Why is GUI code so computationally expensive?

Question

All you Stackoverflowers,

I was wondering why GUI code is responsible for sucking away many, many cpu cycles. In principle, the graphical rendering is far less complex than Doom (although most corporate GUIs will introduce lots of window dressing). The event handling layer is also seemingly a heavy cost, however, it seems that a well-written implementation should switch between contexts efficiently on modern processors with a lot of memory/cache.

If anybody has run a profiler on their big GUI application, or a common API itself, I'm interested in where the bottlenecks lie.

Possible explanations (that I imagine) may be:

1. High levels of abstraction between hardware and application interface
2. Lots of levels of indirection to the correct code to execute
3. Low priority (compared to other processes)
4. Misbehaving applications flooding API with calls
5. Excessive object orientation?
6. Complete poor design choices in API (not just issues, but design philosophy)

Some GUI frameworks are much better than others, so I'd like to hear varied perspectives. For example, the Unix/X11 system is much different than Windows and even than WinForms.

Edit: Now a community wiki - go for it. I have one more thing to add -- I'm an algorithms guy in school and would be interested if there are inefficient algorithms in GUI code and which they are. Then again, it's probably just the implementation overhead.

Answer 1

I've no idea generally, but I'd like to add another item to your list - font rendering and calculations. Finding vector glyphs in a font and converting them to bitmap representations with anti-aliasing is no small task. And often it needs to be done twice - first to calculate the width/height of the text for positioning, and then actually drawing the text at the right coordinates.

Also, most drawing code today relies on clipping mechanisms to update just a part of the GUI. So, if just one part needs to be redrawn, the code actually redraws the whole window behind the scenes, and then takes just the needed part to actually update.

Added:

In the comments I found this:

I'm also very interested in this. It can't be that the gui is rendered using only the cpu because if you don't have proper drivers for your gfx-card, desktop graphics render incredibly slow. If you have gfx-drivers however desktop-gfx go kinda fast but never as fast as a directx/opengl app.

Here's the deal as I understand it: every graphic card out there today supports a generic interface for drawing. I'm not sure if it's called "VESA", "SVGA", or if those are just old names from the past. Anyway, this interface involves doing everything through interrupts. For every pixel there is an interrupt call. Or something like that. The proper VGA driver however is able to take advantage of DMA and other enhancements that make the whole process WAY less CPU-intensive.

Added 2: Ah, and for OpenGL/DirectX - that's another feature of today's graphics cards. They are optimized for 3D operations in exclusive mode. That's why the speed. The normal GUI just utilizes basic 2D drawing procedures. So it gets to send the contents of the whole screen every time it wants an update. 3D applications however send a bunch of textures and triangle definitions to the VRAM (video-RAM) and then just reuse them for drawing. They just say something like "take the triangle set #38 with the texture set #25 and draw them". All these things are cached in the VRAM so this is again way faster.

I'm not sure, but I would suspect that the modern 3D-accelerated GUIs (Vista Aero, compiz on Linux, etc.) also might take advantage of this. They could send common bitmaps to the VGA up front and then just reuse them directly from the VRAM. Any application-drawn surfaces however would still need to be sent directly every time for updates.

Added 3: More ideas. :) The modern GUI's for Windows, Linux, etc. are widget-oriented (that's control-oriented for Windows speakers). The problem with this is that each widget has its own drawing code and associated drawing surface (more or less). When the window needs to get redrawn, it calls the drawing code for all its child-widgets, who in turn call the drawing code for their child-widgets, etc.. Every widget redraws its whole surface, even though some of it is obscured by other widgets. With above mentioned clipping techniques some of this drawn information is immediately discarded to reduce flickering and other artifacts. But still it's lots of manual drawing code that includes bitmap blitting, stretching, skewing, drawing lines, text, flood-filling, etc.. And all this gets translated to a series of putpixel calls that get filtered through clipping filters/masks and other stuff. Ah, yes, and alpha blending has also become popular today for nice effects which means even more work. So... yes, you could say this is because of lots of abstraction and indirection. But... could you really do it any better? I don't think so. Only 3D techniques might help, because they take advantage of GPU for alpha-calculations and clipping.

