Scheduling Tasks and Threads | Web Browser Engineering

Modern browsers must run sophisticated applications while staying responsive to user actions. Doing so means choosing which of its many tasks to prioritize and which to delay until later—tasks like JavaScript callbacks, user input, and rendering. Moreover, browser work must be split across multiple threads, with different threads running events in parallel to maximize responsiveness.

Tasks and task queues

So far, most of the work our browser’s been doing has come from user actions like scrolling, pressing buttons, and clicking on links. But as the web applications our browser runs get more and more sophisticated, they begin querying remote servers, showing animations, and prefetching information for later. And while users are slow and deliberative, leaving long gaps between actions for the browser to catch up, applications can be very demanding. This requires a change in perspective: the browser now has a never-ending queue of tasks to do.

Modern browsers adapt to this reality by multitasking, prioritizing, and deduplicating work. Every bit of work the browser might do—loading pages, running scripts, and responding to user actions—is turned into a task, which can be executed later, where a task is just a function (plus its arguments) that can be executed:By writing *args as an argument to Task, we indicate that a Task can be constructed with any number of arguments, which are then available as the list args. Then, calling a function with *args unpacks the list back into multiple arguments.

The point of a task is that it can be created at one point in time, and then run at some later time by a task runner of some kind, according to a scheduling algorithm.The event loops we discussed in Chapter 2 and Chapter 11 are task runners, where the tasks to run are provided by the operating system. In our browser, the task runner will store tasks in a first-in first-out queue:

When the time comes to run a task, our task runner can just remove the first task from the queue and run it:First-in-first-out is a simplistic way to choose which task to run next, and real browsers have sophisticated schedulers which consider many different factors.

To run those tasks, we need to call the run method on our TaskRunner, which we can do in the main event loop:

Here I’ve chosen to only run tasks on the active tab, which means background tabs can’t slow our browser down.

With this simple task runner, we can now queue up tasks and execute them later. For example, right now, when loading a web page, our browser will download and run all scripts before doing its rendering steps. That makes pages slower to load. We can fix this by creating tasks for running scripts:

Now our browser will not run scripts until after load has completed and the event loop comes around again.

Timers and setTimeout

Tasks are also a natural way to support several JavaScript APIs that ask for a function to be run at some point in the future. For example, setTimeout lets you run a JavaScript function some number of milliseconds from now. This code prints “Callback” to the console one second from now:

We can implement setTimeout using the Timer class in Python’s threading module. You use the class like this:An alternative approach would be to record when each Task is supposed to occur, and compare against the current time in the event loop. This is called polling, and is what, for example, the SDL event loop does to look for events and tasks. However, that can mean wasting CPU cycles in a loop until the task is ready, so I expect the Timer to be more efficient.

This runs callback one second from now on a new Python thread. Simple! But it’s going to be a little tricky to use Timer to implement setTimeout because multiple threads will be involved.

As with addEventListener in Chapter 9, the call to setTimeout will save the callback in a JavaScript variable and create a handle by which the Python-side code can call it:

The exported setTimeout function will create a timer, wait for the requested time period, and then ask the JavaScript runtime to run the callback. That last part will happen via __runSetTimeout:Note that we never remove callback from the SET_TIMEOUT_REQUESTS dictionary. This could lead to a memory leak, if the callback is holding on to the last reference to some large data structure. We saw a similar issue in Chapter 9. In general, avoiding memory leaks when you have data structures shared between the browser and the browser application takes a lot of care.

The Python side, however, is quite a bit more complex, because threading.Timer executes its callback on a new Python thread. That thread can’t just call evaljs directly: we’ll end up with JavaScript running on two Python threads at the same time, which is not ok.JavaScript is not a multi-threaded programming language. It’s possible on the web to create workers of various kinds, but they all run independently and communicate only via special message-passing APIs. Instead, the timer will have to merely add a new Task to the task queue for our primary thread will execute later:This code has a very subtle bug, wherein a page might create a setTimeout, and then have that timer trigger later, when a user is visiting another web page. In our browser, that would allow one page to run JavaScript that modifies a different page—a huge security vulnerability! I think you can avoid this by resetting self.js.tab when you navigate to a new page, but ideally you’d do something more careful, like keeping track of all the child threads spawned by a JSContext and ending all of them before navigating. As our browser gets more complex, our bugs, and their associated fixes, get more complex too!

