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CPU PhysX: Multi-Threading?

Analysis: PhysX On Systems With AMD Graphics Cards
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Does CPU PhysX Really Not Support Multiple Cores?

Our next problem is that, in almost all previous benchmarks, only one CPU core has really been used for PhysX in the absence of GPU hardware acceleration--or so some say. Again, this seems like somewhat of a contradiction given our measurements of fairly good CPU-based PhysX scaling in Metro 2033 benchmarks.

Graphics card
GeForce GTX 480 1.5 GB
Dedicated PhysX card
GeForce GTX 285 1 GB
Graphic drivers
GeForce 258.96
PhysX
9.10.0513


First, we measure CPU core utilization. We switch to DirectX 11 mode with its multi-threading support to get a real picture of performance. The top section of the graph below shows that CPU cores are rather evenly utilized when extended physics is deactivated.

In order to widen the bottleneck effect of the graphics card, we start out with a resolution of just 1280x1024. The less the graphics card acts as a limiting factor, the better the game scales with more cores. This would change with the DirectX 9 mode, as it limits the scaling to two CPU cores.

We notice a small increase in CPU utilization when activating GPU-based PhysX because the graphics card needs to be supplied with data for calculations. However, the increase is much larger with CPU-based PhysX activated, indicating a fairly successful parallelization implementation by the developers.

Looking at Metro 2033, we also see that a reasonable use of PhysX effects is playable, even if no PhysX acceleration is available. This is because Metro 2033 is mostly limited by the main graphics card and its 3D performance, rather than added PhysX effects. There is one exception, though: the simultaneous explosions of several bombs. In this case, the CPU suffers from serious frame rate drops, although the game is still playable. Most people won’t want to play at such low resolutions, so we switched to the other extreme.

Performing these benchmarks with a powerful main graphics card and a dedicated PhysX card was a deliberate choice, given that a single Nvidia card normally suffers from some performance penalties with GPU-based PhysX enabled. Things would get quite bad in this already-GPU-constrained game. In this case, the difference between CPU-based PhysX on a fast six-core processor with well-implemented multi-threading and a single GPU is almost zero.

Assessment

Contrary to some headlines, the Nvidia PhysX SDK actually offers multi-core support for CPUs. When used correctly, it even comes dangerously close to the performance of a single-card, GPU-based solution. Despite this, however, there's still a catch. PhysX automatically handles thread distribution, moving the load away from the CPU and onto the GPU when a compatible graphics card is active. Game developers need to shift some of the load back to the CPU.

Why does this so rarely happen?

The effort and expenditure required to implement coding changes obviously works as a deterrent. We still think that developers should be honest and openly admit this, though. Studying certain games (with a certain logo in the credits) begs the question of whether this additional expense was spared for commercial or marketing reasons. On one hand, Nvidia has a duty to developers, helping them integrate compelling effects that gamers will be able to enjoy that might not have made it into the game otherwise. On the other hand, Nvidia wants to prevent (and with good reason) prejudices from getting out of hand. According to Nvidia, SDK 3.0 already offers these capabilities, so we look forward to seeing developers implement them.

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