"The point is ladies and gentlemen that greed, for lack of a better word, is good. Greed is right. Greed works. Greed clarifies, cuts through and captures the essence of the evolutionary spirit. Greed, in all of it's forms - greed for life, for money, knowledge - has marked the upward surge of mankind."
- Gordon Gekko
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EDRAM
Saturday, May 29, 2010 | Permalink

It's funny how sometimes as technology develops what was originally a good can become a bad idea. EDRAM for video cards is such a thing. AFAIK no video card for the PC ever had it, but it's been used occasionally in some game consoles, most recently in the Xbox 360. However, I recall back when bitboys were trying to enter the PC video card industry using EDRAM based designs. Unfortunately they never succeeded and none of their products ever saw the light of the day. Had they been able to produce one at the time though, chances are it would have worked very well. However, how people render the frames today and back in the DX7 era is very different, which makes EDRAM far less ideal today. Back in the DX7 days you'd probably render to the backbuffer almost all the time, and given no shaders and as many ROP as there were pipelines the biggest bottleneck was generally bandwidth to the backbuffer. That's hardly the case anymore. Today shaders are long, bottlenecks are usually ALU or texture fetches, and even if you end up being limited at the backend you're normally not bandwidth bound but ROP bound.

Having worked a fair amount with the Xbox 360 the last 2.5 years I find that EDRAM mostly is standing in the way, and rarely providing any benefit. Unfortunately, you can't just render "normally" to a buffer in memory if you so prefer, nope, you always have to render to the EDRAM. Once you're done with your rendering the results have to be resolved to the video memory. So even if we assume we are never ROP bound and EDRAM gets to shine with its awesome bandwidth, it really would not buy us much. Each render target operation is immediately followed by a resolve operation copying the data to video memory. During this copying phase the GPU is busy just copying rather than rendering. If the rendering was targetting a video memory buffer to begin those writes to memory would be nicely interleaved with the rendering work that the GPU does and no resolve would be necessary, so once the rendering is done all that's needed is to flush whatever data is residing in the backend cache to memory and you're done.

Sadly it's not just that it doesn't really provide so much of a benefit as it might look on paper, but it also alters the rendering model that we are all familiar with and adds a bunch of new restrictions. Because EDRAM is still quite expensive in hardware it's not something we get an awful lot of. The Xbox 360 has 10MB. But if you render to the typical 1280x720 resolution with 2xAA, that's 14MB needed for the color and depth buffer. So this is generally solved by "tiling", which means you render for instance to the top of the screen first, then resolve, and then the bottom, and resolve. The DirectX9 for Xbox helps out a bit here to let you do this stuff quite automatically by entering a tiling section of the rendering, which is then submitted twice to the hardware, or how many times necessary depending on how many tiles are require for your render target configuration. Sounds fine huh? Well, until you want to squeeze in another render target operation somewhere in that stream. Say you want to apply some SSAO. You need the complete depth buffer for opaque stuff and then apply before rendering transparent stuff. SSAO can be quite expensive, so instead of using oodles of samples you probably want to take a few, then blur the result to half-res, and then apply that. Well, to blur you need to switch render target, which breaks the model. In order for everything to work you need to first resolve everything, do your SSAO passes, then copy that back to EDRAM again and enter a new tiling section. This is of course way too costly so nobody bothers doing that kind of stuff, but instead just try to live with the limitations imposed. So one may attempt to just resolving the depth-buffer without switching render target and then apply SSAO in one pass. Unfortunately, not even this is ideal. The problem is that when the top tile enters this code only the top tiles has been rendered, so the depth buffer texture it will use for the effect is incomplete. So when it samples the neighborhood around pixels close to the edge it will sample over the edge and get data from the previous frame. This often results in visible seams when in motion. It's common to copy the backbuffer for refraction effects. In many games on the Xbox 360 you'll see visible seems when traveling on water for this reason.

