Power, Performance and Diagnostics: What’s new in GCD and XPC 

Session 716 WWDC 2014

Learn about new features in GCD and XPC that help you write responsive energy-efficient apps and diagnose their interactions with the system.

Welcome to Power, Performance and Diagnostics: What’s new in GCD and XPC.

I’m Daniel Steffen, I’m one of the Engineers responsible for GCD and XPC in Core OS, and today we’ll go over some background.

Some a new concept called Quality of Service Classes that we’re introducing this year, the new APIs associated to that, and the concept of propagation of this quality of service and execution context across threads and processes, and finally, some pointers to great new features around diagnostics and queue debugging this year.

So GCD, for those who might be new to the topic even though given the number of people, maybe everybody knows about it [laughter], GCD is a low-level frame maker on asynchronous execution, concurrent execution, and synchronization.

Today, we are mostly going to be focusing on the asynchronous execution aspect of it, and you can think of asynchronous execution with GCD as a way to run code in a separate environment in your process.

The reasons you might want to do that are things like avoid interfering with the current thread, a typical example would be the main thread of your application, or execute at a different priority level, which is something we’ll talk a lot more about in this session, or coordination between multiple clients in the process.

This leads us to XPC, which is our low level IPC framework on the system, and that can be thought of as a way to asynchronously execute code in a separate process, for which you might do for very similar reasons: avoid interfering with the current process as a whole, say if you’re running on un-trusted data that you might not want to crash the main application for if it goes wrong, or you might need to run at a different privilege level say in a different sandbox, and maybe you need to coordinate with multiple clients if you’re writing a Daemon on OS X.

And that’s really all I’m going to go over in terms of background for these two topics.

This is sort of a “what’s new session” this year, so I here are a number of sessions from past years if you’re new to this technology or to the platforms.

That will get you up to speed, you should be able to see all of those in your WWDC app or on the developer website.

So let’s take a step back and think about what our goal should be as application developers.

We’ll see one of the primary goals is to provide the best user experience for the person using the device.

What do they care about?

The frontmost app and its user interface, that that be as responsive as possible.

What do you need to provide on the system as application developers to make this possible?

This responsive user interface well, there must be enough resources available so that the main thread of the frontmost app, which is where all the UI event handling and UI drawing occurs, can proceed unimpeded, as well as all the associated infrastructure that is involved in pushing pixels to the screen or getting events from the input devices.

Other work that is not directly related to this task should execute off the main thread of the application independently off the main thread, and ideally at lower-priority.

Let’s talk about priorities.

Very generically priorities are a mechanism to resolve resource contention on the system.

The idea is that under contention, the high priorities win, but if there’s no contention, the low priorities aren’t really a restriction they will proceed normally.

So an example of that that you are probably familiar with is scheduling priority.

This is something you can set on your threads that tells the Kernel scheduler how you would like access to the CPU prioritized, and the idea is that under contention, high priorities get to the CPU first.

But even if you set low-priority there’s no restriction to your execution if they’re no contention, but then if something high-priority comes along like a UI action, then you might not run for a period of time if you’re at low-priority.

Similar concept for I/O that we’ve had for a long time that you might be familiar with on the GDC background queue, the I/O that you perform on that queue are tagged as low-priority.

Again, this is no restriction if there’s no high-priority I/O present, it will just proceed normally in that case.

But if there is say, the main thread of an application loading an image for display in the UI, if such high-priority I/O is present, the low-priority I/O will be deprioritized.

But it turns out that our system actually has many other resource controls of this type, and to configure all of this correctly is very complex and a lot of knobs involved.

There isn’t really any unified approach for you to know what settings that should be used in all of these cases, and no good way for you to tell the system your intent behind setting specific configuration values, and this is what we wanted to address this year with the introduction of Quality of Service Classes.

Quality of Service Classes are a new concept whose goal is to allow you the developer to communicate intent to the system by giving an explicit classification of work that your application performs with a single abstract parameter, and move away from this situation of having to dictate very specific configuration values for all the possible things that you could configure.