This way it’s ultimately the primary thread that calls evaljs. That’s good, but now we have two threads accessing the task_runner: the primary thread, to run tasks, and the timer thread, to add them. This is a race condition that can cause all sorts of bad things to happen, so we need to make sure only one thread accesses the task_runner at a time.

To do so we use a Condition object, which can only held by one thread at a time. Each thread will try to acquire condition before reading or writing to the task_runner, avoiding simultaneous access:The blocking parameter to acquire indicates whether the thread should wait for the lock to be available before continuing; in this chapter you’ll always set it to True. (When the thread is waiting, it’s said to be blocked.)

The Condition class is actually a Lock, plus functionality to be able to wait until a state condition occurs. If you have no more work to do right now, acquire condition and then call wait. This will cause the thread to stop at that line of code. When more work comes in to do, such as in schedule_task, a call to notify_all will wake up the thread that called wait.

It’s important to call wait at the end of the run loop if there is nothing left to do. Otherwise that thread will tend to use up a lot of the CPU, plus constantly be acquiring and releasing condition. This busywork not only slows down the computer, but also causes the callbacks from the Timer to happen at erratic times, because the two threads are competing for the lock.Try removing this code and observe. The timers will become quite erratic.

When using locks, it’s super important to remember to release the lock eventually and to hold it for the shortest time possible. The code above, for example, releases the lock before running the task. That’s because after the task has been removed from the queue, it can’t be accessed by another thread, so the lock does not need to be held while the task is running.

Long-lived threads

Threads can also be used to add browser multitasking. For example, in Chapter 10 we implemented the XMLHttpRequest class, which lets scripts make requests to the server. But in our implementation, the whole browser would seize up while waiting for the request to finish. That’s obviously bad.For this reason, the synchronous version of the API that we implemented in Chapter 10 is not very useful and a huge performance footgun. Some browsers are now moving to deprecate synchronous XMLHttpRequest.

creates a new thread that runs the callback function. Importantly, this code returns right away, and callback runs in parallel with any other code. We’ll implement asynchronous XMLHttpRequest calls using threads. Specifically, we’ll have the browser start a thread, do the request and parse the response on that thread, and then schedule a Task to send the response back to the script.

Like with setTimeout, we’ll store the callback on the JavaScript side and refer to it with a handle:

When a script calls the open method on an XMLHttpRequest object, we’ll now allow the is_async flag to be true:In browsers, the default for is_async is true, which the code below does not implement.

On the browser side, the XMLHttpRequest_send handler will have three parts. The first part will resolve the URL and do security checks:

Then, we’ll define a function that makes the request and enqueues a task for running callbacks:

Note that the task runs dispatch_xhr_onload, which we’ll define in just a moment.

Finally, depending on the is_async flag the browser will either call this function right away, or in a new thread:

Note that in the async case, the XMLHttpRequest_send method starts a thread and then immediately returns. That thread will run in parallel to the browser’s main work until the request is done.

To communicate the result back to JavaScript, we’ll call a __runXHROnload function from dispatch_xhr_onload:

The __runXHROnload method just pulls the relevant object from XHR_REQUESTS and calls its onload function:

As you can see, tasks allow not only the browser but also applications running in the browser to delay tasks until later.

The cadence of rendering

There’s more to tasks than just implementing some JavaScript APIs. Once something is a Task, the task runner controls when it runs: perhaps now, perhaps later, or maybe at most once a second, or even at different rates for active and inactive pages, or according to its priority. A browser could even have multiple task runners, optimized for different use cases.

Now, it might be hard to see how the browser can prioritize which JavaScript callback to run, or why it might want to execute JavaScript tasks at a fixed cadence. But besides JavaScript the browser also has to render the page, and as you may recall from Chapter 2, we’d like the browser to render the page exactly as fast as the display hardware can refresh. On most computers, this is 60 times per second, or 16ms per frame.

Let’s establish 16ms our ideal refresh rate:16 milliseconds isn’t that precise, since it’s 60 times 16.66666…ms that is just about equal to 1 second. But it’s a toy browser!

