Does a Ruby mutex allow two threads to execute concurrently if one thread is waiting on blocking I/O?
This is my understanding of how the GIL works for MRI. I'm curious if there's any difference between a mutex and GIL?
Yes, this works. Only because of this, using threads in MRI is actually useful for many workloads, even though only a single thread can concurrently execute "code".
A common example is a web application such as a Rails app. Here, you can run multiple threads with e.g. Puma in a single process, each handling a single request. Since you often wait for the database here, a different thread can also execute. This works because the database adapter (e.g. mysql2 or pg) releases the GIL with a call to the database and re-acquires it once the reply arrives and is delivered up to the caller.
With a Mutex however, you are ensuring that a certain block of code is only executed by a single thread at a time. A common example is an adder:
class Adder
attr_reader :number
def initialize
#number = 0
end
def add(number)
new_number = #number
new_number = new_number + number
#number = new_number
# The above code is extremely verbose to show what's happening here.
# It is equivalent to
# #number += number
end
end
Here, the Adder#add method is not thread save. If multiple threads try to concurrently add numbers, some updates will be lost since the operation is not atomic (but consists of a read, an operation and a write). With a Mutex around add, you can ensure that the operation finishes in one thread and the shared data structures are updated consistently.
As a general advice, you should always use a Mutex if you are reading or updating any data shared across thread boundaries. To ensure correctness, you should also strictly control which data structures are passed across thread boundaries and avoid it if possible.
If you still need this, the concurrent-ruby gem provides some thread-safe data structures which can help with sharing data across thread-boundaries.
The Global Interpreter Lock on the default implementation (MRI) prevents any two Ruby threads from running concurrently. A Mutex prevents specific threads from running concurrently, where others are free to do whatever.
Note that the rules are different in implementations that do not have a GIL, like JRuby, where threads can run independently and concurrently.
In effect the Global Interpreter Lock is itself a mutex, but it's one that's engaged by default, not explicitly in your code, like with synchronize.
If a thread is blocked waiting on a mutex then other threads continue to run normally, they're able to do whatever they need independently.
The Global Interpreter Lock is also different in that your Ruby thread will be periodically interrupted so that other threads can run. This is to prevent one thread from monopolizing the lock.
Related
I'm using Ruby on Rails 5 although I'm fairly a novice to Ruby/Rails. I have read about creating threads using
t = Thread.new {
sleep(rand(0)/10.0)
Thread.current["mycount"] = count
count += 1
}
However, I'm wondering if there is a standard way of managing a bunch of application-created threads in Ruby/Rails. I'm familiar with Java, which has a thread factory. That allows a certain number of threads to concurrently run while others must wait in a queue. I'm wondering how I would do something similar in Ruby/Rails.
Note that I'm not talking about the types of threads that are generated automatically when a web page is requested. I'm talking about threads that I (the application owner) creates.
I think https://github.com/ruby-concurrency/concurrent-ruby is the most used library with utils for concurrency in ruby.
It has a lot of useful things including Thread Pools (http://ruby-concurrency.github.io/concurrent-ruby/file.thread_pools.html) which I think is what you are looking for.
Keep in mind that MRI Ruby has GIL, so you only have parallelism if your threads are waiting for IO. For heavy computation you might want to use jRuby or look elsewhere :)
Especially if you're using ActiveRecord inside your threads, you need to be careful about concurrency issues with for example database connections leaking.
Usually if you want to launch a new thread, you want to do something asynchronously in the background without having the user's request hang on an expensive action. For this there are some great libraries like Sucker Punch and Sidekiq. I'd recommend using one of these instead of creating and managing threads manually.
Hope this helps
what i s the difference between SetEvent() and Thread Lock() function? anyone please help me
Events are used when you want to start/continue processing once a certain task is completed i.e. you want to wait until that event occurs. Other threads can inform the waiting thread about the completion of this task using SetEvent.
On the other hand, critical section is used when you want only one thread to execute a block of code at a time i.e. you want a set of instructions to be executed by one thread without any other thread changing the state at that time. For example, you are inserting an item into a linked list which involves multiple steps, at that time you don't want another thread to come and try to insert one more object into the list. So you block the other thread until first one finishes using critical sections.
Events can be used for inter-process communication, ie synchronising activity amongst different processes. They are typically used for 'signalling' the occurrence of an activity (e.g. file write has finished). More information on events:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms686915%28v=vs.85%29.aspx
Critical sections can only be used within a process for synchronizing threads and use a basic lock/unlock concept. They are typically used to protect a resource from multi-threaded access (e.g. a variable). They are very cheap (in CPU terms) to use. The inter-process variant is called a Mutex in Windows. More info:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms682530%28v=vs.85%29.aspx
I was flicking through the Pickaxe, looking for the documentation on Thread, and came across ThreadGroup.