Among the effects of setting Quality of Service, the two we talked about, CPU scheduling priority and I/O priority, but also configuration of timer coalescing and hints to the CPU that it should prefer throughput versus more energy efficient modes of execution, and potentially more parameters today or in the future that you don’t need to know about as an application developer or can’t even know about yet because they don’t exist yet.

In particular, we might be tuning these configuration values that are actually used underneath the colors differently for different platforms of different devices, but you don’t have to know about that.

You can just specify this abstract parameter.

So the core Quality of Service Classes we’re introducing are user-interactive, user-initiated, utility, and background.

I’ll go through each of those in turn.

User-Interactive is the quality of service of the main thread of the application, and we set that up for you automatically.

It’s should be used for anything that’s directly involved in event handling, UI drawing, and anything of that nature, but overall for an application we expect that this should be a small fraction of the total work that an application does, especially in the case where the user isn’t directly interacting with the application.

The User-Initiated Quality of Service class is intended for work that’s asynchronous to the UI but directly UI-initiated, or anything that a user is waiting for immediate results.

This could be things that are required for the user to be able to continue his interaction with the current action that he’s doing in the UI.

Anything that is not of that nature should run at the lower Quality of Service class like Utility which is intended for long running tasks but user visible progress such as a long learning computation, intensive I/O, or networking, but anything that really feeds data to the user interface on a long running basis.

You might also put things like getting ready for the next UI request if you can confidently predict that that will be needed very soon, that Quality of Service.

But this is already one of the energy efficient Quality of Service Classes, so it’s important to put as much work as feasible in your application at this Quality of Service Class or lower in order to maximize your user’s battery life.

The next level is Background.

That is intended for work that the user is unaware of, that the work is currently occurring.

He may have opted into the performance of that work in the past saying like, “I want to have an hourly backup occurring,” but he doesn’t see that occurring currently when it’s going on.

Anything that might be prefetching, opportunistic prefetching of data, or that might be work that could be deferrable for long periods of time or just generally maintenance or cleanup where I can tunnel to an application.

So how do you go about choosing one of these Quality of Service classes?

There’s a couple of questions you can ask yourself that will help with that.

Going through the list for User-Interactive, you should ask yourself, “Is this work actively involved in updating the UI?”

If that’s not the case it probably shouldn’t run at this Quality of Service.

For User Initiated, similarly, is this work required to continue the user interaction?

Like we said, if that’s not the case, this is not the right level.

Utility, the question is, is the user aware of the progress of the work?

So, there’s exceptions to this but that’s typically the criteria for being at this level, and for Background, should you be at Background or not.

Can this work be deferred to start at a better time?

If the answer to that question is, yes, then you probably shouldn’t actually be scheduling that work right now.

You should be using an alternative mechanism to start the work in the first place like this background activity scheduler.

For more on that, please see the Writing Energy Efficient Code, Part one session from yesterday.

So once you’ve picked one of these Quality of Service classes, say User-Initiated, another way to think about your choices are to compare with the other classes above and below you.

So, you could ask questions like, Is it okay for user-interactive work to happen before my work at User-Initiated, or, is it okay for my work at User-Initiated to compete with other work at User-Initiated Quality of Service?

If the answer to that question is, no, then you should probably move on below User-Initiated, and similarly, is it okay for my work to take precedence over work at Utility Quality of Service?

So to recap this section, we talked about the facilities we have and need for being able to provide a responsive user interface, particularly asynchronous execution, at the correct priority, but it wasn’t really very easy until now to express your intent as far as priority is concerned, and that we were addressing that with the Quality of Service Classes which provide an explicit classification of work for you and we talked about the questions you can ask yourself to choose the right QoS Class.

So let’s look at the Quality of Service Class API that you’ll be writing code with.

You can provide Quality of Service Classes at a number of levels in the system, starting with threads if you use manually-created NSThreads or Pthreads, you can provide Quality of Service Class on those at creation.

We won’t talk about this in detail here, but it’s pretty simple.

