I'm an Android engineer trying to port some iOS code that uses 5 SERIAL dispatch queues. I want to make sure I'm thinking about things the right way.
dispatch_sync to a SERIAL queue is basically using the queue as a synchronized queue- only one thread may access it and the block that gets executed can be thought of as a critical region. It happens immediately on the current thread- its the equivalent of
get_semaphore()
queue.pop()
do_block()
release_semaphore()
dispatch_async to a serial queue- performs the block on another thread and lets the current thread return immediately. However since its a serial queue it promises only one of these asynchronous thread is going to execute at a time (the next call to dispatch_async will wait until all other threads are finished). That block can also be thought of as a critical region, but it will occur on another thread. So the same code as above, but its passed to a worker thread first.
Am I off in any of that, or did I figure it out correctly?
This feels like an overly complicated way of thinking of it and there are lots of little details of that description that aren't quite right. Specifically, "it happens immediately on the current thread" is not correct.
First, let's step back: The distinction between dispatch_async and dispatch_sync is merely whether the current thread waits for it or not. But when you dispatch something to a serial queue, you should always imagine that it's running on a separate worker thread of GCD's own choosing. Yes, as an optimization, sometimes dispatch_sync will use the current thread, but you are in no way guaranteed of this fact.
Second, when you discuss dispatch_sync, you say something about it running "immediately". But it's by no means assured to be immediate. If a thread does dispatch_sync to some serial queue, then that thread will block until (a) any block currently running on on that serial queue finish; (b) any other queued blocks for that serial queue run and complete; and (c) obviously, the block that thread A itself dispatched runs and completes.
Now, when you use a serial queue for synchronization for, some thread-safe access to some object in memory, often that synchronization process is very quick, so the waiting thread will generally be blocked for a negligible amount of time as its dispatched block (and any prior dispatched blocks) to finish. But in general, it's misleading to say that it will run immediately. (If it always could run immediately, then you wouldn't need a queue to synchronize access).
Now your question talks about a "critical region", to which I assume you're talking about some bit of code that, in order to ensure thread-safety or for some other reason like that, must be synchronized. So, when running this code to be synchronized, the only question re dispatch_sync vs dispatch_async is whether the current thread must wait. A common pattern, for example, is to say that one may dispatch_async writes to some model (because there's no need to wait for the model to update before proceeding), but dispatch_sync reads from some model (because you obviously don't want to proceed until the read value is returned).
A further optimization of that sync/async pattern is the reader-writer pattern, where concurrent reads are permissible but concurrent writes are not. Thus, you'll use a concurrent queue, dispatch_barrier_async the writes (achieving serial-like behavior for the writes), but dispatch_sync the reads (enjoying concurrent performance with respect to other read operations).
To pick nits, dispatch_sync doesn't necessarily run the code on the current thread, but if it doesn't, it still blocks the current thread until the task completes. The distinction is only potentially important if you're relying on thread IDs or thread-local storage.
But otherwise, yes, unless I missed something subtle.
Related
I encountered a problem with non-iOS developer while describing the flow of OperationQueue. We already know that with OperationQueue we can start as many threads as we want to execute the tasks in sync/async way.
But in practical, some of the people want proof for the OperationQueue is getting executed in the background and not with UI(Main) thread.
I just want to demonstrate that when operation queue starts, it already starts its execution in the background.
I already have convinced that when we try to set qos for the operationQueue that we create, it has all the parameters of global queue's qos viz: default userInitiated userInteractive background and utility.
So that is already perfect example in code to prove that all the OperationQueue operations mentioned are run on global thread. Unless we declare the OperationQueue.main
As Shadowrun said, you can add assertions for Thread.isMainThread. Also, if you ever want to add test that you are not on the main queue, you can add a precondition:
dispatchPrecondition(.notOnQueue(.main))
But it should be noted that the whole purpose of creating an operation queue is to get things off the main thread. E.g., the old Concurrency Programming Guide: Operation Queues says [emphasis added]:
Operations are designed to help you improve the level of concurrency in your application. Operations are also a good way to organize and encapsulate your application’s behavior into simple discrete chunks. Instead of running some bit of code on your application’s main thread, you can submit one or more operation objects to a queue and let the corresponding work be performed asynchronously on one or more separate threads.
This is not to say that operation queues cannot contribute to main thread responsiveness problems. You can wait from the main thread for operations added to an operation queue (which is obviously a very bad idea). Or, if you neglect to set a reasonable maxConcurrentOperationCount and have thread explosion, that can introduce all sorts of unexpected behaviors. Or you can entangle the main thread with operation queues through a misuse of semaphores or dispatch groups.
