On 2D Performance Measurement

Trying to get a handle on 2D graphics rendering performance can be a difficult task. Obviously, people care about the performance of their 2D applications. Nobody wants to wait for a web browser to scroll past tacky banner ads or for an email client to render a screen full of spam. And it's easy for users to notice "my programs aren't rendering as fast with the latest drivers". But what developers need is a way to quantify exactly what that means, in order to track improvements and avoid regressions. And that measurement is the hard part. Or at least it always has been hard, until Chris Wilson's recent cairo-perf-trace.

Previous attempts at 2D benchmarking

Various attempts at 2D-rendering benchmark suites have appeared and even become popular. Notable examples are x11perf and gtkperf. My claim is that these tools range from useless to actively harmful when the task is understanding performance of real applications.

These traditional benchmarks suites are collections of synthetic micro-benchmarks. Within a given benchmark, some tiny operation, (such as "render a line of text" or "draw a radio box"), is performed hundreds of times in a tight loop and the total time is measured. The hope is that these operations will simulate the workload of actual applications.

Unfortunately, the workload of things like x11perf and gtkperf rarely come close to simulating practical workloads. In the worst case, the operation being tested might never be used at all in modern applications, (notice that x11perf tests things like stippled fills and wide ellipses which are obsolete graphics operations). Similarly, even if the operation is used, (such as a GTK+ radio button), it might not represent a significant fraction of time spent rendering by the application, (which might spend most of its time drawing its primary display area rather than any stock widget).

So that's just the well-known idea to not focus on the performance of things other than the primary bottlenecks. But even when we have identified a bottleneck in an application, x11perf can still be the wrong answer for measurement. For example, "text rendering" is a common bottleneck for 2D applications. However, a test like "x11perf aa10text" which seems like a tempting way to measure text performance is far from ideal. This benchmark draws a small number of glyphs from a single font at a single size over and over. Meanwhile, a real application will use many glyphs from many fonts at many sizes. With layers and layers of caches throughout the graphics stack, it's really not possible to accurately simulate what "text rendering" means for a real application without actually just running the actual application.

And yes, I myself have used and perhaps indirectly advocated for using things like x11perf in the past. I won't recommend it again in the future. See below for what I suggest instead.

What do the 3D folks do?

For 3D performance, everybody knows this lesson already. Nobody measures the performance of "draw the same triangles over and over". And if someone does, (by seriously quoting glxgear fps numbers, for example), then everybody gets a good laugh. In fact, the phrase "glxgears is not a benchmark" is a catchphrase among 3D developers. Instead, 3D measurement is made with "benchmark modes" in the 3D applications that people actually care about, (which as far as I can tell is just games for some reason). In the benchmark mode, a sample session of recorded input is replayed as quickly as possible and a performance measurement is reported.

As a rule, our 2D applications don't have similar benchmark modes. (There are some exceptions such as the trender utility for mozilla and special command-line options for the swfdec player.) And coding up application-specific benchmarking code for every interesting application isn't something that anyone is signing up to do right now.

Introducing cairo-perf-trace

Over the past year or so, Chris "ickle" Wilson has been putting a lot of work into a debugging utility known as cairo-trace, (inspired by work on an earlier tool known as libcairowrap by Benjamin Otte and Jeff Muizelaar). The cairo-trace utility produces a trace of all cairo-based rendering operations made by an application. The trace is complete and accurate enough to allow all operations to be replayed with a separate tool.

The cairo-trace utility has long proven invaluable as a way to capture otherwise hard-to-reproduce test cases. People with complex applications that exhibit cairo bugs can generate a cairo-trace and often easily trim it down to a minimal test case. Then after submitting this trace, a developer can replicate this bug without needing to have a copy of the complex application nor its state.

More recently, Chris wrote a new "cairo-trace --profile" mode and a tool named cairo-perf-trace for replaying traces for benchmarking purposes. These tools are currently available by obtaining the cairo source code, (either from git or in the 1.9.2 development snapshot or eventually the 1.10 release or later). Hopefully we'll see them get packaged up so they're easier to use soon.

With cairo-perf-trace, it's a simple matter to get rendering performance measurements of real applications without having to do any modification of the application itself. And you can collect a trace based on exactly the workload you want, (as long as the application you are interested in performs its rendering with cairo). Simply run:

cairo-trace --profile some-application

Which will generate a compressed file named something like some-application.$pid.lzma. To later benchmark this trace, first uncompress it:

lzma -cd some-application.$pid.lzma > some-application.trace

And then run cairo-perf-trace on the trace file:

cairo-perf-trace some-application.trace

The cairo-perf-trace utility will replay several iterations of the trace, (waiting for the standard deviation among reported times to drop below a threshold), and will report timing results for both the "image" backend (cairo's software backend) and whatever native backend is compiled into cairo, (xlib, quartz, win32, etc.). So one immediately useful result is its obvious to see if the native backend is slower than the all-software backend. Then, after making changes to the graphics stack, subsequent runs can be compared to ensure regressions are avoided and performance improvements actually help.

Finally, Chris has also established a cairo-traces git repository which collects useful traces that can be shared and compared. It already contains several different browsing sessions with firefox, swfdec traces (one with youtube), and traces of poppler, gnome-terminal, and evolution. Obviously, anyone should feel free to generate and propose new traces to contribute.

Putting cairo-perf-trace to use

In the few days that cairo-perf-traces has existed, we're already seeing great results from it. When Kristian Høgsberg recently proposed a memory-saving patch for the Intel driver, Chris Wilson followed up with a cairo-perf-trace report showing that the memory-saving had no negative impact on a traced firefox session, which addressed the concern that Eric had about the patch.

As another example, we've known that there's been a performance regression in UXA (compared to EXA) for trapezoid rendering. The problem was that UXA was allocating a pixmap only to then use software-based rasterization to that pixmap (resulting in slow read-modify-write cycles). The obvious fix I implemented is to simply malloc a buffer, do the rasterization, and only then copy the result to a pixmap.

After I wrote the patch, it was very satisfying to be able to validate its real-world impact with a swfdec-based trace. This trace is based on using swfdec to view the Giant Steps movie. When running this trace, sysprof makes it obvious that trapezoid rendering is the primary bottleneck. Here is the output of cairo-perf-trace on a GM965 machine before my patch:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image           swfdec-giant-steps   45.766   45.858  0.11%   6
[  0]     xlib           swfdec-giant-steps  194.422  194.602  0.05%   6

The performance problem is quite plain here. Replaying the swfdec trace to the X server takes 194 seconds compared to only 45 seconds to replay it only to cairo's all-software image backend. Note that 194 seconds is longer than the full video clip, meaning that my system isn't going to be able to keep up without skipping here. That's obviously not what we want.

Then, after my simple just-use-malloc patch I get:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image           swfdec-giant-steps   45.792   46.014  0.37%   6
[  0]     xlib           swfdec-giant-steps   81.505   81.567  0.03%   6

Here the xlib result has improved from 194 seconds to 81 seconds. That's a 2.4x improvement, and fast enough to now play the movie without skipping. It's very satisfying to validate performance patches with real-world application code like this. This commit is in the recent or the Intel driver, by the way. (Of course, there's still a 1.8x slowdown of the xlib backend compared to the image backend, so there's still more to be fixed here.)

The punchline is that we now have an easy way to benchmark 2D rendering in actual, real-world applications. If you see someone benchmarking with only toys like x11perf or gtkperf, go ahead and point them to this post, or the the cairo-perf-trace entry in the cairo FAQ, and insist on benchmarks from real applications.