Pascal GPU Architecture

The Pascal GP102-350 GPU

The GPU is based on DX12 compatible architecture called Pascal. Much like in the past designs you will see pre-modelled SMX clusters that hold what is 2x64 shader processors per cluster. Pascal GPUs are composed of different configurations of Graphics Processing Clusters (GPCs), Streaming Multiprocessors (SMs), and memory controllers. Each SM is paired with a PolyMorph Engine that handles vertex fetch, tessellation, viewport transformation, vertex attribute setup, and perspective correction. The GP102 PolyMorph Engine also includes a new Simultaneous Multi-Projection units. There are 28 active (SM) clusters for a fully enabled Pascal GP102 GPU. The NVIDIA GeForce GTX 1080 Ti however is not fully enabled. Thus two out of two GPCs have been disabled.

• The GeForce GTX 1060 (GP104-400) has 10 x 128 shader processors makes a total of 1,280 shader processors.
• The GeForce GTX 1070 (GP104-200) has 15 x 128 shader processors makes a total of 1,920 shader processors.
• The GeForce GTX 1080 (GP104-400) has 20 x 128 shader processors makes a total of 2,560 shader processors.
• The GeForce GTX 1080 Ti (GP102-350) has 28 x 128 shader processors makes a total of 3,584 shader processors.
• The Nvidia Titan X (GP102-400) has 28 x 128 shader processors makes a total of 3,584 shader processors.

Each SM has a cluster of 64 shader / stream / cuda processors doubled up. Don't let that confuse you, it is 128 shader units per SM. Each GPC ships with a dedicated raster engine and 28 SMs. Each SM contains 128 CUDA cores, 256 KB of register file capacity, a 96 KB shared memory unit, 48 KB of total L1 cache storage, and eight texture units. The 1080 Ti card will be released with a core clock frequency of 1.48 GHz with a Boost frequency that can run up to 1.58 GHz (and even higher depending on load and thermals).
  

 
As far as the memory specs of the GP102 GPU are concerned, these boards will feature a 352-bit memory bus connected to a nice 11 GB of GDDR5X video memory, AKA VRAM AKA framebuffer AKA graphics memory for the graphics card. The GeForce GTX 1000 series are DirectX 12 ready, in our testing we'll address some Async compute tests as well as Pascal now has Enhanced Async compute. The latest revision of DX12 is a Windows 10 feature only, yet will bring in significant optimizations. With 11 GB graphics memory available for one GPU, the card is very attractive for both modern and future games no matter what resolution you game at. 

Color Compression

The card uses GDDR5X memory, the really good stuff at 11,000 MHz (effective), 11 GB and all on a 352-bit wise memory bus. Well, you can never have too much bandwidth so Nvidia applied some more tricks, color compression being one of them. The GPU’s compression pipeline has a number of different algorithms that intelligently determine the most efficient way to compress the data. One of the most important algorithms is delta color compression. With delta color compression, the GPU calculates the differences between pixels in a block and stores the block as a set of reference pixels plus the delta values from the reference. If the deltas are small then only a few bits per pixel are needed. If the packed together result of reference values plus delta values is less than half the uncompressed storage size, then delta color compression succeeds and the data is stored at half size (2:1 compression). Pascal GPUs include a significantly enhanced delta color compression capability:

• 2:1 compression has been enhanced to be effective more often
• A new 4:1 delta color compression mode has been added to cover cases where the per pixel deltas are very small and are possible to pack into ¼ of the original storage
• A new 8:1 delta color compression mode combines 4:1 constant color compression of 2x2 pixel blocks with 2:1 compression of the deltas between those blocks

With that additional memory bandwidth combined with new advancements in color compression Nvidia can claim even more bandwidth as Pascal cards now use 4^th generation delta color compression thanks to enhanced color compression and enhanced caching techniques. Up-to Maxwell the GPU could handle 2:1 color compression ratios, newly added are 4:1 and 8:1 delta color compression. So on one hand the Raw memory bandwidth increases 1.4x~1.5x (for the GeForce GTX 1080 / Ti / Titan X with GDDR5X) and then there's a compression benefit of 1.2x for the GeForce GTX 1000 series which is a nice step up in this generation technology wise. Overall there is an increase of roughly 1.7x in memory bandwidth thanks to the faster memory and new color compression technologies. More effective bandwidth thanks to enhanced color compression and enhanced caching techniques. The effectiveness of delta color compression depends on the specifics of which pixel ordering is chosen for the delta color calculation. The GPU is able to significantly reduce the number of bytes that have to be fetched from memory per frame.