But operations on an operation queue (other than OperationQueue.main) simply do not run on the main thread. Technically, you could get yourself in trouble if you started messing with the target queue of the underlying queue, but I can’t possibly imagine that you are doing that. If you are having main thread problems, your problem undoubtedly rests elsewhere.
I'd worked on Java, and pretty much clear with the working of threads and thread pool.
I was wondering if anyone can explain the working of how thread's are created and allocated space in the thread pool in swift ?.
Also, does
Dispatch.main.async {
// some code
}
Creates a new Thread or Asynchronously executes the task ?
Thanks in advance =)
Queues and threads are separate concepts. Queues are ordered (sometimes prioritized) sequences of blocks to execute. As (mostly) an implementation detail, blocks must be scheduled onto threads in order to execute, but this is not the major point of them.
So Dispatch.main.async dispatches (appends) a block to the main queue. The main queue is serial and somewhat special in that it is promised to also be run exclusively on the main thread (as noted by Paulw11). It also promises to be associated with the main runloop. Understanding this "appends a block to a queue" concept is critical, because it has significant impact on how you design things in queues vs how you design things in threads. async does not mean "start running this now." It means "stick this on a queue, but don't wait for it."
As a good example of how the designs can be different, placing something on a queue doesn't mean it will ever run (even without bugs or deadlocks). It is possible and useful to suspend queues so that it stops scheduling blocks. It's possible to tie queues to other queues so that when a queue "schedules" something, it just puts it onto another queue rather than executing it. There are lots of things you can do with queues unrelated to "run things in the background." You can attach completion handlers to blocks. You can use groups to wait on collections of blocks. GCD is a way of thinking about concurrency. Parallelism is just a side benefit. (A great discussion of this concept is Concurrency is not parallelism by Rob Pike. It's in Go, but the concepts still apply.)
If you call Dispatch.main.async while running on the main queue, then that block is absolutely certain to not execute until the current block finishes. In UIKit and AppKit, "the current block finishes" often means "you return from a method that was called by the OS." While not implemented this way, you can pretend that every time you're called from the OS, it was wrapped in a call to Dispatch.main.async.
This is also why you must never call Dispatch.main.sync (note sync) from the main queue. That block will wait for you to return, and you'll wait until the block finishes. A classic deadlock.
As a rule, the thread pool is not your business in iOS. It is an implementation detail. Occasionally you need to think about it for performance reasons, but if you are thinking too much about it, you probably are designing your concurrency incorrectly.
If you're coming from Java, you definitely want to read Migrating Away From Threads in the Concurrency Programming Guide. It's the definitive resource for how to rethink thread-based patterns in queues.
Your code queues the block of code on the main queue (Dispatch.main) and returns immediately (.async), before executing the code.
You do not have control over which thread is used by the queue. Even if you create an own queue:
let serialQueue = DispatchQueue(label: "queuename")
serialQueue.async {
...
}
you do not know which thread your code will be running on.
Update:
As correctly stated by Paulw11 in the comment,
... if you dispatch a task on the main queue, it is guaranteed to execute on the main thread. If you dispatch a task on any other queue, you don't know which thread it will execute on; it may execute on the main thread or some other thread.
I want to understand something about GCD and Threads.
I have a for loop in my view controller which asks my model to do some async network request.
So if the loop runs 5 times, the model sends out 5 network requests.
Is it correct to state that 5 threads have been created by my model considering the fact that I'm using NSURLConnection's sendAsyncRequest and the completion handlers will be called on an additional 5 threads ?
Now, If I ask my view controller to execute this for loop on a different thread and in every iteration of the loop, the call to the model should be dependent on the previous iteration, would I be creating an "Inception" of threads here ?
Basically, I want the subsequent async requests to my server only if the previous thread has completed entirely (By entirely I mean all of its sub threads should have finished executing too.)
I can't even frame the question properly because I'm massively confused myself.
But if anybody could help with anything, that would be helpful.
It is not correct to state that five threads have been created in the general case.
There is no one-to-one mapping between threads and blocks. GCD is an implementation of thread pooling.
A certain number of threads are created according to the optimal setup for that device — the cost of creating and maintaing threads under that release of the OS, the number of processor cores available, the number of threads it already has but which are presently blocked and any other factors Apple cares to factor in may all be relevant.
GCD will then spread your blocks over those threads. Or it may create new threads. But it won't necessarily.
Beyond that queues are just ways of establishing the sequencing between blocks. A serial dispatch queue does not necessarily own its own thread. All concurrent dispatch queues do not necessarily own their own threads. But there's no reason to believe that any set of queues shares any threads.