Now, there’s some complexity here, because we have multiple tabs. We don’t need each tab redrawing itself every 16ms, because the user only sees one tab at a time. We just need the active tab redrawing itself. Therefore, it’s the Browser that should control when we update the display, not individual Tabs.

Let’s make that happen. First, let’s write a schedule_animation_frame methodIt’s called an “animation frame” because sequential rendering of different pixels is an animation, and each time you render it’s one “frame”—like a drawing in a picture frame. on Browser that schedules a render task to run the Tab half of the rendering pipeline:

Note how every time a frame is scheduled, we set up a timer to schedule the next one. We can kick off the process when we start the Browser:

Next, let’s put the rastering and drawing tasks that the Browser does into their own method:

In the top-level loop, after running a task on the active tab the browser will need to raster-and-draw, in case that task was a rendering task:

Now we’re scheduling a new rendering task every 16 milliseconds, just as we wanted to.

Optimizing with dirty bits

If you run this on your computer, there’s a good chance your CPU usage will spike and your batteries will start draining. That’s because we’re calling render every frame, which means our browser is now constantly styling elements, building layout trees, and painting display lists. Most of that work is wasted, because on most frames, the web page will not have changed at all, so the old styles, layout trees, and display lists would have worked just as well as the new ones.

Let’s fix this using a dirty bit, a piece of state that tells us if some complex data structure is up to date. Since we want to know if we need to run render, let’s call our dirty bit needs_render:

One advantage of this flag is that we can now set needs_render when the HTML has changed instead of calling render directly. The render will still happen, but later. This makes scripts faster, especially if they modify the page multiple times. Make this change in innerHTML_set, load, click, and keypress. For example, in load, do this:

Another problem with our implementation is that the browser is now doing raster_and_draw every time the active tab runs a task. But sometimes that task is just running JavaScript that doesn’t touch the web page, and the raster_and_draw call is a waste.

We can avoid this using another dirty bit, which I’ll call needs_raster_and_draw:The needs_raster_and_draw dirty bit doesn’t just make the browser a bit more efficient. Later in this chapter, we’ll add multiple browser threads, and at that point this dirty bit is necessary to avoid erratic behavior when animating. Try removing it later and see for yourself!

We will need to call set_needs_raster_and_draw every time either the Browser changes something about the browser chrome, or any time the Tab changes its rendering. The browser chrome is changed by event handlers:

Now the rendering pipeline is only run if necessary, and the browser should have acceptable performance again.

Animating frames

One big reason for a steady rendering cadence is so that animations run smoothly. Web pages can set up such animations using the requestAnimationFrame API. This API allows scripts to run code right before the browser runs its rendering pipeline, making the animation maximally smooth. It works like this:

By calling requestAnimationFrame, this code is doing two things: scheduling a rendering task, and asking that the browser call callback at the beginning of that rendering task, before any browser rendering code. This lets web page authors change the page and be confident that it will be rendered right away.

The implementation of this JavaScript API is straightforward. We store the callbacks on the JavaScript side:

In JSContext, when that method is called, we need to schedule a new rendering task:

Then, when render is actually called, we need to call back into JavaScript, like this:

Note that __runRAFHandlers needs to reset RAF_LISTENERS to the empty array before it runs any of the callbacks. That’s because one of the callbacks could itself call requestAnimationFrame. If this happens during such a callback, the spec says that a second animation frame should be scheduled. That means we need to make sure to store the callbacks for the current frame separately from the callbacks for the next frame.

This situation may seem like a corner case, but it’s actually very important, as this is how pages can run an animation: by iteratively scheduling one frame after another. For example, here’s a simple counter “animation”:

This script will cause 100 animation frame tasks to run on the rendering event loop. During that time, our browser will display an animated count from 0 to 99. Serve this example web page from our HTTP server:

One flaw with our implementation so far is that an inattentive coder might call requestAnimationFrame multiple times and thereby schedule more animation frames than expected. If other JavaScript tasks appear later, they might end up delayed by many, many frames.

Luckily, rendering is special in that it never makes sense to have two rendering tasks in a row, since the page wouldn’t have changed in between. To avoid having two rendering tasks we’ll add a dirty bit called needs_animation_frame to the Browser which indicates whether a rendering task actually needs to be scheduled:

Note how I also checked for not having an animation timer object; this avoids running two at once.