The documentation describes what it does, but it doesn't explain what it's for.
Is a thread group related to a thread pool, which I assumed Ruby doesn't have?
New threads are created in their parent's ThreadGroup. You can use the ThreadGroup to organize the implicit tree structure given by the parent threads spawning other threads, and use the list instance method to get all threads which have not terminated yet, i.e. to define methods operating on all threads in the group.
Additionaly, you can use enclose to prohibit adding (or removing) threads to this group, if you run untrusted code and want to keep an eye on the threads it spawns.
Is there a way to force a context switch in C++ to a specific thread, assuming I have the thread handle or thread ID?
No, you won't be able to force operating system to run the thread you want. You can use yield to force a context switch though...
yield in Win32 API is function SwitchToThread. If there is no other thread available for running, then a ZERO value will be returned and current thread will keep running anyway.
You can only encourage the Windows thread scheduler to pick a certain thread, you can't force it. You do so first by making the thread block on a synchronization object and signaling it. Secondary by bumping up its priority.
Explicit context switching is supported, you'll have to use fibers. Review SwitchToFiber(). A fiber is not a thread by a long shot, it is similar to a co-routine of old. Fibers' heyday has come and gone, they are not competitive with threads anymore. They have very crappy cpu cache locality and cannot take advantage of multiple cores.
The only way to force a particular thread to run is by using process/thread affinity, but I can't imagine ever having a problem for which this was a reasonable solution.
The only way to force a context switch is to force a thread onto a different processor using affinity.
In other words, what you are trying to do isn't really viable.
Calling SwitchToThread() will result in a context switch if there is another thread ready to run that are eligible to run on this processor. The documentation states it as follows:
If calling the SwitchToThread function
causes the operating system to switch
execution to another thread, the
return value is nonzero.
If there are no other threads ready to
execute, the operating system does not
switch execution to another thread,
and the return value is zero.
You can temporarily bump the priority of the other thread, while looping with Sleep(0) calls: this passes control to other threads. Suppose that the other thread has increased a lock variable and you need to wait until it becomes zero again:
// Wait until other thread releases lock
SetThreadPriority(otherThread, THREAD_PRIORITY_HIGHER);
while (InterlockedRead(&lock) != 0)
Sleep(0);
SetThreadPriority(otherThread, THREAD_PRIORITY_NORMAL);
I would check out the book Concurrent Programming for Windows. The scheduler seems to do a few things worth noting.
Sleep(0) only yields to higher priority threads (or possibly others at the same priority). This means you cannot fix priority inversion situations with just a Sleep(0), where other lower priority threads need to run. You must use SwitchToThread, Sleep a non-zero duration, or fully block on some kernel HANDLE.
You can create two synchronization objects (such as two events) and use the API SignalObjectAndWait.
If the hObjectToWaitOn is non-signaled and your other thread is waiting on the hObjectToSignal, the OS can theoretically perform quick context switch inside this API, before end of time slice.
And if you want the current thread to automatically resume, simply inform a small value (such as 50 or 100) on the dwMilliseconds.
I'm trying to learn about threading and I'm thoroughly confused. I'm sure all the answers are there in the apple docs but I just found it really hard to breakdown and digest. Maybe somebody could clear a thing or 2 up for me.
1)performSelectorOnMainThread
Does the above simply register an event in the main run loop or is it somehow a new thread even though the method says "mainThread"? If the purpose of threads is to relieve processing on the main thread how does this help?
2) RunLoops
Is it true that if I want to create a completely seperate thread I use
"detachNewThreadSelector"? Does calling start on this initiate a default run loop for the thread that has been created? If so where do run loops come into it?
3) And Finally , I've seen examples using NSOperationQueue. Is it true to say that If you use performSelectorOnMainThread the threads are in a queue anyway so NSOperation is not needed?
4) Should I forget about all of this and just use the Grand Central Dispatch instead?
Run Loops
You can think of a Run Loop to be an event processing for-loop associated to a thread. This is provided by the system for every thread, but it's only run automatically for the main thread.
Note that running run loops and executing a thread are two distinct concepts. You can execute a thread without running a run loop, when you're just performing long calculations and you don't have to respond to various events.
If you want to respond to various events from a secondary thread, you retrieve the run loop associated to the thread by
[NSRunLoop currentRunLoop]
and run it. The events run loops can handle is called input sources. You can add input sources to a run-loop.