The exact means of picking threads for blocks has changed between versions of the OS. E.g. iOS 4 was highly profligate in thread creation, in a way that iOS 5+ definitely haven't been.
GCD will just try to do whatever is best in the circumstances. Don't waste your time trying to second guess it.
"Basically, I want the subsequent async requests to my server only if the previous thread has completed entirely (By entirely I mean all of its sub threads should have finished executing too.)"
Only focusing on the above statement to avoid confusion. Simple solution would be create a queue. feed the queue with 5 loops. Each loop will be making network request synchronously(you can use sendSynchronousRequest: method available in NSURLConnection), performing the operations after request completion and then start the next loop. queue following FIFO will execute the your requests subsequently.
GCD : Think of this as a simple queue that can accept tasks. Tasks are blocks of your code. You can put in as many tasks as you want in a queue (permitting system limits). Queues come in different flavors. Concurrent vs Serial. Main vs Global. High Priority vs Low Priority. A queue is not a thread.
Thread : It is a single line of execution of code in sequence. You can have multiple threads working on your code at the same time. A thread is not a queue.
Once you separate the 2 entities things start become clear.
GCD basically uses the threads in the process to work on tasks. In a serial queue everything is processed in sequence. So you don't need to have synchronization mechanisms in your code, the very nature of serial queue ensures synchronization. If this is a concurrent queue (i.e. 2 or more tasks being processed at the same time, then you need to ensure critical sections of your code are protected with synchronization).
Here is how you queue work to be done.
dispatch_async(_yourDispatchQueue, ^() {
NSLog (#"work queued");
});
The above NSLog will now get executed in a background thread in a near future time, but in a background thread.
If you notice when we put a request in we use dispatch_async. The other variation is dispatch_sync. The different between the 2 is after you put the request in the queue, the async variation will move on. The sync variation will not !!
If you are going to use a GCD for NSURLConnection you need to be careful on which thread you start the connection. Here is a SO link for more info. GCD with NSURLConnection
I am using Grand Central Dispatch to run a process in background. I want know how can i suspend, resume and stop that background thread. I have tried
dispatch_suspend(background_thread);
dispatch_resume(background_thread);
but these functions doesn't help me, it keeps on running. Please someone help me.
You seem to have some confusion. Direct manipulation of threads is not part of the GCD API. The GCD object you normally manipulate is a queue, not a thread. You put blocks in a queue, and GCD runs those blocks on any thread it wants.1
Furthermore, the dispatch_suspend man page says this:
The dispatch framework always checks the suspension status before executing a block, but such changes never affect a block during execution (non-preemptive).
In other words, GCD will not suspend a queue while the queue is running a block. It will only suspend a queue while the queue is in between blocks.
I'm not aware of any public API that lets you stop a thread without cooperation from the function running on that thread (for example by setting a flag that is checked periodically on that thread).
If possible, you should break up your long-running computation so that you can work on it incrementally in a succession of blocks. Then you can suspend the queue that runs those blocks.
Footnote 1. Except the main queue. If you put a block on the main queue, GCD will only run that block on the main thread.
You are describing a concurrent processing model, where different processes can be suspended and resumed. This is often achieved using threads, or in some cases coroutines.
GCD uses a different model, one of partially ordered blocks where each block is sequentially executed without pre-emption, suspension or resumption directly supported.
GCD semaphores do exist, and may suit your needs, however creating general cooperating concurrent threads with them is not the goal of GCD. Otherwise look at a thread based solution using NSThread or even Posix threads.
Take a look at Apple's Migrating Away from Threads to see if your model is suited to migration to GCD, but not all models are.
I was reading OReilly's iOS6 Programming Cookbook and am confused about something. Quoting from page 378, chapter 6 "Concurrency":
For any task that doesn’t involve the UI, you can use global concurrent queues in GCD.
These allow either synchronous or asynchronous execution. But synchronous execution
does not mean your program waits for the code to finish before continuing. It
simply means that the concurrent queue will wait until your task has finished before it
continues to the next block of code on the queue. When you put a block object on a
concurrent queue, your own program always continues right away without waiting for
the queue to execute the code. This is because concurrent queues, as their name implies,
run their code on threads other than the main thread.
I bolded the text that intrigues me. I think it is false because as I've just learned today synchronous execution means precisely that the program waits for the code to finish before continuing.
Is this correct or how does it really work?
How is this paragraph wrong? Let us count the ways:
For any task that doesn’t involve the UI, you can use global
concurrent queues in GCD.