You can look that up in the documentation.

We’ll talk about how to provide Quality of Service on dispatch queues and dispatch blocks, and yesterday’s session on writing energy efficient code talked about how to provide Quality of Service on NSOperation queue and NSOperation.

In rare cases, it’s also useful to provide Quality of Service on processes at a whole.

Again, that’s something that you can look up in the documentation.

So here under Quality of Service Class Constants that we’ve provided in the headers that will pass through the APIs, these are the four classes we talked about.

The sys/qos.h header has constants that you typically use at the lower level APIs, and foundation.h has a coolant, and in fact, interchangeable constants for user at the NS APIs.

But the QS.h header has two additional values that we’ll talk about right now.

The QoS Class default is a class that fits in the middle between User-Interactive the UI classes, rather, and the non UI classes, and this is what we use when we have no more specific QoS information.

For instant, for a thread that was created without any specific QoS, it will run at the default Quality of Service.

Similarly, for the PCD Global Default Queue, that runs at Quality of Service default.

It’s not in itself intended as a work classification for you to use to specify intent, it’s just so you shouldn’t typically set it, except maybe when you’re resetting to a previous state or maybe propagating a state from one place to another.

The other special value is QoS underscore Class underscore Unspecified.

This isn’t an actually class, this is the absence of QoS information, the nil value if you will, and this indicates to us that maybe they should be inferring the Quality of Service from a different place like the work origin.

It is also something that you’ll might see returned from the thread header APIs if a thread was opted out of Quality of Service by use of a legacy API.

These are things that might manipulate these underlying knobs directly that we talked about that then become incompatible with the Quality of Service unified concept in which case, so this would be things like skipparam, in which case we will opt out the thread out of Quality of Service and you will see this value as the one returned from the current QoS.

In addition to the classes, we also provide you an initial parameter that indicates relative position within a QoS class band, or relative priority.

So rather than having five discreet classes, we can think of QoS really as a set of five priority bands where you can position yourself inside one of these bands, and you can only lower yourself from the default.

So, you can provide a value between minus 15 and zero to position yourself lower than most other people in that band, and it’s really only intended for unusual situations.

We expect that in most cases, the zero value default zero value will be perfectly sufficient, but if you have special situations like interdependent work at the same Quality of Service class that needs slightly differing priority or produce a consumer scenario so one or other side might need to be slightly high-priority to get a good flow, this is the tool for that.

Now let’s talk about the API you’ll use with threads.

As mentioned, QoS is kind of a thread specific concept, and you can get the QoS class off the currently running thread with the qos class self function.

This will return what the thread is currently running at.

This is not only in the cases of manually-created threads, but if work starts running where your specified QoS would, the GCD or NSOperation APIs, once it starts running the thread will have a QoS value and this is how you get it.

The other thread concept we have is the Initial QoS Class of the main thread.

This is something the system chooses for you when it brings up the main thread, and that’s selected depending on what kind of process you are.

If you’re an App, that will be the User-Interactive Quality of Service.

If you were an XPC service or Daemon, it will be the default Quality of Service, and because that can change later on if the main thread changes itself, then you can go back to that original value with this API.

For the APIs in GCD and QoS, let’s look at the existing global queues that we’ve had since the beginning, and you’ll see we’ll be mapping those to Quality of Service Classes.

So the main queue, which in an application maps through the main thread of the application obviously runs that User Interactive quality of service.

You know, mapping the high default and low queue to User Initiated, Default and Utility respectively, and the Background priority, concurrent queue is mapped to the background QoS class.

That one is pretty much a one-to-one mapping.

The others, it’s worth noting, are a slightly larger spread of behavior than what you’ve had before with high, default, and low, which were very similar.

So, this might be something to watch out for when you move up to current releases.

Getting a Global Queue with QoS directly is also easy.

Just use the existing dispatch to get global Queue API with the utility QoS constant, that’s the first constant in this example rather than the existing priority constants.

And this is really what we recommend you start doing from now on to be able to express that intent directly of what you want rather than take advantage of the compatibility mapping.