PerformSelector
performSelectorOnMainThread: adds the target and the selector to a special input source called performSelector input source. The run loop of the main thread dequeues that input source and handles the method call one by one, as part of its event processing loop.
NSOperation/NSOperationQueue
I think of NSOperation as a way to explicitly declare various tasks inside an app which takes some time but can be run mostly independently. It's easier to use than to detach the new thread yourself and maintain various things yourself, too. The main NSOperationQueue automatically maintains a set of background threads which it reuses, and run NSOperations in parallel.
So yes, if you just need to queue up operations in the main thread, you can do away with NSOperationQueue and just use performSelectorOnMainThread:, but that's not the main point of NSOperation.
GCD
GCD is a new infrastructure introduced in Snow Leopard. NSOperationQueue is now implemented on top of it.
It works at the level of functions / blocks. Feeding blocks to dispatch_async is extremely handy, but for a larger chunk of operations I prefer to use NSOperation, especially when that chunk is used from various places in an app.
Summary
You need to read Official Apple Doc! There are many informative blog posts on this point, too.
1)performSelectorOnMainThread
Does the above simply register an event in the main run loop …
You're asking about implementation details. Don't worry about how it works.
What it does is perform that selector on the main thread.
… or is it somehow a new thread even though the method says "mainThread"?
No.
If the purpose of threads is to relieve processing on the main thread how does this help?
It helps you when you need to do something on the main thread. A common example is updating your UI, which you should always do on the main thread.
There are other methods for doing things on new secondary threads, although NSOperationQueue and GCD are generally easier ways to do it.
2) RunLoops
Is it true that if I want to create a completely seperate thread I use "detachNewThreadSelector"?
That has nothing to do with run loops.
Yes, that is one way to start a new thread.
Does calling start on this initiate a default run loop for the thread that has been created?
No.
I don't know what you're “calling start on” here, anyway. detachNewThreadSelector: doesn't return anything, and it starts the thread immediately. I think you mixed this up with NSOperations (which you also don't start yourself—that's the queue's job).
If so where do run loops come into it?
Run loops just exist, one per thread. On the implementation side, they're probably lazily created upon demand.
3) And Finally , I've seen examples using NSOperationQueue. Is it true to say that If you use performSelectorOnMainThread the threads are in a queue anyway so NSOperation is not needed?
These two things are unrelated.
performSelectorOnMainThread: does exactly that: Performs the selector on the main thread.
NSOperations run on secondary threads, one per operation.
An operation queue determines the order in which the operations (and their threads) are started.
Threads themselves are not queued (except maybe by the scheduler, but that's part of the kernel, not your application). The operations are queued, and they are started in that order. Once started, their threads run in parallel.
4) Should I forget about all of this and just use the Grand Central Dispatch instead?
GCD is more or less the same set of concepts as operation queues. You won't understand one as long as you don't understand the other.
So what are all these things good for?
Run loops
Within a thread, a way to schedule things to happen. Some may be scheduled at a specific date (timers), others simply “whenever you get around to it” (sources). Most of these are zero-cost when idle, only consuming any CPU time when the thing happens (timer fires or source is signaled), which makes run loops a very efficient way to have several things going on at once without any threads.
You generally don't handle a run loop yourself when you create a scheduled timer; the timer adds itself to the run loop for you.
Threads
Threads enable multiple things to happen at the exact same time on different processors. Thing 1 can happen on thread A (on processor 1) while thing 2 happens on thread B (on processor 0).
This can be a problem. Multithreaded programming is a dance, and when two threads try to step in the same place, pain ensues. This is called contention, and most discussion of threaded programming is on the topic of how to avoid it.
NSOperationQueue and GCD
You have a thing you need done. That's an operation. You can't have it done on the main thread, or you'd simply send a message like normal; you need to run it in the background, on a secondary thread.
To achieve this, express it as either an NSOperation object (you create a subclass of NSOperation and instantiate it) or a block (or both), then add it to either an NSOperationQueue (NSOperations, including NSBlockOperation) or a dispatch queue (bare block).
GCD can be used to make things happen on the main thread, as well; you can create serial queues and add blocks to them. A serial queue, as its name suggests, will run exactly one block at a time, rather than running a bunch of them in parallel.
So what should I do?
I would not recommend creating threads directly. Use NSOperationQueue or GCD instead; they force you into better thinking habits that will reduce the risk of your threaded code inducing headaches.
For things that run periodically, not fitting into the “thing I need done” model of NSOperations and GCD blocks, consider just using the run loop on the main thread. Chances are, you don't need to put it on a thread after all. A rendering loop in a 3D game, for example, can be a simple timer.