This is overly specific and inaccurate. Certain UI centric tasks, such as loading images, could be done off the main thread. This would be better said as "In most cases, don't interact with UIKit classes except from the main thread," but there are exceptions (for instance, drawing to a UIGraphicsContext is thread-safe as of iOS 4, IIRC, and drawing is a great example of a CPU intensive task that could be offloaded to a background thread.) FWIW, any work unit you can submit to a global concurrent queue you can also submit to a private concurrent queue too.
These allow either synchronous or asynchronous execution. But
synchronous execution does not mean your program waits for the code to
finish before continuing. It simply means that the concurrent queue
will wait until your task has finished before it continues to the next
block of code on the queue.
As iWasRobbed speculated, they appear to have conflated sync/async work submission with serial/concurrent queues. Synchronous execution does, by definition, mean that your program waits for the code to return before continuing. Asynchronous execution, by definition, means that your program does not wait. Similarly, serial queues only execute one submitted work unit at a time, executing each in FIFO order. Concurrent queues, private or global, in the general case (more in a second), schedule submitted blocks for execution, in the order in which they were enqueued, on one or more background threads. The number of background threads used is an opaque implementation detail.
When you put a block object on a concurrent queue, your own program
always continues right away without waiting for the queue to execute
the code.
Nope. Not true. Again, they're mixing up sync/async and serial/concurrent. I suspect what they're trying to say is: When you enqueue a block asynchronously, your own program always continues right away without waiting for the queue to execute the code.
This is because concurrent queues, as their name implies, run their code on threads other than the main thread.
This is also not correct. For instance, if you have a private concurrent queue that you are using to act as a reader/writer lock that protects some mutable state, if you dispatch_sync to that queue from the main thread, your code will, in many cases, execute on the main thread.
Overall, this whole paragraph is really pretty horrible and misleading.
EDIT: I mentioned this in a comment on another answer, but it might be helpful to put it here for clarity. The concept of "Synchronous vs. Asynchronous dispatch" and the concept of "Serial vs. Concurrent queues" are largely orthogonal. You can dispatch work to any queue (serial or concurrent) in either a synchronous or asynchronous way. The sync/async dichotomy is primarily relevant to the "dispatch*er*" (in that it determines whether the dispatcher is blocked until completion of the block or not), whereas the Serial/Concurrent dichotomy is primarily relevant to the dispatch*ee* block (in that it determines whether the dispatchee is potentially executing concurrently with other blocks or not).
I think that bit of text is poorly written, but they are basically explaining the difference between execution on a serial queue with execution on a concurrent queue. A serial queue is run on one thread so it doesn't have a choice but to execute one task at a time, whereas a concurrent queue can use one or more threads.
Serial queue's execute one task after the next in the order in which they were put into the queue. Each task has to wait for the prior task to be executed before it can then be executed (i.e. synchronously).
In a concurrent queue, tasks can be run at the same time that other tasks are also run since they normally use multiple threads (i.e. asynchronously), but again they are still executed in the order they were enqueued and they can effectively be finished in any order. If you use NSOperation, you can also set up dependencies on a concurrent queue to ensure that certain tasks are executed before other tasks are.
More info:
https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html
The author is Vandad Nahavandipoor, I don't want to affect this guy's sales income, but all his books contain the same mistakes in the concurrency chapters:
http://www.amazon.com/Vandad-Nahavandipoor/e/B004JNSV7I/ref=sr_tc_2_rm?qid=1381231858&sr=8-2-ent
Which is ironic since he's got a 50-page book exactly on this subject.
http://www.amazon.com/Concurrent-Programming-Mac-iOS-Performance/dp/1449305636/ref=la_B004JNSV7I_1_6?s=books&ie=UTF8&qid=1381232139&sr=1-6
People should STOP reading this guy's books.
When you put a block object on a concurrent queue, your own program
always continues right away without waiting for the queue to execute
the code. This is because concurrent queues, as their name implies,
run their code on threads other than the main thread.
I find it confusing, and the only explanation I can think of, is that she is talking about who blocks who. From man dispatch_sync:
Conceptually, dispatch_sync() is a convenient wrapper around
dispatch_async() with the addition of a semaphore to wait for
completion of the block, and a wrapper around the block to signal its
completion.
So execution returns to your code right away, but the next thing the dispatch_sync does after queueing the block, is wait on a semaphore until the block is executed. Your code blocks because it chooses to.
The other way your code would block is when the queue chooses to run a block using your thread (the one from where you executed dispatch_sync). In this case, your code wouldn't recover control until the block is executed, so the check on the semaphore would always find the block is done.
Erica Sadun for sure knows better than me, so maybe I'm missing some nuance here, but this is my understanding.