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I'm learning about concurrent programming for iOS. So far I've read about NSOperation/NSOperationQueue and GCD. What are the reasons for using NSOperationQueue over GCD and vice versa?
Sounds like both GCD and NSOperationQueue abstract away the explicit creation of NSThreads from the user. However the relationship between the two approaches isn't clear to me so any feedback to appreciated!
GCD is a low-level C-based API that enables very simple use of a task-based concurrency model. NSOperation and NSOperationQueue are Objective-C classes that do a similar thing. NSOperation was introduced first, but as of 10.5 and iOS 2, NSOperationQueue and friends are internally implemented using GCD.
In general, you should use the highest level of abstraction that suits your needs. This means that you should usually use NSOperationQueue instead of GCD, unless you need to do something that NSOperationQueue doesn't support.
Note that NSOperationQueue isn't a "dumbed-down" version of GCD; in fact, there are many things that you can do very simply with NSOperationQueue that take a lot of work with pure GCD. (Examples: bandwidth-constrained queues that only run N operations at a time; establishing dependencies between operations. Both very simple with NSOperation, very difficult with GCD.) Apple's done the hard work of leveraging GCD to create a very nice object-friendly API with NSOperation. Take advantage of their work unless you have a reason not to.
Caveat:
On the other hand, if you really just need to send off a block, and don't need any of the additional functionality that NSOperationQueue provides, there's nothing wrong with using GCD. Just be sure it's the right tool for the job.
In line with my answer to a related question, I'm going to disagree with BJ and suggest you first look at GCD over NSOperation / NSOperationQueue, unless the latter provides something you need that GCD doesn't.
Before GCD, I used a lot of NSOperations / NSOperationQueues within my applications for managing concurrency. However, since I started using GCD on a regular basis, I've almost entirely replaced NSOperations and NSOperationQueues with blocks and dispatch queues. This has come from how I've used both technologies in practice, and from the profiling I've performed on them.
First, there is a nontrivial amount of overhead when using NSOperations and NSOperationQueues. These are Cocoa objects, and they need to be allocated and deallocated. In an iOS application that I wrote which renders a 3-D scene at 60 FPS, I was using NSOperations to encapsulate each rendered frame. When I profiled this, the creation and teardown of these NSOperations was accounting for a significant portion of the CPU cycles in the running application, and was slowing things down. I replaced these with simple blocks and a GCD serial queue, and that overhead disappeared, leading to noticeably better rendering performance. This wasn't the only place where I noticed overhead from using NSOperations, and I've seen this on both Mac and iOS.
Second, there's an elegance to block-based dispatch code that is hard to match when using NSOperations. It's so incredibly convenient to wrap a few lines of code in a block and dispatch it to be performed on a serial or concurrent queue, where creating a custom NSOperation or NSInvocationOperation to do this requires a lot more supporting code. I know that you can use an NSBlockOperation, but you might as well be dispatching something to GCD then. Wrapping this code in blocks inline with related processing in your application leads in my opinion to better code organization than having separate methods or custom NSOperations which encapsulate these tasks.
NSOperations and NSOperationQueues still have very good uses. GCD has no real concept of dependencies, where NSOperationQueues can set up pretty complex dependency graphs. I use NSOperationQueues for this in a handful of cases.
Overall, while I usually advocate for using the highest level of abstraction that accomplishes the task, this is one case where I argue for the lower-level API of GCD. Among the iOS and Mac developers I've talked with about this, the vast majority choose to use GCD over NSOperations unless they are targeting OS versions without support for it (those before iOS 4.0 and Snow Leopard).
GCD is a low-level C-based API.
NSOperation and NSOperationQueue are Objective-C classes.
NSOperationQueue is objective C wrapper over GCD.
If you are using NSOperation, then you are implicitly using Grand Central Dispatch.