A tab will set the needs_animation_frame flag when an animation frame is requested:

Note that set_needs_animation_frame will only actually set the dirty bit if called from the active tab. This guarantees that inactive tabs can’t interfere with active tabs. Besides preventing scripts from scheduling too many animation frames, this system also makes sure that if our browser consistently runs slower than 60 frames per second, we won’t end up with an ever-growing queue of rendering tasks.

Profiling rendering

We now have a system for scheduling a rendering task every 16ms. But what if rendering takes longer than 16ms to finish? Before we answer this question, let’s instrument the browser and measure how much time is really being spent rendering. It’s important to always measure before optimizing, because the result is often surprising.

Let’s implement some simple instrumentation to measure time. We’ll want to average across multiple raster-and-draw cycles:

We’ll measure the time for something like raster and draw by just calling start and stop methods on one of these MeasureTime objects:

(Naturally you’ll need to call these methods before quitting, from the main event loop, so it has a chance to print its timing data.)

Fire up the server, open our timer script, wait for it to finish counting, and then exit the browser. You should see it output timing data. On my computer, it looks like this:

On every animation frame, my browser spent about 20ms in render and about 66ms in raster_and_draw. That clearly blows through our 16ms budget. So, what can we do?

Well, one option, of course, is optimizing raster-and-draw, or even render. And if we can, that’s the right choice.See the go further at the end of this section for some ideas on how to do this. But another option—complex, but worthwhile and done by every major browser—is to do the render step in parallel with the raster-and-draw step by adopting a multi-threaded architecture.

Two threads

Running rendering in parallel would allow us to produce a new frame every 66ms, instead of every 88ms. That’s good, but there’s more. Since there’s no point to running render more often than raster-and-draw, after the 20ms spent rendering the rendering thread would 46ms left over, which could be used for running JavaScript. And that in turn means many tasks could be handled with a delay of no more than 20ms (and the other thread 66ms), which makes the browser much more responsive. That’s reason enough to add a second thread.

Let’s call our two threads the browser threadIn modern browsers the analogous thread is often called the compositor thread, though modern browsers have lots of threads and the correspondence isn’t exact. and the main thread.Here I’m going with the name real browsers often use. A better name might be the “JavaScript” or “DOM” thread (since JavaScript can sometimes run on other threads). The browser thread corresponds to the Browser class and will handle raster and draw. It’ll also handle interactions with the browser chrome. The main thread, on the other hand, corresponds to a Tab and will handle running scripts, loading resources, and rendering, along with associated tasks like running event handlers and callbacks. If you’ve got more than one tab open, you’ll have multiple main threads (one per tab) but only one browser thread.

To start, the one thread that exists already—the one that runs when you start the browser—will be the browser thread. We’ll make a main thread every time we create a tab. These two threads will need to communicate to handle events and draw to the screen.

When the browser thread needs to communicate with the main thread, to inform it of events, it’ll place tasks on the main thread’s TaskRunner. The main thread will need to communicate with the browser thread to request animation frames and to send it a display list to raster and draw, and the main thread will do that via two methods on browser: set_needs_animation_frame to request an animation frame and commit to send it a display list.

Let’s implement this design. To start, we’ll add a Thread to each TaskRunner, which will be the tab’s main thread. This thread will need to run in a loop, pulling tasks from the task queue and running them. We’ll put that loop inside the TaskRunner’s run method.

Remove the call to run from the top-level while True loop, since that loop is now going to be running in the browser thread. And run will have its own loop:

Because this loop runs forever, the main thread will live on indefinitely.Or until the browser quits, at which point it should ask the main thread to quit as well.

The Browser should no longer call any methods on the Tab. Instead, to handle events, it should schedule tasks on the main thread. For example, here is loading:

Event handlers are mostly similar, except that we need to be careful to distinguish events that affect the browser chrome from those that affect the tab. For example, consider handle_click. If the user clicked on the browser chrome (meaning e.y < CHROME_PX), we can handle it right there in the browser thread. But if the user clicked on the web page, we must schedule a task on the main thread:

So now we have the browser thread telling the main thread what to do Communication in the other direction is a little subtler.

