Drilling Down: DX11 And The Multi-Threaded Game Engine
In spite of the fact that multi-threaded programming has been around for decades, mainstream programmers didn't start focusing on parallel programming until multi-core CPUs started coming along. Much general purpose code is straightforward as a single thread; extracting performance via parallel programming can be difficult and isn't always obvious. Even with talented programmers, Amdahl's Law is a bitch: your speed up from parallelization is limited by the percent of code that is necessarily sequential.
Currently, in game development, rendering is one of those "necessarily" sequential tasks. DirectX 10 isn't set up to appropriately handle multiple threads all throwing commands at the GPU. That doesn't mean parallelization of renderers can't happen, but it does limit speed up because costly synchronization techniques or management threads need to be implemented in order to make sure nothing steps out of line. All this limits the benefit of parallelization and discourages programmers from trying too hard. After all, it's a better idea to put more of your effort into areas where performance can be improved more significantly. (John Carmack put it really well once, but I can't remember the quote... and I'm doing too much benchmarking to go look for it now. :-P)
No matter what anyone does, some stuff in the renderer will need to be sequential. Programs, textures, and resources must be loaded up; geometry happens before pixel processing; draw calls intended to be executed while a certain state is active must have that state set first and not changed until completion. Even in such a massively parallel machine, order must be maintained for many things. But order doesn't always matter.
Making more things thread-safe through an extended device interface using multiple contexts and making a lot of synchronization overhead the responsibility of the API and/or graphics driver, Microsoft has enabled game developers to more easily and effortlessly thread not only their rendering code, but their game code as well. These things will also work on DX10 hardware running on a system with DX11, though some missing hardware optimizations will reduce the performance benefit. But the fundamental ability to write code differently will go a long way to getting programmers more used to and better at parallelization. Let's take a look at the tools available to accomplish this in DX11.
First up is free threaded asynchronous resource loading. That's a bit of a mouthful, but this feature gives developers the ability to upload programs, textures, state objects, and all resources in a thread-safe way and, if desired, concurrent with the rendering process. This doesn't mean that all this stuff will get pushed up in parallel with rendering, as the driver will manage what gets sent to the GPU and when based on priority, but it does mean the developer no longer has to think about synchronizing or manually prioritizing resource loading. Multiple threads can start loading whatever resources they need whenever they need them. The fact that this can also be done concurrently with rendering could improve performance for games that stream in data for massive open worlds in addition to enabling multi-threaded opportunities.
In order to enable this and other threading, the D3D device interface is now split into three separate interfaces: the Device, the Immediate Context, and the Deferred Context. Resource creation is done through the Device. The Immediate Context is the interface for setting device state, draw calls, and queries. There can only be one Device and one Immediate Context. The Deferred Context is another interface for state and draw calls, but many can exist in one program and can be used as the per-thread interface (Deferred Contexts themselves are thread unsafe though). Deferred Contexts and the free threaded resource creation through the device are where DX11 gets it multi-threaded benefit.
Multiple threads submit state and draw calls to their Deferred Context which complies a display list that is eventually executed by the Immediate Context. Games will still need a render thread, and this thread will use the Immediate Context to execute state and draw calls and to consume the display lists generated by Deferred Contexts. In this way, the ultimate destination of all state and draw calls is the Immediate Context, but fine grained synchronization is handled by the API and the display driver so that parallel threads can be better used to contribute to the rendering process. Some limitations on Deferred Contexts include the fact that they cannot query the device and they can't download or read back anything from the GPU. Deferred Contexts can, however, consume the display lists generated by other Deferred Contexts.
The end result of all this is that the future will be more parallel friendly. As two and four core CPUs become more and more popular and 8 and 16 (logical) core CPUs are on the horizon, we need all the help we can get when trying to extract performance from parallelism. This is a good move for DirectX and we hope it will help push game engines to more fully utilize more than two or even four cores when the time comes.
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just4U - Wednesday, February 4, 2009 - link
While there might be some programs out there that definitely do take advantage of 8G of ram.. I haven't noticed that for Vista. It seems to hit a sweet spot at 4Gigs (or 3.3 for 32bit) I don't think 2Gigs of ram is optimal for Vista at all tho, and always push for 4 with all my builds.dzmcm - Tuesday, February 3, 2009 - link
Do yourself the biggest favor and Google kat mouse. I used ubuntu for a while and got spoiled by the mouse wheel functionality. This little program will allow your to scroll windows not in focus. And it adds extra functionality to the middle mouse button (which I just dissable). Plus you can set up per application rules.SoCalBoomer - Monday, February 2, 2009 - link
Fail. Sigh.A: 2Gigs works great. 3 Works FINE. 4 works happily with 64.
B: DECENT video cards work fine. By Decent, I mean anything in the past couple of years - nVidia 7xxx cards, for instance. PCI vid cards? Prolly not. . .
C: Why not just do it within Windows? UAC is easy to turn off. Windows Defender actually works okay. Readyboost doesn't do anything if you don't turn it on, etc.
"Nothing like Ubuntu though" - true. I have Ubuntu on my laptop (although it's got Win7 on it and is a TON more functional now) but then Ubuntu doesn't have ANY common use programs written for it - Open Office is about it and . . . much as I like it, it's missing stuff that I use every day on my desktop.