GCD advantage over NSOperation:
i. implementation
For GCD implementation is very light-weight
NSOperationQueue is complex and heavy-weight
NSOperation advantages over GCD:
i. Control On Operation
you can Pause, Cancel, Resume an NSOperation
ii. Dependencies
you can set up a dependency between two NSOperations
operation will not started until all of its dependencies return true for finished.
iii. State of Operation
can monitor the state of an operation or operation queue.
ready ,executing or finished
iv. Max Number of Operation
you can specify the maximum number of queued operations that can run simultaneously
When to Go for GCD or NSOperation
when you want more control over queue (all above mentioned) use NSOperation
and for simple cases where you want less overhead
(you just want to do some work "into the background" with very little additional work) use GCD
ref:
https://cocoacasts.com/choosing-between-nsoperation-and-grand-central-dispatch/
http://iosinfopot.blogspot.in/2015/08/nsthread-vs-gcd-vs-nsoperationqueue.html
http://nshipster.com/nsoperation/
Another reason to prefer NSOperation over GCD is the cancelation mechanism of NSOperation. For example, an App like 500px that shows dozens of photos, use NSOperation we can cancel requests of invisible image cells when we scroll table view or collection view, this can greatly improve App performance and reduce memory footprint. GCD can't easily support this.
Also with NSOperation, KVO can be possible.
Here is an article from Eschaton which is worth reading.
GCD is indeed lower-level than NSOperationQueue, its major advantage is that its implementation is very light-weight and focused on lock-free algorithms and performance.
NSOperationQueue does provide facilities that are not available in GCD, but they come at non-trivial cost, the implementation of NSOperationQueue is complex and heavy-weight, involves a lot of locking, and uses GCD internally only in a very minimal fashion.
If you need the facilities provided by NSOperationQueue by all means use it, but if GCD is sufficient for your needs, I would recommend using it directly for better performance, significantly lower CPU and power cost and more flexibility.
Both NSQueueOperations and GCD allow executing heavy computation task in the background on separate threads by freeing the UI Application Main Tread.
Well, based previous post we see NSOperations has addDependency so that you can queue your operation one after another sequentially.
But I also read about GCD serial Queues you can create run your operations in the queue using dispatch_queue_create. This will allow running a set of operations one after another in a sequential manner.
NSQueueOperation Advantages over GCD:
It allows to add dependency and allows you to remove dependency so for one transaction you can run sequential using dependency and for other transaction run concurrently while GCD
doesn't allow to run this way.
It is easy to cancel an operation if it is in the queue it can be stopped if it is running.
You can define the maximum number of concurrent operations.
You can suspend operation which they are in Queue
You can find how many pending operations are there in queue.
GCD is very easy to use - if you want to do something in the background, all you need to do is write the code and dispatch it on a background queue. Doing the same thing with NSOperation is a lot of additional work.
The advantage of NSOperation is that (a) you have a real object that you can send messages to, and (b) that you can cancel an NSOperation. That's not trivial. You need to subclass NSOperation, you have to write your code correctly so that cancellation and correctly finishing a task both work correctly. So for simple things you use GCD, and for more complicated things you create a subclass of NSOperation. (There are subclasses NSInvocationOperation and NSBlockOperation, but everything they do is easier done with GCD, so there is no good reason to use them).
Well, NSOperations are simply an API built on top of Grand Central Dispatch. So when you’re using NSOperations, you’re really still using Grand Central Dispatch.
It’s just that NSOperations give you some fancy features that you might like. You can make some operations dependent on other operations, reorder queues after you sumbit items, and other things like that.
In fact, ImageGrabber is already using NSOperations and operation queues! ASIHTTPRequest uses them under the hood, and you can configure the operation queue it uses for different behavior if you’d like.
So which should you use? Whichever makes sense for your app. For this app it’s pretty simple so we just used Grand Central Dispatch directly, no need for the fancy features of NSOperation. But if you need them for your app, feel free to use it!
I agree with #Sangram and other answers but want to add few points. Correct me if I am wrong.
I think now a days first two points of #Sangram's answer are not valid (i. Control On Operation ii. Dependencies). We can achieve these two by using GCD also. Trying to explain by code(do not focus on quality of code, this is for reference purpose only)
func methodsOfGCD() {
let concurrentQueue = DispatchQueue.init(label: "MyQueue", qos: .background, attributes: .concurrent)
//We can suspend and resume Like this
concurrentQueue.suspend()
concurrentQueue.resume()
//We can cancel using DispatchWorkItem
let workItem = DispatchWorkItem {
print("Do something")
}
concurrentQueue.async(execute: workItem)
workItem.cancel()
//Cam add dependency like this.
//Operation 1
concurrentQueue.async(flags: .barrier) {
print("Operation1")
}
//Operation 2
concurrentQueue.async(flags: .barrier) {
print("Operation2")
}
//Operation 3.