Committing a display list

We already have a set_needs_animation_frame method, but we also need a commit method that a Tab can call when it’s finished creating a display list. And if you look carefully at our raster-and-draw code, you’ll see that to draw a display list we also need to know the URL (to update the browser chrome), the document height (to allocate a surface of the right size), and the scroll position (to draw the right part of the surface).

When running an animation frame, the Tab should construct one of these objects and pass it to commit. To keep render from getting too confusing, let’s put this in a new run_animation_frame method, and move __runRAFHandlers there too:

Think of the CommitData object as being sent from the main thread to browser thread. That means the main thread shouldn’t access it any more, and for this reason I’m resetting the display_list field. The Browser should now schedule run_animation_frame:

On the Browser side, the new commit method needs to read out all of the data it was sent and call set_needs_raster_and_draw as needed. Because this call will come from another thread, we’ll need to acquire a lock. Another important step is to not clear the animation_timer object until after the next commit occurs. Otherwise multiple rendering tasks could be queued at the same time.

Note that commit is called on the main thread, but acquires the browser thread lock. As a result, commit is a critical time when both threads are both “stopped” simultaneously.For this reason commit needs to be as fast as possible, to maximize parallelism and responsiveness. In modern browsers, optimizing commit is quite challenging, because their method of caching and sending data between threads is much more sophisticated. Also note that, it’s possible for the browser thread to get a commit from an inactive tab,That’s because even inactive tabs are still running their main threads and responding to callbacks from setTimeout or XMLHttpRequest, and might be processing one last animation frame. so the tab parameter is compared with the active tab before copying over any committed data.

Now that we have a browser lock, we also need to acquire the lock any time the browser thread accesses any of its variables. For example, in set_needs_animation_frame, do this:

In schedule_animation_frame you’ll need to do it both inside and outside the callback:

Add locks to raster_and_draw, handle_down, handle_click, handle_key, and handle_enter as well.

We also don’t want the main thread doing rendering faster than the browser thread can raster and draw. So we should only schedule animation frames once raster and draw are done.The technique of controlling the speed of the front of a pipeline by means of the speed of its end is called back pressure. Luckily, that’s exactly what we’re doing:

And that’s it: we should now be doing render on one thread and raster and draw on another!

Threaded scrolling

Splitting the main thread from the browser thread means that the main thread can run a lot of JavaScript without slowing down the browser much. But it’s still possible for really slow JavaScript to slow the browser down. For example, imagine our counter adds the following artificial slowdown:

Now, every tick of the counter has an artificial pause during which the main thread is stuck running JavaScript. This means it can’t respond to any events; for example, if you hold down the down key, the scrolling will be janky and annoying. I encourage you to try this and witness how annoying it is, because modern browsers usually don’t have this kind of jank.Adjust the loop bound to make it pause for about a second or so on your computer.

To fix this, we need to the browser thread to handle scrolling, not the main thread. This is harder than it might seem, because the scroll offset can be affected by both the browser (when the user scrolls) and the main thread (when loading a new page or changing the height of the document via innerHTML). Now that the browser thread and the main thread run in parallel, they can disagree about the scroll offset.

What should we do? The best we can do is to use the browser thread’s scroll offset until the main thread tells us otherwise, because the scroll offset is incompatible with the web page (by, say, exceeding the document height). To do this, we’ll need the browser thread to inform the main thread about the current scroll offset, and then give the main thread the opportunity to override that scroll offset or to leave it unchanged.

Let’s implement that. To start, we’ll need to store a scroll variable on the Browser, and update it when the user scrolls:

This code sets needs_raster_and_draw to raster and draw with changed scroll offset, and sets needs_animation_frame to cause the main thread to receive the scroll offset asynchronously in the future. (Even with threaded scroll, it’s important to synchronize the new value back to the main thread soon, since APIs like click handling depend on it.)