Once you have a queue, you can also ask for its QoS class with the dispatch queue get QoS class getter.

Not that QoS class is an immutable property of the queue that is specified when the queue is created.

For a queue that you create yourself, how do you do that?

With the dispatch queue adder make with QoS class API.

This will really turn an attribute for the QoS Class that you have requested, like Utility in this example, and you then pass that attribute to the dispatch you create API and get a Utility serial queue in this example.

Now let’s move onto a new concept that we’re introducing this year for QoS and other reasons called Dispatch Block Objects.

We’ve always had blocks in GCD as sort of a fundamental unit of work, of course.

We are enhancing that concept slightly this year with Dispatch Block Objects to allow you to configure properties of individual units of work on a queue directly.

And it will also allow you to address individual work units for the purposes of waiting for their completion or getting notified about their completion or being able to cancel them.

So, this is something that lots of people have requested over year that you be able to cancel blocks in the GCD queue.

Hopefully this helps out with that.

Otherwise, we are the goal was to integrate transparently with the existing API that we already had without having to introduce a lot of additional functionality.

So, the way we achieve that is by using the concept of a wrapper block.

You start with an existing GCD block of type dispatch block t which is that function at the right of a block that takes no arguments and returns no return value, and we wrap that in another block of the same type which contains these additional configuration parameters of QoS Class and Flags.

That operation creates a heap object of course, so this is really like similar to calling block copy on the nested block, so in a seed program you will have to call block release on the return object to get rid of it, or in Objective-C programs, send a release message or let arch do that for you.

Quick example of that API in action, we create a local variable of this type dispatch block t and send it to the result of the dispatch block create function.

Here passing no flags and just a block literal, and this is very similar to block create at this point, and then we can just pass that block object to the existing dispatch, async API and do some work while that is synchronous and this log is occurring, and finally maybe we need to wait on that result, so we call the dispatch wait API, passing in that block object directly and now we don’t need any additional setup to wait for the result to face [inaudible] like we might have in the patch with dispatch group or dispatch centerfolds.

And finally as mentioned, in a C program you have to block release that reference created by dispatch block create.

Second example here we use the dispatch block create with QoS Class API to create a block object that has a specific assigned Quality of Service Class that we want for just that block.

So here we’ve chosen Utility minus 8 derivative just as an example, and we pass, again, that block, to dispatch async, and maybe we do some work and then decide, “Oh we really didn’t need this Utility Quality of Service work at all,” so we then pass it to the block to dispatch cancel which will mark that block as cancelled and if it hasn’t started executing yet when it gets de-queued it will just return straight away.

So, this allows you to sort of take back the end queue that we thought in the past was not possible.

It’s important to note this cancellation is not preemptive.

It’s very similar to the dispatch source cancellation that we’ve had.

If the block is started, cancellation will not stop it from doing anything.

The block can check for cancellation on itself with the test cancel API of course.

Finally, last example here we’ll be showing the use of a flag when we call the dispatch block create API.

We’re using the detached flag here which is something that you might have heard about in the Activity Tracing Session if you went to that this morning.

It’s a concept of being able to disassociate that block that you’re going to schedule from what is currently going on in the thread that caused the dispatch block create for work that should not be correlated such as internal work to the application like clean caches in this example.

You know of course, we pass that again to dispatch async, and in this case we will use the dispatch notify API to schedule a notification block on the main queue to tell us when that clean cache block is completed.

This is very similar to the dispatch group notifier API that we’ve had.

Now that we’ve talked about the interaction at least talked about the various levels where you can specify Quality of Service, we have to talk about how they interact when you specify them at multiple levels at once, and for Asynchronous Blocks, the default behavior is that we will always prefer the Quality of Service Class of the queue if it has a Quality of Service Class.

Or if it doesn’t, we will look at the immediate target queue if that’s one of the global target queues with Quality of Service like the default sorry, like the high/low Background but not the default, or one of the ones that you specifically requested with Quality of Service.