//Operation 3 have dependency on Operation1 and Operation2. Once 1 and 2 will finish will execute Operation 3. Here operation queue work as a serial queue.
concurrentQueue.async(flags: .barrier) {
print("Operation3")
}
}
I am making an app for website. I use JSON to get data. I want to load all posts in threads (1 post - 1 thread). How many threads I can make? Should I control the number of threads?
With Cocoa you usually don't work with Threads directly. Grand Central Dispatch (GCD) is an API that handles this for your. You just have to partition your task into small managable chunks, dispatch them onto a background queue and the rest is handled for you. You don't need to worry about creating threads, how many are currently running etc. When you dispatch enough work on one (or possibly more) queues, the CPU is going to run at maximum load.
You can also use NSOperationQueue, which has the ability to throttle the execution to some extend or cancel currently running tasks (not possible with GCD).
Unless you are doing anything unusual there is no need to use NSThread directly. Use GCD when you just need to perform a simple small task asynchronously. Use NSOperationQueue when you need more control, like cancelling submitted tasks or setting priorities. It's API is also a bit higher level and in Objective-C. GCD is a C level API, so for example it can't catch ObjC-Exceptions. NSOperationQueue uses GCD internally, so both should work equally well.
I just start using NSOperation and NSOperationQueue. I really want to know what should be better practice for using them.
Should I create a only one queue for whole app, or should I create one queue for each specific type of task? What is the effect if I create so many queues?
It depends on what you're using them for. If you're using them for performance, maxing out the CPU, then there is little point going much beyond how many cores you expect to have.
If you're offloading processing just to keep the UI responsive, but you're not particularily concerned about maxing out performance, I'd just use the default background queue for simplicity.
If you're using sequential background queues to trigger actions on external events ( network request completion, for example), then it doesn't do any harm to use a bunch, as they won't lead to a lot of threads trying to run concurrently.
There are many instances where you can simplify the design using an operation queue.
NSOperationQueues are built on top of GCD, and AFAIK, there is no performance impact of having loads of queues, as long as they're not trying to execute concurrently.
Read up on the Executing Operations section of the Apple Concurrency Guide. It depends of course on your implementation and application but in summary ther can only be a certain amount of NSOperations running concurrently and increasing the number of NSOperationQueues won't serve any benefit to you. So my recommendation would be to opt for 1 or fewer queues if your design can allow it.
I'm learning about concurrent programming for iOS. So far I've read about NSOperation/NSOperationQueue and GCD. What are the reasons for using NSOperationQueue over GCD and vice versa?
Sounds like both GCD and NSOperationQueue abstract away the explicit creation of NSThreads from the user. However the relationship between the two approaches isn't clear to me so any feedback to appreciated!
GCD is a low-level C-based API that enables very simple use of a task-based concurrency model. NSOperation and NSOperationQueue are Objective-C classes that do a similar thing. NSOperation was introduced first, but as of 10.5 and iOS 2, NSOperationQueue and friends are internally implemented using GCD.
In general, you should use the highest level of abstraction that suits your needs. This means that you should usually use NSOperationQueue instead of GCD, unless you need to do something that NSOperationQueue doesn't support.
Note that NSOperationQueue isn't a "dumbed-down" version of GCD; in fact, there are many things that you can do very simply with NSOperationQueue that take a lot of work with pure GCD. (Examples: bandwidth-constrained queues that only run N operations at a time; establishing dependencies between operations. Both very simple with NSOperation, very difficult with GCD.) Apple's done the hard work of leveraging GCD to create a very nice object-friendly API with NSOperation. Take advantage of their work unless you have a reason not to.
Caveat:
On the other hand, if you really just need to send off a block, and don't need any of the additional functionality that NSOperationQueue provides, there's nothing wrong with using GCD. Just be sure it's the right tool for the job.
In line with my answer to a related question, I'm going to disagree with BJ and suggest you first look at GCD over NSOperation / NSOperationQueue, unless the latter provides something you need that GCD doesn't.
Before GCD, I used a lot of NSOperations / NSOperationQueues within my applications for managing concurrency. However, since I started using GCD on a regular basis, I've almost entirely replaced NSOperations and NSOperationQueues with blocks and dispatch queues. This has come from how I've used both technologies in practice, and from the profiling I've performed on them.