The scroll offset also needs to change when the user switches tabs, but in this case we don’t know the right scroll offset yet. We need the main thread to run in order to commit a new display list for the other tab, and at that point we will have a new scroll offset as well. Move tab switching (in load and handle_click) to a new method set_active_tab that simply schedules a new animation frame:

So far, this is only updating the scroll offset on the browser thread. But the main thread eventually needs to know about the scroll offset, so it can pass it back to commit. So, when the Browser creates a rendering task for run_animation_frame, it should pass in the scroll offset. The run_animation_frame function can then store the scroll offset before doing anything else. Add a scroll parameter to run_animation_frame:

But the main thread also needs to be able to modify the scroll offset. We’ll add a scroll_changed_in_tab flag that tracks whether it’s done so, and only store the browser thread’s scroll offset if scroll_changed_in_tab is not already true.Two-threaded scroll has a lot of edge cases, including some I didn’t anticipate when writing this chapter. For example, it’s pretty clear that a load should force scroll to 0 (unless the browser implements scroll restoration for back-navigations!), but what about a scroll clamp followed by a browser scroll that brings it back to within the clamped region? By splitting the browser into two threads, we’ve brought in all of the challenges of concurrency and distributed state.

We’ll set scroll_changed_in_tab when loading a new page or when the browser thread’s scroll offset of past the bottom of the page:

If the main thread hasn’t overridden the browser’s scroll offset, we’ll set the scroll offset to None in the commit data:

That’s it! If you try the counting demo now, you’ll be able to scroll even during the artificial pauses. As you’ve seen, moving tasks to the browser thread can be challenging, but can also lead to a much more responsive browser. These same trade-offs are present in real browsers, at a much greater level of complexity.

Threaded style and layout

Now that we have separate browser and main threads, and now that some operations are performed on the browser thread, our browser’s thread architecture has started to resemble that of a real browser.Note that many browsers now run some parts of the browser thread and main thread in different processes, which has some advantages for security and error handling. But why not move even more browser components into even more threads? Wouldn’t that make the browser even faster?

In a word, yes. Modern browsers have dozens of threads, which together serve to make the browser even faster and more responsive. For example, raster-and-draw often runs on its own thread so that the browser thread can handle events even while a new frame is being prepared. Likewise, modern browsers typically have a collection of network or IO threads, which move all interaction with the network or the file system off of the main thread.

On the other hand, some parts of the browser can’t be easily threaded. For example, consider the earlier part of the rendering pipeline: style, layout and paint. In our browser, these run on the main thread. But could they move to their own thread?

In principle, yes. The only thing browsers have to do is implement all the web API specifications correctly, and draw to the screen after scripts and requestAnimationFrame callbacks have completed. The specification spells this out in detail in what it calls the update-the-rendering steps. The specification doesn’t mention style or layout at all—because style and layout, just like paint and draw, are implementation details of a browser. The specification’s update-the-rendering steps are the JavaScript-observable things that have to happen before drawing to the screen.

Nevertheless, in practice, no current modern browser runs style or layout on off the main thread.The Servo rendering engine uses multiple threads to take advantage of parallelism in style and layout, but those steps still block, for example, JavaScript execution on the main thread. The reason is simple: there are many JavaScript APIs that can query style or layout state. For example, getComputedStyle requires first computing style, and getBoundingClientRect requires first doing layout.There is no JavaScript API that allows reading back state from anything later in the rendering pipeline than layout. This made it relatively easy for us to move raster and draw to the browser thread. If a web page calls one of these APIs, and style or layout is not up-to-date, then it has to be computed then and there. These computations are called forced style or forced layout: style or layout are “forced” to happen right away, as opposed to possibly 16ms in the future, if they’re not already computed. Because of these forced style and layout situations, browsers have to be able to layout and style on the main thread.Or the main thread could force the compositor thread to do that work, but that’s even worse, because forcing work on the compositor thread will make scrolling janky unless you do even more work to avoid that somehow.

One possible way to resolve these tensions is to optimistically move style and layout off the main thread, similar to optimistically doing threaded scrolling if a web page doesn’t preventDefault a scroll. Is that a good idea? Maybe, but forced style and layout aren’t just caused by JavaScript execution. One example is our implementation of click, which causes a forced render before hit testing:

It’s possible (but very hard) to move hit testing off the main thread or to do hit testing against an older version of the layout tree, or to come up with some other technological fix. Thus it’s not impossible to move style and layout off the main thread “optimistically”, but it is challenging. That said, browser developers are always looking for ways to make things faster, and I expect that at some point in the future style and layout will be moved to their own thread. Maybe you’ll be the one to do it?

