How Valve Made L4D2 Faster On Linux Than Windows

Following this morning's Here Is Valve's Source Engine Left 4 Dead 2 On Linux article, here is most of the details that were shared during yesterday's SIGGRAPH presentation about Left 4 Dead 2 running natively on Linux with OpenGL and outperforming the Windows version.

Valve's big SIGGRAPH Linux presentation centered around their OpenGL work and how the Linux version is faster than the Windows version -- for both the OpenGL and Direct3D renderers.

Valve is supposed to be posting the slides (presumably on their blog) and hopefully a video will emerge, but since there's nothing yet, it will probably take until next week before they share anything. As a result, below are the pertinent details they shared from their slides that were presented last night at SIGGRAPH Los Angeles. With SIGGRAPH being an industry-leading graphics conference, most of the talk was filled with low-level technical details. Enjoy!

- The presentation was made by Rich Geldreich of Valve and entitled "Left 4 Dead 2 Linux: From 6 to 300 FPS in OpenGL."

- The Source Engine with OpenGL is on average about 11% faster with OpenGL than Direct3D 9 on a NVIDIA GeForce GTX 680. It's believed that another 5% higher performance for GL is still easily obtainable by reducing overhead in their Direct3D -> OpenGL layer.

- Yes, the way the Source Engine is hitting on OpenGL right now is through a non-deferring, locally-optimizing abstraction layer to basically convert their longstanding Direct3D calls into OpenGL. However, it's not the same way that Wine does Direct3D to OpenGL conversion. The Source Engine targets a D3D9-like API with extensions that translates GL calls dynamically. This also works for Shader Model 2.0b with Shader Model 3.0 support coming soon. Valve's implementation is nearly a 1:1 mapping between D3D and GL concepts.

- The overhead attributed to the Direct3D to OpenGL translation is about 50/50 split between CPU cycles spent calling GL vs. translation overhead. For single-threaded graphics drivers on the other hand, it's about 80% in GL and 20% translation overhead. But again, even with this extra layer, OpenGL is faster. NVIDIA's proprietary Linux team has done a lot of work with their driver's multi-threading abilities.

- Valve's worked with all major vendors (Intel, AMD, and NVIDIA) for improved driver support and optimizations. Valve's Linux team originally had "little practical OpenGL experience." Their process came down to devising/conducting experiments, test results with known workloads, refining/updating mental model of system's behavior, repeat. The goal was to account for every micro-second spent in the Direct3D to OpenGL layer and render thread.

- Interpreting the experimental results were a bit challenging with the game being multi-threaded, the driver's server thread is invisible to most profiling tool, and the Source Engine is extremely configurable/scalable.

- RAD Game Tools' Telemetry was used a lot plus a custom batch trace recording mode for analyzing their translation layer. Telemtry offers cross-platform performance visualization systems via a visualizer app, run-time component, and server.

- Some of Valve's optimizations made so far is multi-threading support with the GL mode, removing most calls to glXMakeCurrent, pthreads usage fixes, reducing translation overhead by rewriting the hottest D3D->GL code paths, improved dirty range tracking, added separate uniform array for bone matrices, dynamic buffer updating improvements, and compiler optimizations. The compiler improvements were building the game/engine with -ffast-math and removing -fPIC.

- Extra details not covered during the presentation will be shared via their Linux blog in the coming days.