Archive for October, 2014

While I’m at it….

Wednesday, October 15th, 2014

“Interestingly enough, on desktop-like platforms, PhysX3 is faster than Box2D”

Random PhysX stuff

Tuesday, October 14th, 2014

So, yes, PhysX 3 is well optimized (and PhysX 3.4 even more). But I’d like to go back to the old “2.8.4 is crippled” myth, since this is mentioned here again (”PhyX 2.x was garbage because a ton of sites outed nVidia for using x87 and lacking multi-threading on the old CPU code”).

This is not what happened, at all. I’m afraid you’ve been spoon-fed utter bullshit by websites that love an easy dramatic headline. I suppose it makes a good story to reveal nasty things about “big” companies like MS, Google, Apple, Nvidia, whatever. But the reality behind it here is terribly mundane and ordinary. There is no story. There is no conspiracy. There is no evil plan to cripple this or that.

NovodeX (on which PhysX was initially based) was written by Adam and me. The code was super optimized, to the best of my knowledge at the time. But it did not support SIMD or multi-threading. At that time, none of us knew how to write SSE code, and I also had no experience with multi-threading. Also, IIRC SSE2 was not widely supported (only SSE). From our perspective the gains from SSE seemed limited, using SSE2 would have made the SDK incompatible with most of our users’ machines, and we simply didn’t have the time or resources to learn SSE, support SIMD and non-SIMD versions, etc.

Then came Ageia. In the beginning, we had to make the SDK feature-complete before thinking about making it run faster. NovodeX did not even support convex objects! And that’s the first thing I had to implement in the post-NovodeX days. NovodeX was the fusion of two hobby projects from two independent developers. In a number of ways the result was still just that: a hobby project. We loved it, we worked very hard on it, but we basically had no customers and no actual games making actual requests for random features that are actually useful in games. This all changed when the hobby project became PhysX under Ageia. That’s when it became an actual product. An actual middleware. With actual customers actually using the thing in actual games. Especially when it got picked up by Epic and integrated in the Unreal engine. Suddenly we got TONS of bug reports, feedback, feature requests, you name it. Tons of stuff to do. But as far as I can remember nobody asked for “the SSE version”, and so none of us worked on it. There was no time for that, no incentive to worry about it, and we still didn’t have a SIMD expert at the time anyway. We briefly looked at the new compiler options in MSVC (/SSE2, etc) but the gains were minimal, maybe 15 to 20% at best. If you believe that recompiling with such a flag will magically make your code run 4X faster, I am sorry but you are just a clueless fool misguided soul. At the time, with the available compiler, we never saw more than 20% in the very best of case. And most of the time, for actual scenes running in actual games, we saw virtually no gains at all. Enabling the flag would have given us marginal gains, but would have increased our support burden significantly (forcing us to provide both SIMD and non-SIMD versions). It would have been stupid and pointless. Hence, no SIMD in PhysX2. Simple as that.

For proper SIMD gains you need to design the data structures accordingly and think about that stuff from the ground up, not as an afterthought. And this is exactly what we did for PhysX3. After making PhysX2 stable and complete enough, after making it a real, useable, feature-complete middleware, it was finally time to think about optimizations again. It took time to make the software mature enough for this to be even on the roadmap. It took time for us (and for me) to actually learn SIMD and multi-threading. It took time for compilers to catch up (/SSE2 on latest versions of MSVC is way way way better and produces way more efficient code than what it did when we first tried it). It took time for SSE2 support to spread, and be available in all machines (these days we only have a SIMD version - there is no non-SIMD version. It would have been unthinkable before). And still, even after all this happened, a better algorithm, or better data structures or less cache misses, still give you more gains than SIMD. SIMD itself does not guarantee that your code is any good. Any non-SIMD code can kick SIMD code’s ass any day of the week if SIMD code is clueless about everything else. Anybody claiming that PhysX2 is “garbage” because it doesn’t use SIMD is just a ridiculous moron (pardon my French but hey, I’m French) clearly not a programmer worth his salt (or not a programmer at all for that matter).

So there was no crippling. The old version is just that: old. The code I wrote 10 years ago, as fast as it appeared to be at the time, is not a match for the code I write today. Opcode 2 (which will be included in PhysX 3.4) is several times faster than Opcode 1.3, even though that collision library is famous for its performance. It’s the same for PhysX. PhysX 2 was faster than NovodeX/PhysX 1. PhysX 3 is faster than PhysX 2. We learn new tricks. We find new ideas. We simply get more time to try more options and select the best one.

As the guy in the article says, PhysX3 is so fast that it changed his mind about the whole GPU Physics thing. Does that sound like we’re trying to promote GPU Physics by crippling PhysX3? Of course not. And in the same way we did not try to promote Ageia Physics by crippling PhysX2. We were and we are a proud bunch of engineers who love to make things go fast - software or hardware.

EDIT: I forgot something. Contrary to what people also claim, PhysX works just fine on consoles and it is a multi-platforms library. That is, writing SIMD is not as easy as hardcoding a bunch of SSE2 intrinsics in the middle of the code. It has to be properly supported on all platforms, including some that do not like basic things like shuffles, or do not support very useful instructions like movemask. Converting something to SIMD means writing the converted code several times, possibly in different ways, making sure that the SIMD versions are faster than their non-SIMD counterparts on each platform - which is not a given at all. It takes a lot of time and a lot of effort, and gains vary a lot from one platform to the next. cialis