You want threads, why not Zoidberg?

Søren Sandmann Pedersen, maintainer of the pixman library that is at the heart of the software rasteriser in Cairo and X, argues that with all things being equal an integrated graphics processor should never be able to outperform the main processor for rendering Cairo. Ideally, when using either the GPU or CPU, we want the memory being the rate-limiting component. On the Sandybridge processor, the memory is attached to the system agent, along with the large L3 caches, and is shared between the CPU and GPU. So he argues that any performance advantage cairo-xlib might show over cairo-image whilst using Intel graphics is really an opportunity for improvement of the rasteriser and pixman kernels. (But also bear in mind that the GPU should be considerably more efficient and be able to saturate the memory bus at a fraction of the power cost of the CPU). Using SSE2, we are indeed able to fully saturate the bus to L1, L2 and main memory for a variety of common operations in pixman. However, the bilinear scaling kernels take longer to compute than it does to write the results to memory, and so fail to completely utilize the available bandwidth. Furthermore, for a slightly more complex transformation than a simple scale and translation, we do not yet have a fast SSE2 kernel that operates directly on the destination and so must incur some extra work. If we ignore, for the time being, making further optimisations to those kernels to improve performance, we can instead ask the question: if using one SSE2 unit is not enough to saturate the memory bus, what happens if we throw more cores at the problem?

To gain the most improvement from adding threads to cairo, you need to design a rasteriser and usage model with threading in mind in. One such design is the vector renderer O by Øyvind Kolås. Despite being an experiment, it does show quite a bit of promise, but in its raw form just throwing threads at the problem does not beat using the SIMD compositing routines provided by pixman. However, it did raise the question whether we can make improvements to the existing image backend without impacting upon its immediate mode nature and so could be used by existing applications without alteration. To preserve the existing semantics, we can break up the individual composite and scan conversion operations into small pieces and feed those to a pool of threads, and then wait for the threads to complete before returning back to the application. As such we then never run the threads for very long, and risk that the overhead in thread management outweighs any benefit from splitting the operation over multiple cores.

To gain the greatest advantage from adding threads, I used a desktop Sandybridge processor (an i5-2500) for benchmarking this prototype cairo-image backend:

About half the test cases regress by about 10%, but around one third are improved by 2-3x. Not bad – the regressions are unacceptable, but it does offer a tantalising suggestion that we can make some significant improvements with only a minor change. (Remember not all processors are equal, and thanks to Sandybridge’s turbo some cores are more equal than others. I tried running the test cases on a Sandybridge ultrabook, and as soon as more one core was active, the entire package was throttled to keep it within its thermal constraints. The performance regression was dire.)

Having tuned the software backend to make better use of the average resources, what can we now say about the relative merits of the integrated graphics processor?

Indeed, for the cases that are almost entirely GPU bound (for example the firefox-fishbowl, -fishtank, -paintball, -particles), we have virtually eliminated all the previous advantage that the GPU held. In a notable couple of cases, we have improved the image backend to outperform SNA, and for all cases now the threaded image backend beats UXA. However, as can be seen there is still plenty of room for improvement of the image backend, and we can’t let the hardware acceleration be merely equal to a software rasteriser…

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