
I'm learning VB.NET coming from a VB6 and Java background.
In my app, I've got a function that validates the fields on a form. All it is doing is reading them, not updating. I've searched and see info about the backgroundWorker class, but all the examples are about updating the fields.
I understand the idea of threading and how it works, but have never written code that spawned threads myself. I've always let the language handle it. It seems like a lot of work that I would have to write a sub using the backgroundWorker for every time I wanted to read or update each field. The couple of books I've got that introduce you to the language show you reading or updating the field directly.
How do I know what threads are running other than writing the code like I'm used to then running through debugger to figure out what variables are on which thread?
Thanks.
Here and here is some reading on the BackgroundWorkerProcess. My advice, don't use this unless you have to i.e. only when you have a long running process and want to
Have the user switch between screens while that task is running.
Use a progress indicator on the form
That being said, I find it useful in cases like processing invoices. When I have to generate say, 4k invoices, while that task is running I can put an indacator on the form.
I find the following book helpful "Visual Basic 2008 Recipes" in explaining several use of threading, including the BackGroundWorker
The background worker does a lot of work for you. Certainly easier than managing threads and marshalling callbacks yourself. However, I agree with Saif... no point in doing any work unless there is some benefit to be had. Use it only for processes that may potentially take a lot of time.
Hopefully you're using VS2010, as it added some threading features. For example, use the Debug Location toolbar to select the thread of interest.
Is there a way to execute a whole objective-c function atomic?
As far as I know, using synchronized only protects a specific peace of code from being executed on multiple threads at the same time. But what I want is stop ALL other threads from doing ANYTHING, as long as I execute the function.
There is a wealth of info in the Threading Programming Guide. It specifically mentions to avoid synchronization (which is funny, cause you cant sometimes) but they offer some suggestions around the problem.
You will have serious problems with your design if you start running your software on multicore. It is a VERY expensive operation to stop all cores from running to run your bit of code. Mutexes, semaphores, run loop events, and atomic operations are the way to go.
Nope. Can't do that.
Or, well, you probably could if you dipped deep enough in the Mach APIs (on Mac OS X anyway).
But you shouldn't do that.
Why do you think you want to do that?