In that case, we really use that as sort of a backwards compatibility method with the existing way to specify priority in GCD, or the target queue.

If you don’t have any of these two pieces of information, we will use the Block Quality of Service class if you’ve specified it with the creation API, or otherwise we will use Quality of Service inferred from the submitting thread.

What do we mean by that inferred QoS?

This is the Quality of Service that we captured at the time the block was submitted to the queue, so this is the Quality of Service that was active on the thread that called dispatch async at the time the block was submitted.

We will because this is an automatic mechanism, we will translate User Incorrective to User Initiated for you to make sure that you don’t propagate the main thread priority inadvertently to lots of places in the application.

But otherwise, if there’s no Quality of Service specified on the queue, we will use this mechanism.

This is intended for queues that might not have a specific identity that it can assign a Quality of Service to or that don’t really serve a single purpose where it is appropriate for the Quality of Service Class from the client, if you roll off the queue, to actually determine what you run it.

So things that mediate between many different clients would be a good candidate for that.

For synchronous blocks, the rules are slightly different, but you will default to the Quality of Service Class off the block if there’s such a thing, or otherwise use the one off the current thread.

This is very similar to what has always happened with this batch sync.

It actually executes the block that you pass on the calling thread itself.

Note that this will only ever raise the Quality of Service, so as not to prevent any work later on in the thread after the dispatch sync returns from making progress.

These are just default.

We also provide you explicit control over these options.

You can use the Dispatch Block Inherent QoS flag when you create a block a special block object to tell us to prefer, always prefer the QoS of the queue or the thread, or conversely to pass the Dispatch Block Enforce QoS Class flag so that we will always prefer the block’s Quality of Service even if we go to a queue that has a Quality of Service itself, but again, in these cases we only ever raise the Quality of Service to something higher.

Now that we’ve talked about all these different ways of introducing different priorities into your process, we have to talk about the priority inversions.

What is a priority inversion?”

In general, it’s just some situation where the progress of high-priority work depends on either the results of some low-priority work or a resource held by low-priority work.

And in the debugging scenario you would see this as high-priority threats that are either blocked or maybe even spinning or polling results from a low-priority thread that you might also see present.

So in a synchronous situation like that it would be high Quality of Service thread waiting on lower Quality of Service work.

We will actually try to resolve inversions in very specific cases for you, namely when you call dispatch sync and dispatch wait the block on a serial queue, or when you call pthread mutex lock or any facilities built on top it like NSLock.

In those cases, the system will try to raise the Quality of Service of the work that is being waited on to the Quality of Service of the waiter.

The asynchronous case is obviously also possible.

Say you have submitted a high Quality of Service block to a serial queue that was created with lower Quality of Service or that contains some blocks with lower Quality of Service earlier on.

Now this block is some high-priority work is backed up behind lower-priority work asynchronously.

In the case of a serial queue specifically again, the system will attempt to automatically resolve that for you by raising the Quality of Service of the queue temporarily until you have reached that high Quality of Service work.

But of course, rather than relying on the system to try and resolve these situations for you, it’s much better if you can avoid these inversions in the first place.

So if that’s possible, you should attempt to do that if you see that type of problem.

One technique is to decouple shared data between multiple priority levels as much as you can by using finer grade synchronization mechanisms, finer granularity, and move work outside of blocks or serial queues if that is possible.

Another technique is to prefer asynchronous execution over synchronous waiting, because synchronous waiting typically leads to chain soft waiters in situations like this where one guy is waiting on the next is waiting on something else, etcetera.

But in asynchronous execution, so now that is much easier to resolve.

And also something worth looking at here is spinning or polling for completion where my high quality of service thread might actually be, by doing that, holding off the low-priority work that it’s waiting for, and particularly look at for timer-based “synchronization” in quotes, which is some kind of checking for a result after an amount of time, that might not immediately appear to be a polling loop, but in fact is, especially if it’s some priority inversion situation.

