I'm rewriting existing real-time kernel TNKernel; I have used it for a couple of years, but I don't like many of its design decisions (as well as implementation details), so I decided to fork it and have fun implementing what I want. Anyone who is interested might read additional information at the project page on bitbucket.
TNKernel has one strange, in my opinion, feature: it has service tn_task_sleep(int timeout) which puts current task to sleep, and it has tn_task_wakeup(struct TN_Task *task) which wakes currently sleeping task up.
The strangeness is that it is legal to call tn_task_wakeup() on non-sleeping task; in this case, special flag like wakeup_request will be set, and on the next call to tn_task_sleep() this flag will be cleared, and task won't sleep.
All of this seems to me as a completely dirty hack, it probably might be used as a workaround to avoid race condition problems, or as a hacky replacement for semaphore.
It just encourages programmer to go with hacky approach, instead of creating straightforward semaphore and provide proper synchronization. So, I'm willing to remove this service from my project. Is this good idea to get rid of it, or have I missed something important? Why do we ever need that?
Since no one said that I'm wrong, I assumed that I'm right and removed these strange "features" from my kernel.
Related
So here's my use case: I have an async task wrapped in a Signal<String, NSError> that effectively executes once, and is no longer relevant (one-shot, essentially).
I'd like to observe or bind the output of this to a text field in my UI, but I can't see a way to do this that doesn't involve hanging on to a reference to the Signal's disposable and manually disposing it myself once the task has completed.
Is there a simpler way of doing this? Ideally there'd be a way to make the Signal release itself when it completes.
I feel like I'm missing something conceptually here.
Signals release their resources once they complete, error out, or if their observer(s) disconnect(s).
The disposeBag is there to disconnect the observer from the signal, but in your case, it is likely that the signal will have already released all its resources before that happens, because it will complete first.
In other words, you are overthinking it. The SDK does the right thing without you having to do anything special.
I'm using lua as the scripting language for handling events in my application, and I don't want to restrict users to writing short handlers - e.g. someone might want to have one handler run an infinite loop, and another handler would interrupt the first one. Obviously, lua doesn't directly support such behavior, so I'm looking for workarounds.
First of all, I'd like to avoid modifying the engine. Is it possible to set up a debug hook that would yield once the state has reached its quota? Judging by the documentation, it shouldn't be hard at all, but I don't know if there are any caveats to this.
And second, can I use lua_close to terminate a thread as I would in actual multithreading?
I've done something similar in the past. Its completely possible to multi-thread on separate Lua states. Be sure to take a look at luaL_lock() and luaL_unlock() (plus associated setup/cleanup), as you will no doubt need this setup (a simple mutex should do the trick).
After that, it should be a fairly simple matter of creating a lock/wait/interrupt API for your handlers.
I have been asked to debug, and improve, a complex multithreaded app, written by someone I don't have access to, that uses concurrent queues (both GCD and NSOperationQueue). I don't have access to a plan of the multithreaded architecture, that's to say a high-level design document of what is supposed to happen when. I need to create such a plan in order to understand how the app works and what it's doing.
When running the code and debugging, I can see in Xcode's Debug Navigator the various threads that are running. Is there a way of identifying where in the source-code a particular thread was spawned? And is there a way of determining to which NSOperationQueue an NSOperation belongs?
For example, I can see in the Debug Navigator (or by using LLDB's "thread backtrace" command) a thread's stacktrace, but the 'earliest' user code I can view is the overridden (NSOperation*) start method - stepping back earlier in the stack than that just shows the assembly instructions for the framework that invokes that method (e.g. __block_global_6, _dispatch_call_block_and_release and so on).
I've investigated and sought various debugging methods but without success. The nearest I got was the idea of method swizzling, but I don't think that's going to work for, say, queued NSOperation threads. Forgive my vagueness please: I'm aware that having looked as hard as I have, I'm probably asking the wrong question, and probably therefore haven't formed the question quite clearly in my own mind, but I'm asking the community for help!