Answer 2

Uhm, that's quite a lot.

The most simple but probably obvious answer is that the programmers behind these GUI apps, are really bad programmers. You can go along way in writing code which does the most bizarre things and it will be faster but few people seem to care how to do this or they deem it to be an expensive non-profitable time wasted effort.

To set things straight off-loading computations to the GPU won't necessarily fix any problems. The GPU is just like the CPU except it's less general purpose and more a data paralleled processor. It can do graphics computations exceptionally well. Whatever graphics API/OS and driver combination you have doesn't really matter that much... well OK, with Vista as an example, they changed the desktop composition engine. This engine is far better composting only that which has changed, and since the number one bottle neck for GUI apps is redrawing is a neat optimization strategy. This idea of virtualizing your computational needs and only update the smallest change every time.

Win32 sends WM_PAINT messages to windows when they need to be redrawn, this can be a result of windows occluding each other. However it's up to the window itself to figure out whats actually changed. More than so nothing did change or the change that was made was trivial enough so that it could have been just preformed on top of what ever top most surface you had.

This kind of graphics handling doesn't necessarily exist today. I would say that people have refrained from writing really efficient and virtualizing rendering solutions because the benefit/cost ration is rather low/high (bad).

Something Windows Presentation Foundation (WPF) does, which I think is far superior to most other GUI API is that it splits layout updates and rendering updates into two separate passes. And while WPF is managed code the rendering engine is not. What happens with rendering is that the managed WPF rendering engine builds a command queue (this is what DirectX and OpenGL does) which is then handed of to the native rendering engine. What's a bit more elegant here is that WPF will then try to retain any computation which didn't change the visual state. A trick if you may, where you avoid costly rendering calls for things that doesn't have to be rendered (virtualizing).

In contrast to WM_PAINT which tells a Win32 window to repaint itself a WPF app would check what parts of that window requires repainting and only repaint the smallest change.

Now WPF is not supreme, it's a solid effort from Microsoft but it's not the holy grail yet... the code which runs the pipeline could still be improved and the memory footprint of any managed app is still more than I would want. But I hope this is the kind of answer you are looking for.

WPF is able to do some things asynchronously rather decent, which is a huge deal if you wanna make a really responsive low-latency/low-cpu UI. Asynchronous operations is more than off-loading work on a different thread.

To summarize things slow and expensive GUI means too much repainting and the kind of repainting which is very expensive i.e. the entire surface area.

Answer 3

I does to some degree depend on the language. You might have noticed that Java and RealBasic applications are a fair bit slower than their C-based (C++, C#, Objective-C) counterparts.

However GUI applications are much more complex than command line apps. The Terminal window needs only to draw a simple window that doesn't support buttons.

There are also multiple loops for extra inputs and features.

Answer 4

Let's begin by saying that writing libraries is much harder than writing a stand-alone code. The requirement that your abstraction be reusable in as many contexts as possible, including contexts which you haven't though of yet, makes the task challenging even for experienced programmers.

Amongst libraries, writing a GUI toolkit library is a famously difficult problem. This is because the programs which use GUI libraries range over a very wide variety of domains with very different needs. Mr Why and Martin DeMollo discussed the requirements placed of GUI libraries a little while ago.

Writing GUI widgets themselves is difficult because computer users are very sensitive minute details of the behavior of the interface. Non-native widget never feel right, don't they? In order to get non-native widget right -- in order to get any widget right, in fact -- you need to spend an inordinate amount of time tweaking the details of the behavior.

So, GUI are slow because of the inefficiencies introduced by the abstraction mechanisms used to create highly-reusable components, that added to shortness of time available to optimize the code once so much time has been spent just getting the behavior right.

Answer 5

I think that you can find some interesting thoughts on this topic in "Window System Design: If I had it to do over again in 2002" by James Gosling (the Java guy, also known for his work on pre-X11 windowing systems). Available online here[pdf].

The article focuses on the positive side (how to make it fast), not on the negative side (what's making it slow), but it is still a good read on the topic.