Pascal Graphics Architecture

Let's place the more important data on the GPU into a chart to get an idea and better overview of changes in terms of architecture like shaders, ROPs and where we are at frequencies wise:

Reference GeForce	GTX 1080 Ti	Titan X	GTX 1080	GTX 1070	GTX 1060	GTX 980 Ti
		(2016 edition)
GPU	GP102-350-A1	GP102-400-A1	GP104-400-A1	GP104-200-A1	GP106-400-A1	GM200
Architecture	Pascal	Pascal	Pascal	Pascal	Pascal	Maxwell
Transistor count	12 Billion	12 Billion	7.2 Billion	7.2 Billion	4.4 Billion	8 Billion
Fabrication Node	TSMC 16 nm	TSMC 16 nm	TSMC 16 nm	TSMC 16 nm	TSMC 16 nm	TSMC 28 nm
CUDA Cores	3,584	3,584	2,560	1,920	1,280	2,816
SMMs / SMXs	28	28	20	15	10	22
ROPs	88	96	64	64	48	96
GPU Clock Core	1,480	1,417 MHz	1,607 MHz	1,506 MHz	1,506 MHz	1,002 MHz
GPU Boost clock	1,582	1,531 MHz	1,733 MHz	1,683 MHz	1,709 MHz	1,076 MHz
Memory Clock	2752 MHz	2500 MHz	1,250 MHz	2,000 MHz	2,000 MHz	1,753 MHz
Memory Size	11 GB	12 GB	8 GB	8 GB	3 GB / 6 GB	6 GB
Memory Bus	352-bit	384-bit	256-bit	256-bit	192-bit	384-bit
Memory Bandwidth	484 GB/s	480 GB/s	320 GB/s	256 GB/s	192 GB/s	337 GB/s
FP Performance	11.5 TFLOPS	11.0 TFLOPS	9.0 TFLOPS	6.45 TFLOPS	4.61 TFLOPS	6.4 TFLOPS
GPU Thermal Threshold	91 Degrees C	91 Degrees C	94 Degrees C	94 Degrees C	94 Degrees C	91 Degrees C
TDP	250 Watts	250 Watts	180 Watts	150 Watts	120 Watts	250 Watts
Launch MSRP ref	$699	$1200	$499	$379/$449	$249/$299	$699

So we talked about the core clocks, specifications and memory partitions. However, to be able to better understand a graphics processor you simply need to break it down into tiny pieces. Let's first look at the raw data that most of you can understand and grasp. This bit will be about the architecture. NVIDIA’s “Pascal” GPU architecture implements a number of architectural enhancements designed to extract even more performance and more power efficiency per watt consumed. Above, in the chart photo, we see the block diagram that visualizes the architecture, Nvidia started developing the Pascal architecture around 2013/2014 already. The GPCs have 28 SMX/SMM (streaming multi-processors) clusters in total. You'll spot twelve 32-bit memory interfaces, bringing in a 384-bit path to the graphics GDDR5X memory pipeline. Tied to each 32-bit memory controller are eight ROP units and 256 KB of L2 cache. Simple math now tells us the full GP102 chip used in Titan X thus has a total of 96 ROPs and 3,072 KB of L2 cache. What about double-precision? It's dumbed down to not interfere with Quadro sales -- double-precision instruction throughput is 1/32 the rate of single-precision instruction throughput. So the compute capabilities are:

• FP32 : 11TF, INT8 : 44TOPS
• FP64 : 11TF x 1/32 
• FP16 : 11TF x 1/64 (but can use lib to convert FP16 to FP32)

An important thing to focus on is the SM (block of shader processors) clusters (SMX), which have 128 shader processors. One SMX holds 128 single‐precision shader cores, double‐precision units, special function units (SFU), and load/store units. So based on a full 28 SM (3.584 shader proc) core chip the looks are fairly familiar in design. In the pipeline we run into the ROP (Raster Operation) engine and the GP102 has 96 engines for features like pixel blending and AA. The GPU has 64 KB of L1 cache for each SMX plus a special 48 KB texture unit memory that can be utilized as a read-only cache. The GPU’s texture units are a valuable resource for compute programs with a need to sample or filter image data. The texture throughput then, each SMX unit contains 8 texture filtering units.

• GeForce GTX 960 has 8 SMX x 8 Texture units = 64
• GeForce GTX 970 has 13 SMX x 8 Texture units = 104
• GeForce GTX 980 has 16 SMX x 8 Texture units = 128
• GeForce Titan X has 24 SMX x 8 Texture units = 192
• GeForce GTX 1060 has 10 SMX x 8 Texture units = 80
• GeForce GTX 1070 has 15 SMX x 8 Texture units = 120
• GeForce GTX 1080 has 20 SMX x 8 Texture units = 160
• NVIDIA GTX 1080 Ti has 28 SMX x 8 Texture units = 224
• NVIDIA Titan X (2016) has 28 SMX x 8 Texture units = 224

So there's a total of up-to 28 SMX x 8 TU = 224 texture filtering units available for the silicon itself. Actually 240 if all SMXes would have been enabled for the SKU.