Summary

This chapter explained in some detail the two-thread rendering system at the core of modern browsers. The main points to remember are:

Additionally, you’ve seen how hard it is to move tasks between the two threads, such as the challenges involved in scrolling on the browser thread, or how forced style and layout makes it hard to fully isolate the rendering pipeline from JavaScript.

Outline

The complete set of functions, classes, and methods in our browser should now look something like this:

WIDTH

HEIGHT

HSTEP

VSTEP

SCROLL_STEP

def print_tree(node, indent)

class HTMLParser:
    def __init__(body)

    def parse()

    def get_attributes(text)

    def add_text(text)

    SELF_CLOSING_TAGS

    def add_tag(tag)

    HEAD_TAGS

    def implicit_tags(tag)

    def finish()

BLOCK_ELEMENTS

class CSSParser:
    def __init__(s)

    def whitespace()

    def literal(literal)

    def word()

    def pair()

    def ignore_until(chars)

    def body()

    def selector()

    def parse()

class TagSelector:
    def __init__(tag)

    def matches(node)

    def __repr__()

class DescendantSelector:
    def __init__(ancestor, descendant)

    def matches(node)

    def __repr__()

INHERITED_PROPERTIES

def style(node, rules)

def cascade_priority(rule)

def tree_to_list(tree, list)

CHROME_PX

class Text:
    def __init__(text, parent)

    def __repr__()

class Element:
    def __init__(tag, attributes, parent)

    def __repr__()

INPUT_WIDTH_PX

EVENT_DISPATCH_CODE

COOKIE_JAR

class JSContext:
    def __init__(tab)

    def run(script, code)

    def dispatch_event(type, elt)

    def get_handle(elt)

    def querySelectorAll(selector_text)

    def getAttribute(handle, attr)

    def innerHTML_set(handle, s)

    def XMLHttpRequest_send(method, url, body, isasync, handle)

    def dispatch_settimeout(handle)

    def setTimeout(handle, time)

    def dispatch_xhr_onload(out, handle)

    def requestAnimationFrame()

class URL:
    def __init__(url)

    def request(top_level_url, payload)

    def resolve(url)

    def origin()

def get_font(size, weight, style)

FONTS

class DrawLine:
    def __init__(x1, y1, x2, y2, color, thickness)

    def execute(canvas)

    def __repr__()

class DrawRect:
    def __init__(x1, y1, x2, y2, color)

    def execute(canvas)

    def __repr__()

class DrawOutline:
    def __init__(x1, y1, x2, y2, color, thickness)

    def execute(canvas)

    def __repr__()

def linespace(font)

class DrawText:
    def __init__(x1, y1, text, font, color)

    def execute(canvas)

    def __repr__()

class SaveLayer:
    def __init__(sk_paint, children, should_save, should_paint_cmds)

    def execute(canvas)

class ClipRRect:
    def __init__(rect, radius, children, should_clip)

    def execute(canvas)

class BlockLayout:
    def __init__(node, parent, previous)

    def token(tok)

    def word(node, word)

    def flush()

    def recurse(node)

    def open_tag(tag)

    def close_tag(tag)

    def layout()

    def layout_mode()

    def paint(display_list)

    def get_font(node)

    def __repr__()

    def new_line()

    def input(node)

class DocumentLayout:
    def __init__(node)

    def layout()

    def paint(display_list)

    def __repr__()

class LineLayout:
    def __init__(node, parent, previous)

    def layout()

    def paint(display_list)

    def __repr__()

class TextLayout:
    def __init__(node, word, parent, previous)

    def layout()

    def paint(display_list)

class InputLayout:
    def __init__(node, parent, previous)

    def layout()

    def paint(display_list)

    def __repr__()

def paint_visual_effects(node, cmds, rect)

def parse_blend_mode(blend_mode_str)

def parse_color(color)

class Tab:
    def __init__(browser)

    def load(url, body)

    def draw(canvas)

    def scrolldown()

    def click(x, y)

    def go_back()

    def __repr__()

    def render()

    def submit_form(elt)

    def keypress(char)

    def allowed_request(url)