So to recap this section, we talked about the QoS Class constants that you can use, the concept of relative Quality of Service priority, the APIs for queues and blocks, and the interaction of multiple Quality of Service specifications if you’ve given them to blocks of queues at the same time, along with what we do for priority inversions and what you can do to avoid them.

Our next section is about the Propagation of Execution Context.

What is Execution Context here, this is a set of thread-local attributes that the system maintains for you.

This includes the Activity ID that we heard about this morning in the Activity Tracing session that underlies the correlation aspect of Activity Tracing.

It also includes properties of the current IPC request if you say in a XPC Event Handler, such as the originator of a chain of IPC across multiple processes, or the importance of that originator, and we’ll talk more about that in a while.

The interesting thing about this Execution Context is that we automatically propagate it for you.

We propagate it across threads and processes with GCD, NSOperationQueue, and other Foundation APIs, and we propagate it across processes like XPC and other IPC APIs.

So an example of that graphically would be we have two queues here that are running with two different execution contexts, two different activity IDs as a proxy for one and two.

If you do a Dispatch Async from Q1 to Q3, that will transport that activity ID1 transparently for you to that other Q, or if that Q1 talks to a different processor at XPC we will transport that execution context across process, and then of course inside that process, we can continue to propagate, as well.

Now because this is automatic propagation, sometimes you may need to prevent that because it might be inappropriate in some situations, and that is where the DISPATCH BLOCK DETACHED flag to the dispatch block create API comes in.

This would be used for any type of work that’s asynchronous and to be disassociated from the principle activity that’s currently ongoing, so anything that is not directly related to say, the UI action that an application is undertaking right now, or in a Daemon.

You can’t IPC request if you have to do some related but work that may not be directly attributable to say, the IPC request.

A typical example of that is an asynchronous long running-cleanup or Daemon of that nature.

We have a couple of things that are detached by default.

The Dispatch source handlers, the blocks that specify as Dispatch source handlers, or the blocks that pass the dispatch after are detached by default.

So same animation as before.

The upper half, we have asynchronous propagation of the activities automatically, but say Q3 now discovers that it has to do some maintenance operation that really shouldn’t be associated to this activity.

It uses the Detached Block API to create a separate unit of work that is not related to this activity, which then maybe later on can create its own Activity ID that is separate from the one that it originated from.

We also provide facilities for you to manually propagate this execution context with the DISPATCH BLOCK ASSIGN CURRENT flag.

This assigns the current Quality of Service Class and Execution Context at the time you call the dispatch block create API to the block.

And this is particularly useful in cases where you want to store the block yourself in some data structure.

Say you have your own thread pool or your own threading model, and you then later on want to call that block on one of those threads and we can’t really make that connection that you transported work across threads for you in that case because we don’t understand that relationship.

Similarly, you might decide to later on submit a block to a dispatch queue but you want to capture this state that occurred when you stored the work.

For XPC, as mentioned, the propagation is automatic.

XPC connections propagate both Quality of Service Class and Execution Context automatically to the remote XPC Event Handler.

Worth noting that the capture of the current state happens when the Send operation occurs, so we call the XPC Connection Send API.

That’s what Message Send API, this is what that’s the point at which we captured that state, and note that XPC handlers prefer the propagated Quality of Service over that on the queues that they run on by default.

For XPC Services that you might be writing in OS X, we have talked about in the past about the concept of importance boosting in past years’ sessions.

This is still present this year, but it’s slightly changed.

We this is a mechanism that’s used to initially clamp the XPC Service process to Background Quality of Service and only unclamp it during the IPC with the UI process.

So this allows XPC services to have as little impact on the system as a whole when they’re not directly in use, and XPC manages the lifetime of that boost automatically for you until either the reply is sent or you’ve released the last reference of the XPC message that you’ve received.

Additionally this year, the lifetime of that boost will also be maintained while there is asynchronous work that was submitted from the context of the handler, the XPC Handler was submitted is ongoing, sorry.

So this is done right at propagation of execution context that contains this important state.

So this is typically what you want in a process if you’re creating asynchronous work that is related to the IPC request.