Games? Well, yes, but there are things other than games - like Office. . . yeah, you know, that behemoth office suite that controls the world? Yeah, not necessarily my fav (except for OneNote - which is awesome) but you gotta do it.
You can't make your mouse work the way you want and THEREFORE Vista fails? Dude. . . epic fail.
stmok - Saturday, January 31, 2009 - link
Vista IS a marketing failure.* It failed to get massive adoption as expected by Microsoft. A good majority of people and businesses are sticking with XP. Go and actually search for world wide statistics, and you'll see for yourself.
* No matter how much Microsoft spent on marketing (see Seinfield+Gates ads and Mojave), it never caught on...Viral and deceptive marketing don't work.
* Why do you think the next one is called "Windows 7" (even though you can basically call it Windows Vista Second Edition). Simple. The name "Vista" is a marketing failure. Its kryptonite to the Windows brand name. That's why they dropped it.
Linux works well when the user has strong motivations to learn and adopt it. When they don't, they are better off using something like Linux Mint (which has all the video codec, and Flash, etc pre-installed) or not even using Linux at all. (Which is better for all involved).
When a user treats Linux like Windows, it just doesn't work. Hence, all the complaining about how Linux on the desktop won't happen. (Linux was never intended for the desktop. The point was to create a cheap Unix-like solution for the x86 platform).
Many don't realise that in Linux, the people take responsibility, not a company like Apple or Microsoft. Its about getting your butt off the ground and doing things to make it happen.
Nothing happens when all you do is sit and complain how Linux isn't this all or that. Its up to you (if you choose to), to do something about it...This is identical to life itself.
gochichi - Thursday, February 19, 2009 - link
If Microsoft wants to go Windows 7 early, they need to reconsider their pricing for Vista upgraders. B/c I have 3 Vista computers and I'm unwilling to spend more than $100.00 to upgrade all 3 of them to Windows 7.That's $33.33 per computer as my ceiling. And I'm not going to upgrade one and not all of them. I have more than earned the right to a good price from Microsoft. If they want to charge me full price for Office 2010 (or whatever it'll be called) that's fine, but OS upgrades can't be $100+ a pop, not if Microsoft wants to change OS's every two years.
I kind of wonder if Vista will pick up traction in the piracy circles once 7 is released. I don't even know if I'll want to upgrade period... I mean, Vista really is fine, and why fix what aint broke.
I've tried Windows 7 and other than being unusually unstable (which is "alright" technically because it's a beta) it does offer up some decent usability improvements. I'm quite sure that most Vista users will feel like I do... namely, that we should be getting the cosmetic upgrades for free... since we're footing their bills and all. Windows 7 is just warmed over Vista... and frankly if you're excited about Windows 7 yet you "hate" Vista... you're weird b/c they're the same thing.
Like I said, Windows 7 is UNSTABLE which given the "mission statement" is truly alarming. I mean, I'm running Windows 7 by itself (no apps installed) and yet sometimes Windows Explorer needs to restart (one of my biggest gripes (probably the only legitimate one) with Vista).
I guess those of us even considering buying OSes separately from PCs are crazy, and now with Windows 7 we'll be even crazier (pricing pending). Hopefully Microsoft will offer us a hand so we can all leave Vista (not because it's horrible, just for uniformity) together. Otherwise, Windows 7 will probably have even worse adoption rates than Vista.
Microsoft: Don't be greedy! Don't split your user base so much. Let me have computers with the same OS without charging me an arm and a leg. Let Vista users have access to DirectX 11 too... b/c the "gaming" community is just going to backlash and boycott 7 like they did Vista.
We all "naturally" want the latest version of the OS, coercion isn't necessary at all.
x86 64 - Thursday, February 26, 2009 - link
Nah, Vista is already easier to pirate than XP ever was. All you need is a BIOS with an SLIC table and a VLK (easily done yourself with some research). Then you activate it using Vista's Software License Service (SLsvc.exe the way big OEM's like Dell and HP mass activate PC's) and MS can't tell the difference between a bogus or a legit installation, unless they overhaul their validation methods (not likely).With XP you had to deal with trying to remove the WGA nagware. Also there are some WinXP Pro student editions floating around that don't need activation because they don't contain WGA. The thought was that eventually MS would ban these keys but it has yet to happen.
I imagine Windows 7 will be just as easy as to pirate as Vista, either through a BIOS softmod or foolproof BIOS hardmod.
nilepez - Wednesday, February 4, 2009 - link
WTF are you talking about? Why would MS have expected Vista to get 30 or 40% market share after 2 years?For all the BS that XP is God and sold like gangbusters out the door, a little research shows that XP had less than 40% market share in 2005
You'd have to be a complete idiot to think that Businesses were transition to Vista quicker than they transitioned to XP.
swaaye - Monday, February 2, 2009 - link
I think Vista has "failed" only because there's little reason for the vast majority of users to switch to it. It's very expensive to upgrade to it, and there's just little reason to. The fact of the matter is that for everyone that doesn't care about DX10 gaming, XP can do anything Vista can.Personally, I think Vista is ok. If Win 7 improves on it, there's nowhere to go but up. But I also have no dislike of XP, either, and certainly see that it has its place on every PC with <2GB RAM.
michaelklachko - Monday, February 2, 2009 - link
That's a good post. Especially about Linux. Thanks!