Thanks
The best I can think of is to put breakpoints on dispatch_async, -[NSOperation init], -[NSOperationQueue addOperation:] and so on. You could configure those breakpoints to log their stacktrace, possibly some other info (like the block's address for dispatch_async, or the address of the queue and operation for addOperation:), and then continue running. You could then look though the logs when you're curious where a particular block came from and see what was invoked and from where. (It would still take some detective work.)
You could also accomplish something similar with dtrace if the breakpoints method is too slow.
I've been learning some lua for game development. I heard about coroutines in other languages but really came up on them in lua. I just don't really understand how useful they are, I heard a lot of talk how it can be a way to do multi-threaded things but aren't they run in order? So what benefit would there be from normal functions that also run in order? I'm just not getting how different they are from functions except that they can pause and let another run for a second. Seems like the use case scenarios wouldn't be that huge to me.
Anyone care to shed some light as to why someone would benefit from them?
Especially insight from a game programming perspective would be nice^^
OK, think in terms of game development.
Let's say you're doing a cutscene or perhaps a tutorial. Either way, what you have are an ordered sequence of commands sent to some number of entities. An entity moves to a location, talks to a guy, then walks elsewhere. And so forth. Some commands cannot start until others have finished.
Now look back at how your game works. Every frame, it must process AI, collision tests, animation, rendering, and sound, among possibly other things. You can only think every frame. So how do you put this kind of code in, where you have to wait for some action to complete before doing the next one?
If you built a system in C++, what you would have is something that ran before the AI. It would have a sequence of commands to process. Some of those commands would be instantaneous, like "tell entity X to go here" or "spawn entity Y here." Others would have to wait, such as "tell entity Z to go here and don't process anymore commands until it has gone here." The command processor would have to be called every frame, and it would have to understand complex conditions like "entity is at location" and so forth.
In Lua, it would look like this:
local entityX = game:GetEntity("entityX");
entityX:GoToLocation(locX);
local entityY = game:SpawnEntity("entityY", locY);
local entityZ = game:GetEntity("entityZ");
entityZ:GoToLocation(locZ);
do
coroutine.yield();
until (entityZ:isAtLocation(locZ));
return;
On the C++ size, you would resume this script once per frame until it is done. Once it returns, you know that the cutscene is over, so you can return control to the user.
Look at how simple that Lua logic is. It does exactly what it says it does. It's clear, obvious, and therefore very difficult to get wrong.
The power of coroutines is in being able to partially accomplish some task, wait for a condition to become true, then move on to the next task.
Coroutines in a game:
Easy to use, Easy to screw up when used in many places.
Just be careful and not use it in many places.
Don't make your Entire AI code dependent on Coroutines.
Coroutines are good for making a quick fix when a state is introduced which did not exist before.
This is exactly what java does. Sleep() and Wait()
Both functions are the best ways to make it impossible to debug your game.
If I were you I would completely avoid any code which has to use a Wait() function like a Coroutine does.
OpenGL API is something you should take note of. It never uses a wait() function but instead uses a clean state machine which knows exactly what state what object is at.
If you use coroutines you end with up so many stateless pieces of code that it most surely will be overwhelming to debug.
Coroutines are good when you are making an application like Text Editor ..bank application .. server ..database etc (not a game).
Bad when you are making a game where anything can happen at any point of time, you need to have states.
So, in my view coroutines are a bad way of programming and a excuse to write small stateless code.
But that's just me.
It's more like a religion. Some people believe in coroutines, some don't. The usecase, the implementation and the environment all together will result into a benefit or not.
Don't trust benchmarks which try to proof that coroutines on a multicore cpu are faster than a loop in a single thread: it would be a shame if it were slower!
If this runs later on some hardware where all cores are always under load, it will turn out to be slower - ups...
So there is no benefit per se.
Sometimes it's convenient to use. But if you end up with tons of coroutines yielding and states that went out of scope you'll curse coroutines. But at least it isn't the coroutines framework, it's still you.
We use them on a project I am working on. The main benefit for us is that sometimes with asynchronous code, there are points where it is important that certain parts are run in order because of some dependencies. If you use coroutines, you can force one process to wait for another process to complete. They aren't the only way to do this, but they can be a lot simpler than some other methods.