For the next generation consoles chances are we want 1080p, full HDR, at least 4xMSAA and probably many want additional buffers for deferred rendering or other techniques. I don't think it will be possible to embed enough EDRAM to fit all for many games, so if you're designing a future console now and are thinking of using EDRAM, please don't. Or at least let us render directly to memory. Or only let the EDRAM work as a large cache or something if you really want it.

[ 12 comments | Last comment by mark (2010-07-27 23:27:20) ]

Pac-man turns 30!
Friday, May 21, 2010 | Permalink

Happy anniversary Pac-man!

[ 3 comments | Last comment by fmoreira (2010-05-22 17:10:40) ]

My other project ...
Friday, April 16, 2010 | Permalink

Some of you may have noticed a bit of a decline in my posting rate on my blog. No DX11 demo yet, little news on Framework4, not much commentary on the state of the industry. Most posts recently has been on Just Cause 2, which of course is natural since we approached and passed the launch of the game. However, the main reason is that there's another project that has taken up a considerable amount of time and certainly occupied my mind a lot the last half year, which just recently culminated in this:



Yes, that's me and my fiance's hands. We got engaged on April 4, under the starry sky by the beach on Tenerife. And yesterday we got our rings.

[ 39 comments | Last comment by rebb (2010-05-21 16:51:20) ]

Just Cause 2 tops the UK charts
Monday, March 29, 2010 | Permalink

Woot!

[ 7 comments | Last comment by vis (2010-04-09 19:38:42) ]

56.5 percent have DX10+ system
Friday, March 19, 2010 | Permalink

As I touched on before the lack of XP in Just Cause 2 is not as bad as it may first have seemed. In fact, now that the demo has been released I have not heard any more criticism on that, because what we could do with a DX10 only renderer certainly justified the move and now it has turned more into a selling point of the game.

In addition to this I find the latest Steam survey interesting. The DX10/11 systems around are now 56.5%, which if we still count on about 15% in the bottom being too low-end anyway, that means we now reach 66% of the potential customers. In my last post I counted on a drop for XP by 2 percentage points per months, but the last survey noted a drop of 3.5% in the last month alone.

[ 14 comments | Last comment by ocsi (2010-05-13 18:00:06) ]

OpenGL 3.3 and 4.0
Friday, March 12, 2010 | Permalink

OK, I was not prepared for this one, but Khronos has released not just one but two new versions of OpenGL, 3.3 and 4.0. The 3.3 version is a nice upgrade on 3.2 and the 4.0 is pretty much a match for DirectX 11. To me it's clear that OpenGL is back on track again.

Now we just need drivers.

[ 15 comments | Last comment by mark (2010-04-12 23:01:37) ]

Mac as a gaming platform
Wednesday, March 10, 2010 | Permalink

Recently Valve announced that they will not just deliver Steam on Mac, but also a bunch of actual games for the Mac platform. Even though I'm a PC guy I find this to be excellent news. We need competition in the OS market, and one of the main reasons many people stay on the Windows platform is because it's the only viable platform for gaming. With a heavy-weighter like Valve giving Mac full attention, it could really change that. I don't think every developer will follow, but I think it will mean a substantial increase in games available on the Mac. Another side effect of this is that OpenGL will again become an important API. No bold new initiative from Khronos could ever make the same kind of impact as having Steam and popular games on the Mac platform. I think this is also good news for PC gamers, because it expands that gaming audience on computers. I wish Valve all the success on the Mac platform.

[ 15 comments | Last comment by mark (2010-04-12 23:03:29) ]

Just Cause 2 demo
Friday, March 5, 2010 | Permalink

I didn't get time to post this yesterday, because, well, I was busy playing the demo myself. But anyway, the demo is out for your favorite platform. Personally I like PC, so get it from Steam here.

Whether you already played the demo or not it's worth watching the new demo trailer.

If you're not tired of Just Cause 2 videos yet, the Nvidia added features for the PC version is a recommended view too.

[ 5 comments | Last comment by mark (2010-03-31 04:27:31) ]

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