    def raster(canvas)

    def set_needs_render()

    def run_animation_frame(scroll)

class Browser:
    def __init__()

    def handle_down()

    def handle_click(e)

    def handle_key(char)

    def handle_enter()

    def load(url)

    def paint_chrome()

    def draw()

    def raster_tab()

    def raster_chrome()

    def handle_quit()

    def render()

    def commit(tab, data)

    def set_needs_animation_frame(tab)

    def set_needs_raster_and_draw()

    def raster_and_draw()

    def schedule_animation_frame()

    def set_active_tab(index)

    def schedule_load(url, body)

    def load_internal(url)

class MeasureTime:
    def __init__(name)

    def start_timing()

    def text_current(name)

    def stop_timing()

    def text()

SETTIMEOUT_CODE

XHR_ONLOAD_CODE

def clamp_scroll(scroll, tab_height)

class Task:
    def __init__(task_code)

    def run()

class SingleThreadedTaskRunner:
    def __init__(tab)

    def schedule_task(callback)

    def run_tasks()

    def clear_pending_tasks()

    def start_thread()

    def set_needs_quit()

    def run()

class CommitData:
    def __init__(url, scroll, height, display_list)

class TaskRunner:
    def __init__(tab)

    def schedule_task(task)

    def set_needs_quit()

    def clear_pending_tasks()

    def start_thread()

    def run()

    def handle_quit()

REFRESH_RATE_SEC

if __name__ == "__main__"

If you run it, it should look something like this page; due to the browser sandbox, you will need to open that page in a new tab.

Exercises

setInterval: setInterval is similar to setTimeout but runs repeatedly at a given cadence until clearInterval is called. Implement these. Make sure to test setInterval with various cadences in a page that also uses requestAnimationFrame with some expensive rendering pipeline work to do. Record the actual timing of setInterval tasks; how consistent is the cadence?

Clock-based frame timing: Right now our browser schedules the next animation frame to happen exactly 16ms later than the first time set_needs_animation_frame is called. However, this actually leads to a slower animation frame rate cadence than 16ms, for example if render takes say 10ms to run. Can you see why? Fix this in our browser by using the absolute time to schedule animation frames, instead of a fixed delay between frames. You will need to choose a slower cadence than 16ms so that the frames don’t overlap.

Scheduling: As more types of complex tasks end up on the event queue, there comes a greater need to carefully schedule them to ensure the rendering cadence is as close to 16ms as possible, and also to avoid task starvation. Implement a task scheduler with a priority system that balances these two needs. Test it out on a web page that taxes the system with a lot of setTimeout-based tasks.

Threaded loading: When loading a page, our browser currently waits for each style sheet or script resource to load in turn. This is unnecessarily slow, especially on a bad network. Instead, make your browser sending off all the network requests in parallel. It may be convenient to use the join method on a Thread, which will block the thread calling join until the other thread completes. This way your load method can block until all network requests are complete.

Networking thread: Real browsers usually have a separate thread for networking (and other I/O). Tasks are added to this thread in a similar fashion to the main thread. Implement a third networking thread and put all networking tasks on it.

Fine-grained dirty bits: at the moment, the browser always re-runs the entire rendering pipeline if anything changed. For example, it re-rasters the browser chrome every time (which chapter 11 didn’t do). Add separate dirty bits for raster and draw stages.You can also try adding dirty bits for whether layout needs to be run, but be careful to think very carefully about all the ways this dirty bit might need to end up being set.

Optimized scheduling: On a complicated web page, the browser may not be able to keep up with the desired cadence. Instead of constantly pegging the CPU in a futile attempt to keep up, implement a frame time estimator that estimates the true cadence of the browser based on previous frames, and adjust schedule_animation_frame to match. This way complicated pages get consistently slower, instead of having random slowdowns.

Raster-and-draw thread: Right now, if an input event arrives while the browser thread is rastering or drawing, that input event won’t be handled immediately. This is especially a problem because raster and draw are slow. Fix this by adding a separate raster-and-draw thread controlled by the browser thread. While the raster-and-draw thread is doing its work, the browser thread should be available to handle input events. Be careful: SDL is not thread-safe, so all of the steps that directly use SDL still need to happen on the browser thread.