First, there is a nontrivial amount of overhead when using NSOperations and NSOperationQueues. These are Cocoa objects, and they need to be allocated and deallocated. In an iOS application that I wrote which renders a 3-D scene at 60 FPS, I was using NSOperations to encapsulate each rendered frame. When I profiled this, the creation and teardown of these NSOperations was accounting for a significant portion of the CPU cycles in the running application, and was slowing things down. I replaced these with simple blocks and a GCD serial queue, and that overhead disappeared, leading to noticeably better rendering performance. This wasn't the only place where I noticed overhead from using NSOperations, and I've seen this on both Mac and iOS.
Second, there's an elegance to block-based dispatch code that is hard to match when using NSOperations. It's so incredibly convenient to wrap a few lines of code in a block and dispatch it to be performed on a serial or concurrent queue, where creating a custom NSOperation or NSInvocationOperation to do this requires a lot more supporting code. I know that you can use an NSBlockOperation, but you might as well be dispatching something to GCD then. Wrapping this code in blocks inline with related processing in your application leads in my opinion to better code organization than having separate methods or custom NSOperations which encapsulate these tasks.
NSOperations and NSOperationQueues still have very good uses. GCD has no real concept of dependencies, where NSOperationQueues can set up pretty complex dependency graphs. I use NSOperationQueues for this in a handful of cases.
Overall, while I usually advocate for using the highest level of abstraction that accomplishes the task, this is one case where I argue for the lower-level API of GCD. Among the iOS and Mac developers I've talked with about this, the vast majority choose to use GCD over NSOperations unless they are targeting OS versions without support for it (those before iOS 4.0 and Snow Leopard).
GCD is a low-level C-based API.
NSOperation and NSOperationQueue are Objective-C classes.
NSOperationQueue is objective C wrapper over GCD.
If you are using NSOperation, then you are implicitly using Grand Central Dispatch.
GCD advantage over NSOperation:
i. implementation
For GCD implementation is very light-weight
NSOperationQueue is complex and heavy-weight
NSOperation advantages over GCD:
i. Control On Operation
you can Pause, Cancel, Resume an NSOperation
ii. Dependencies
you can set up a dependency between two NSOperations
operation will not started until all of its dependencies return true for finished.
iii. State of Operation
can monitor the state of an operation or operation queue.
ready ,executing or finished
iv. Max Number of Operation
you can specify the maximum number of queued operations that can run simultaneously
When to Go for GCD or NSOperation
when you want more control over queue (all above mentioned) use NSOperation
and for simple cases where you want less overhead
(you just want to do some work "into the background" with very little additional work) use GCD
ref:
https://cocoacasts.com/choosing-between-nsoperation-and-grand-central-dispatch/
http://iosinfopot.blogspot.in/2015/08/nsthread-vs-gcd-vs-nsoperationqueue.html
http://nshipster.com/nsoperation/
Another reason to prefer NSOperation over GCD is the cancelation mechanism of NSOperation. For example, an App like 500px that shows dozens of photos, use NSOperation we can cancel requests of invisible image cells when we scroll table view or collection view, this can greatly improve App performance and reduce memory footprint. GCD can't easily support this.
Also with NSOperation, KVO can be possible.
Here is an article from Eschaton which is worth reading.
GCD is indeed lower-level than NSOperationQueue, its major advantage is that its implementation is very light-weight and focused on lock-free algorithms and performance.
NSOperationQueue does provide facilities that are not available in GCD, but they come at non-trivial cost, the implementation of NSOperationQueue is complex and heavy-weight, involves a lot of locking, and uses GCD internally only in a very minimal fashion.
If you need the facilities provided by NSOperationQueue by all means use it, but if GCD is sufficient for your needs, I would recommend using it directly for better performance, significantly lower CPU and power cost and more flexibility.
Both NSQueueOperations and GCD allow executing heavy computation task in the background on separate threads by freeing the UI Application Main Tread.
Well, based previous post we see NSOperations has addDependency so that you can queue your operation one after another sequentially.
But I also read about GCD serial Queues you can create run your operations in the queue using dispatch_queue_create. This will allow running a set of operations one after another in a sequential manner.
NSQueueOperation Advantages over GCD:
It allows to add dependency and allows you to remove dependency so for one transaction you can run sequential using dependency and for other transaction run concurrently while GCD
doesn't allow to run this way.
It is easy to cancel an operation if it is in the queue it can be stopped if it is running.
