Monday, January 26, 2015

NVIDIA Discloses Full Memory Architecture and Limitations of GTX 970: Utilizing Only 3.5GB of the 4GB of Ram Frame Buffer Shenanigan, Gamer Madness Will Ensue

Yes, that last 0.5GB of memory on your GeForce GTX 970 does run slower than the first 3.5GB. More interesting than that fact is the reason why it does, and why the result is better than you might have otherwise expected. Last night we got a chance to talk with NVIDIA’s Senior VP of GPU Engineering, Jonah Alben on this specific concern and got a detailed explanation to why gamers are seeing what they are seeing along with new disclosures on the architecture of the GM204 version of Maxwell.
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NVIDIA's Jonah Alben, SVP of GPU EngineeringFor those looking for a little background, you should read over my story from this weekend that looks at NVIDIA's first response to the claims that the GeForce GTX 970 cards currently selling were only properly utilizing 3.5GB of the 4GB frame buffer. While it definitely helped answer some questions it raised plenty more which is whey we requested a talk with Alben, even on a Sunday.
Let’s start with a new diagram drawn by Alben specifically for this discussion.
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GTX 970 Memory System
Believe it or not, every issue discussed in any forum about the GTX 970 memory issue is going to be explained by this diagram. Along the top you will see 13 enabled SMMs, each with 128 CUDA cores for the total of 1664 as expected. (Three grayed out SMMs represent those disabled from a full GM204 / GTX 980.) The most important part here is the memory system though, connected to the SMMs through a crossbar interface. That interface has 8 total ports to connect to collections of L2 cache and memory controllers, all of which are utilized in a GTX 980. With a GTX 970 though, only 7 of those ports are enabled, taking one of the combination L2 cache / ROP units along with it. However, the 32-bit memory controller segment remains.
You should take two things away from that simple description. First, despite initial reviews and information from NVIDIA, the GTX 970 actually has fewer ROPs and less L2 cache than the GTX 980. NVIDIA says this was an error in the reviewer’s guide and a misunderstanding between the engineering team and the technical PR team on how the architecture itself functioned. That means the GTX 970 has 56 ROPs and 1792 KB of L2 cache compared to 64 ROPs and 2048 KB of L2 cache for the GTX 980. Before people complain about the ROP count difference as a performance bottleneck, keep in mind that the 13 SMMs in the GTX 970 can only output 52 pixels/clock and the seven segments of 8 ROPs each (56 total) can handle 56 pixels/clock. The SMMs are the bottleneck, not the ROPs.

*To those wondering how peak bandwidth would remain at 224 GB/s despite the division of memory controllers on the GTX 970, Alben stated that it can reach that speed only when memory is being accessed in both pools.
Second to that, it turns out the disabled SMMs have nothing to do with the performance issues experienced or the memory system complications.
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Full GM204 Block Diagram
In a GTX 980, each block of L2 / ROPs directly communicate through a 32-bit portion of the GM204 memory interface and then to a 512MB section of on-board memory. When designing the GTX 970, NVIDIA used a new capability of Maxwell to implement the system in an improved fashion than would not have been possible with Kepler or previous architectures. Maxwell’s configurability allowed NVIDIA to disable a portion of the L2 cache and ROP units while using a “buddy interface” to continue to light up and use all of the memory controller segments. Now, the SMMs use a single L2 interface to communicate with both banks of DRAM (on the far right) which does create a new concern.
A quick note about the GTX 980 here: it uses a 1KB memory access stride to walk across the memory bus from left to right, able to hit all 4GB in this capacity. But the GTX 970 and its altered design has to do things differently. If you walked across the memory interface in the exact same way, over the same 4GB capacity, the 7th crossbar port would tend to always get twice as many requests as the other port (because it has two memories attached).  In the short term that could be ok due to queuing in the memory path.  But in the long term if the 7th port is fully busy, and is getting twice as many requests as the other port, then the other six must be only half busy, to match with the 2:1 ratio.  So the overall bandwidth would be roughly half of peak. This would cause dramatic underutilization and would prevent optimal performance and efficiency for the GPU.

Let's be blunt here: access to the 0.5GB of memory, on its own and in a vacuum, would occur at 1/7th of the speed of the 3.5GB pool of memory.

To avert this, NVIDIA divided the memory into two pools, a 3.5GB pool which maps to seven of the DRAMs and a 0.5GB pool which maps to the eighth DRAM.  The larger, primary pool is given priority and is then accessed in the expected 1-2-3-4-5-6-7-1-2-3-4-5-6-7 pattern, with equal request rates on each crossbar port, so bandwidth is balanced and can be maximized. And since the vast majority of gaming situations occur well under the 3.5GB memory size this determination makes perfect sense. It is those instances where memory above 3.5GB needs to be accessed where things get more interesting.
Let's be blunt here: access to the 0.5GB of memory, on its own and in a vacuum, would occur at 1/7th of the speed of the 3.5GB pool of memory. If you look at the Nai benchmarks floating around, this is what you are seeing.
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Check the result on the left: 22.35 GB/s is almost exacly 1/7th of 150 GB/s
But the net result for gaming scenarios is much less dramatic than that, so why is that the case? It comes down to the way that memory is allocated by the operating system for applications and games. As memory is requested by a game, the operating system will allocate portions for it depending on many factors. These include the exact data space that the game asked for, what the OS has available and what the heuristic patterns of the software models deem at the time. Not all memory is accessed in the same way, even for PC games.
If a game has allocated 3GB of graphics memory it might be using only 500MB of a regular basis with much of the rest only there for periodic, on-demand use. Things like compressed textures that are not as time sensitive as other material require much less bandwidth and can be moved around to other memory locations with less performance penalty. Not all allocated graphics memory is the same and innevitably there are large sections of this storage that is reserved but rarely used at any given point in time.

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