The process shouldn’t become clamped again until that work is done, but of course, it is also possible that you might have unrelated work generated there and in those cases you should make sure to use that Detached Block flag to submit that work, because otherwise you might be keeping the XPC service unboostered, unclamped, for a longer period of time.

Now to recap, we talked about execution context and the attributes that we track therein.

Automatic propagation of this context along with QoS and how you can control that propagation manually, as well as, the aspects pertaining to XPC propagation and importance boosting.

So finally, I want to give a shout out to a couple of very exciting new features that we’re introducing this year around Diagnostics of Asynchronous Code and Debugging of Asynchronous Code.

First off, the Xcode 6 CPU Report this is what you would use to diagnose or confirm that you have done your adoption of Quality of Service classes correctly.

If you stop in the debugger at the breakpoint say, you can click on the CPU gauge tab to get the CPU report, and here you can see the total amount of CPU used by the process in a graph.

And new this year for each of the threads involved we will also show you the Quality of Service of that thread that it is currently running at.

So in this example, if you want to confirm that you have correctly adopted Utility Quality of Service class, this would provide that confirmation since we see most of the CPU time in the overall graph is actually in that thread that was running at Utility Quality of Service.

Next up, Xcode 6 CPU debugging you may have seen this on Monday.

It’s a really exciting feature that hopefully will help out a lot with debugging asynchronous code.

Not only does the Xcode debugger now show back traces from the currently running code, which in the case of an asynchronously-executed block isn’t always as helpful as it could be.

It will also stitch in the back trace that was captured when the block was enqueued.

So this shows you a past historical back trace of when that block was submitted to the queue, and you can distinguish the two halves by seeing that the icons for the currently live back trace are colored and the historically-captured back traces from the enqueue event are colored on gray.

In addition to showing you currently running blocks, the queue view of the debugger can also show you the set of enqueued blocks on a queue.

So these are things that are not running yet but will be running on that queue in the future, which sometimes other source of something not occurring, and that can be really difficult to track down if you don’t have something like this.

So here, it will also show you how many pending blocks exist on this queue, and for each of the blocks if you just close the triangle we can see where that block was enqueued.

Finally, a really exciting feature we’re introducing this year called Activity Tracing that I’ve mentioned already.

It was covered this morning.

Just a quick reference, this will allow you to have additional information about asynchronous or past events in your crash reports.

You’re adding concept of bread crumb trails which are past high-level events that occurred leading up to the point of the crash, directing into your crash report, as well as the Activity ID.

This is the Activity ID that I was talking about before in being tracked in the execution context.

That is also available directly in your crash report along with the meta data associated to it, and in particular the most interesting part, trace messages for that activity scoped to that specific activity from both the crashing process and any processes that the activity has propagated to by this propagation of execution context that we talked about.

And this is also available directly in the debugger when, say in this case, you have crashed with the sync abort.

You can type the thread info command and see that there was an activity present, with its name and 5 messages, and it will show you the meta data, the bread crumb, and those trace messages directly in the debugger.

So this can be a really powerful way of debugging asynchronous code as well by inserting trace message the system keeps for you and displays to you when you crash in the same activity that we’ve propagated for you.

So in summary, we went over some background, then talked about Quality of Service Classes, the new concept that we’re introducing this year, and then the APIs surrounding that, as well as the propagation of Quality of Service and Execution Context across threads and processes, and finally some exciting news about diagnostics and queue debugging that we’re introducing this year.

For more information, please see Paul Danbold, our Core OS Technologies Evangelist, and the documentation on GCD on the developers site, as well as all the related sessions that have already occurred this week, in particular, the Writing Energy Efficient Code Part 1 session went into more detail on Quality of Service with different set of examples.

If you would like more information on that please see that, as well as the Debugging in Xcode session, view the live demo of the queue debugging feature, and provides some more information on that as well.

The Fix Bugs Faster using Activity Tracing session of this morning goes into lots of detail about that Activity ID and Activity Tracing mechanism that you saw, and that is it.

Thank you.

[ Applause ]

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