You can define the maximum number of concurrent operations.
You can suspend operation which they are in Queue
You can find how many pending operations are there in queue.
GCD is very easy to use - if you want to do something in the background, all you need to do is write the code and dispatch it on a background queue. Doing the same thing with NSOperation is a lot of additional work.
The advantage of NSOperation is that (a) you have a real object that you can send messages to, and (b) that you can cancel an NSOperation. That's not trivial. You need to subclass NSOperation, you have to write your code correctly so that cancellation and correctly finishing a task both work correctly. So for simple things you use GCD, and for more complicated things you create a subclass of NSOperation. (There are subclasses NSInvocationOperation and NSBlockOperation, but everything they do is easier done with GCD, so there is no good reason to use them).
Well, NSOperations are simply an API built on top of Grand Central Dispatch. So when you’re using NSOperations, you’re really still using Grand Central Dispatch.
It’s just that NSOperations give you some fancy features that you might like. You can make some operations dependent on other operations, reorder queues after you sumbit items, and other things like that.
In fact, ImageGrabber is already using NSOperations and operation queues! ASIHTTPRequest uses them under the hood, and you can configure the operation queue it uses for different behavior if you’d like.
So which should you use? Whichever makes sense for your app. For this app it’s pretty simple so we just used Grand Central Dispatch directly, no need for the fancy features of NSOperation. But if you need them for your app, feel free to use it!
I agree with #Sangram and other answers but want to add few points. Correct me if I am wrong.
I think now a days first two points of #Sangram's answer are not valid (i. Control On Operation ii. Dependencies). We can achieve these two by using GCD also. Trying to explain by code(do not focus on quality of code, this is for reference purpose only)
func methodsOfGCD() {
let concurrentQueue = DispatchQueue.init(label: "MyQueue", qos: .background, attributes: .concurrent)
//We can suspend and resume Like this
concurrentQueue.suspend()
concurrentQueue.resume()
//We can cancel using DispatchWorkItem
let workItem = DispatchWorkItem {
print("Do something")
}
concurrentQueue.async(execute: workItem)
workItem.cancel()
//Cam add dependency like this.
//Operation 1
concurrentQueue.async(flags: .barrier) {
print("Operation1")
}
//Operation 2
concurrentQueue.async(flags: .barrier) {
print("Operation2")
}
//Operation 3.
//Operation 3 have dependency on Operation1 and Operation2. Once 1 and 2 will finish will execute Operation 3. Here operation queue work as a serial queue.
concurrentQueue.async(flags: .barrier) {
print("Operation3")
}
}
This question already has answers here:
NSOperation vs Grand Central Dispatch
(9 answers)
Closed 9 years ago.
Which tasks would be better suited to using NSOperation as opposed to using GCD when programming for the iPhone?
To me they seem to do the same thing. I can't see the strengths and weaknesses one has over the other.
NSOperation is built on top of GCD, so the question is more about whether you use NSOperation or pass a block directly to GCD.
An NSOperation is bulky and needs more boiler-plate codes to set it up, but it has a lot more functionality. You can create the same NSOperation subclass in various parts of your code and put them into the queue and run it.
Passing a block to GCD by e.g. dispatch_async is quick and disposable. You typically don't reuse a block anywhere else; you just set up a block which is executed only at that point of the code, passes it to the GCD or other APIs, and quickly go on.
So each has its merits.
Apparently, NSOperationQueue is built on GCD as of iOS 4; the docs just haven't been updated. Check this posting by an Apple employee here: https://devforums.apple.com/message/352770 (You may need to create an account) So, you should follow Mike Abdullah's advice and use the simplest API for the task at hand. dispatch_async is lower level, usually C-type stuff (but not limited to), and is good for one-shot and sequential type deals (fire this block on this queue, FTW). NSOperationQueues are higher level, Objective-C stuff, and are good if you are adding a lot of operations at various points in your code, and/or need to manage concurrency, priorities and dependencies. At least that's how I use them.
As always with such questions, use the simplest API available. Measure if it's a performance problem and then reevaluate if needed.
One thing that I don't believe has been mentioned here is that NSOperations can be cancelled during execution, whereas a block is guaranteed to complete once execution has begun. Having said that, a GCD queue can be suspended (dispatch_suspend()), so that any blocks following the currently executing blocks will not be executed.