I'm just not getting how different they are from functions except that
they can pause and let another run for a second.
That's a pretty important property. I worked on a game engine which used them for timing. For example, we had an engine that ran at 10 ticks a second, and you could WaitTicks(x) to wait x number of ticks, and in the user layer, you could run WaitFrames(x) to wait x frames.
Even professional native concurrency libraries use the same kind of yielding behaviour.
Lots of good examples for game developers. I'll give another in the application extension space. Consider the scenario where the application has an engine that can run a users routines in Lua while doing the core functionality in C. If the user needs to wait for the engine to get to a specific state (e.g. waiting for data to be received), you either have to:
multi-thread the C program to run Lua in a separate thread and add in locking and synchronization methods,
abend the Lua routine and retry from the beginning with a state passed to the function to skip anything, least you rerun some code that should only be run once, or
yield the Lua routine and resume it once the state has been reached in C
The third option is the easiest for me to implement, avoiding the need to handle multi-threading on multiple platforms. It also allows the user's code to run unmodified, appearing as if the function they called took a long time.
What's a good way to leverage TDD to drive out thread-safe code? For example, say I have a factory method that utilizes lazy initialization to create only one instance of a class, and return it thereafter:
private TextLineEncoder textLineEncoder;
...
public ProtocolEncoder getEncoder() throws Exception {
if(textLineEncoder == null)
textLineEncoder = new TextLineEncoder();
return textLineEncoder;
}
Now, I want to write a test in good TDD fashion that forces me to make this code thread-safe. Specifically, when two threads call this method at the same time, I don't want to create two instances and discard one. This is easily done, but how can I write a test that makes me do it?
I'm asking this in Java, but the answer should be more broadly applicable.
You could inject a "provider" (a really simple factory) that is responsible for just this line:
textLineEncoder = new TextLineEncoder();
Then your test would inject a really slow implementation of the provider. That way the two threads in the test could more easily collide. You could go as far as have the first thread wait on a Semaphore that would be released by the second thread. Then success of the test would ensure that the waiting thread times out. By giving the first thread a head-start you can make sure that it's waiting before the second one releases.
It's hard, though possible - possibly harder than it's worth. Known solutions involve instrumenting the code under test. The discussion here, "Extreme Programming Challenge Fourteen" is worth sifting through.
In the book Clean Code there are some tips on how to test concurrent code. One tip that has helped me to find concurrency bugs, is running concurrently more tests than the CPU has cores.
In my project, running the tests takes about 2 seconds on my quad core machine. When I want to test the concurrent parts (there are some tests for that), I hold down in IntelliJ IDEA the hotkey for running all tests, until I see in the status bar that 20, 50 or 100 test runs are in execution. I follow in Windows Task Manager the CPU and memory usage, to find out when all the test runs have finished executing (memory usage goes up by 1-2 GB when they all are running and then slowly goes back down).
Then I close one by one all the test run output dialogs, and check that there were no failures. Sometimes there are failed tests or tests which are in deadlock, and then I investigate them until I find the bug and have fixed it. That has helped me to find a couple of nasty concurrency bugs. The most important thing, when facing an exception/deadlock that should not have happened, is always assuming that the code is broken, and investigating the reason ruthlessly and fixing it. There are no cosmic rays which cause programs to crash randomly - bugs in code cause programs to crash.
There are also frameworks such as http://www.alphaworks.ibm.com/tech/contest which use bytecode manipulation to force the code to do more thread switching, thus increasing the probability of making concurrency bugs visible.
When I test drove an implementation that needed to be thread safe recently I came up with the solution I provided as an answer for this question. Hope that helps even though there are no tests there. Hope link is OK raher than duplicating teh answer...
Chapter 12 of Java Concurrency in Practice is called "Testing Concurrent Programs". It documents testing for safety and liveness, but says this is a hard subject. I am not sure this problem is solvable by the tools of that chapter.
Just off the top of my head could you compare the instances returned to see if they are indeed the same instance or if they are different? That's probably where I would start with C#, I would imagine you can do the same in java