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Does Microsoft purposely slow down OpenGL?

27 posts in this topic

The sad thing though is that apparently Valve was just using a Direct3D wrapper (ala Wine). Ouch if this is the case, that'd mean emulated Direct3D is faster than Direct3D itself...


Reading that I get the impression they were using their own translation layer rather than an external one such as WINE which makes sense as it'll have a much lower code cost than trying to effectively rewrite the renderer so that it can natively target both platforms.


The problem with trying to draw any conclusions from all this is that there is still a frustrating lack of details on system performance over all; L4D is hardly the most demanding of games (as evidenced by the frame rate) but I'd like to know the overall system load from this multi-threaded approach, how many cores is it using (and indeed memory) compared to the D3D version as that would impact other games with more demanding resource requirements.

(It also remains a D3D9 spec'd engine; everyone else is running off into D3D11 land, heck it's been 18months since we killed support for D3D9 in our engine at work, so seeing a more modern D3D11-style engine tested in this manner would also be nice.)


Don't get me wrong, this is good work when all is said and done BUT the lack of some details is bothersome to me as it prevents detailed analysis of just what is going on.


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Honestly, it'd be wrong to even use the results they got as a comparison when the proper way to use OpenGL would be to rewrite the code from scratch rather than faking Direct3D, and some engines already had their own OpenGL pipelines since the beginning. Also you have to take into account the hardware they used... pretty sure OpenGL suffers a lot on AMD hardware (they don't care as much about it). For a proper test usually you use a wide range of hardware. They never said which versions of the APIs they used, either =P Direct3D 9 should only be compared to OpenGL 2.1, if they used OpenGL 3.x or 4.x the comparison probably isn't proper (since that'd be akin to D3D10 and D3D11, respectively - at least in theory...).


Also L4D may not be one of the most demanding games but the framerate is in the low hundreds (at least during that test)...


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The slides say;

- D3D9-like API with extensions and only shader model 2.0b, which implies a GL2 context


Also the framerate, post optimisation work, is ~300 which is very low demand work load both CPU and GPU wise as they can turn out a frame every 3.3ms or so; I'd call that the very definition of not demanding ;)


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It's important to look at the development process of Valve's port.


Their initial working implementation actually ran at 6 FPS - that's not a case of "ZOMG!  OpenGL Is teh slower than D3D!" - that's (as they note) "typical of an initial successful port to a new platform".


Following that they identified 3 key areas for improvement - two relating to the game itself and one relating to the driver.  Of those, only the last one (that relating to the driver) is - strictly speaking - directly relevant to the OpenGL vs D3D performance question, but that would be assuming that all other things are equal, and of course the game's use of the API is also a huge factor.


Working with the hardware vendors on resolving these, as well as fixing up the game's use of the API, gave the end result, following which they again noted that "it comes down to a few additional microseconds overhead per batch in Direct3D which does not affect OpenGL on Windows" - so the specific reason for the performance difference was identified, and it was isolated to one very specific difference between the two APIs; this is quite clearly not a case "my API is faster than your API" but rather a case of "this one part of my API, in the way we use it for this one particular game, on this one particular hardware configuration, is faster than the corresponding one part of your API in the same circumstances".


That's about as far from a meaningful scientific comparison as it's possible to get.


In actual fact, well-coded OpenGL will always smoke poorly-coded D3D, just as well-coded D3D will likewise smoke poorly-coded OpenGL.  I'm not saying that Valve's D3D code is poor (I'd expect it to be close enough to the best D3D code on the planet), but it is still using an ancient version of the API which - guess what - is widely recognized to have more overhead in exactly the area that Valve noted.  And that would be an expected result for any code that particularly stresses that particular area.  So the end result actually does nothing more than confirm what we already knew!  Wowsers!


A more detailed breakdown of performance comparisons between different parts of each renderer would be expected to see D3D pulling ahead in some areas but GL pulling ahead in others.  If they're doing any dynamic vertex buffer updates, for example, then - assuming a GL2.0/2.1 baseline - I'd expect to see D3D doing those faster than the GL code.


Finally, it's necessary to note Valve's conclusion: "now that we know the hardware is capable of more performance, we will go back and figure out how to mitigate this effect under Direct3D."   Again, this is not a case of "my API is faster than your API", this is instead a recognition that their own use of D3D may have room for further performance improvment and that a mitigation of the identified bottleneck may see D3D pulling ahead again.


Like they said at the very start: "performance is a complicated issue".


All references: http://blogs.valvesoftware.com/linux/faster-zombies/


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