Balancing Efficiency and Flexibility for DNN Acceleration via Temporal GPU-Systolic Array Integration
The research interest in specialized hardware accelerators for deep neural networks (DNN) spiked recently owing to their superior performance and efficiency. However, today’s DNN accelerators primarily focus on accelerating specific “kernels” such as convolution and matrix multiplication, which are vital but only part of an end-to-end DNN-enabled application. Meaningful speedups over the entire application often require supporting computations that are, while massively parallel, ill-suited to DNN accelerators. Integrating a general-purpose processor such as a CPU or a GPU incurs significant data movement overhead and leads to resource under-utilization on the DNN accelerators.
We propose Simultaneous Multi-mode Architecture (SMA), a novel architecture design and execution model that offers general-purpose programmability on DNN accelerators in order to accelerate end-to-end applications. The key to SMA is the temporal integration of the systolic execution model with the GPU-like SIMD execution model. The SMA exploits the common components shared between the systolic-array accelerator and the GPU, and provides lightweight reconfiguration capability to switch between the two modes in-situ. The SMA achieves up to 63% performance improvement while consuming 23% less energy than the baseline Volta architecture with TensorCore.
Deep learning has revolutionized the key application domains such as computer vision  and natural-language processing . The GPU architecture was the primary enabler of this second wave of deep learning. Although still widely used for training and inference, GPU’s performance and energy efficiency start to plateau. DNN models have high computation intensity and regular data-flow patterns, which are amenable to specialized hardware accelerators for continuous performance improvements. Prime examples include Eyeriss , Diannao Family , TPU , and TensorCore (TC) .
However, existing accelerators are insufficient to provide meaningful speedup for DNN-enabled applications. This is because they focus only on specific “kernels”, mainly the convolution operation, while largely ignoring the end-to-end characteristics of DNN-enabled applications. As a result, accelerating those fixed kernels leads to insignificant speedup, sometimes even slow-down, at the application-level. We highlight two reasons. First, many emerging DNNs have started moving towards “hybrid” models, which combine conventional GEMM kernels with irregular kernels that are, although massively parallel, ill-suited for specialized DNN accelerators. Second, applications such as autonomous driving adopt not only DNN models but also non-DNN based algorithms , which are not amenable to conventional DNN accelerators.
To support end-to-end applications, the hardware architecture must couple the specialized DNN accelerators with flexibilities for non-GEMM computations. Today’s mainstream solutions fall into three categories, neither being efficient. The first category integrates a DNN accelerator with a host processor that is general-purpose, which is tasked with non-GEMM operations. However, this approach incurs significant data movement overhead and under-utilizes the accelerator resource during non-GEMM computations. Second, systems like cloud TPU convert non-GEMM operations into GEMM-compatible operations that are amenable to acceleration but are inefficient on DNN accelerators. Finally, the GPU-based systems provide general-purpose programmability and execute both GEMM operations and non-GEMM operations, but are inefficient on DNNs compared to specialized accelerators.
We propose SMA, a new architecture that is designed to provide flexibility in accelerating non-GEMM operations while providing efficiency in GEMM operations. As a result, SMA leads to significant end-to-end application speedup compared to state-of-the-art systems. The SMA’s design philosophy starts with a conventional SIMD execution in common GPU architecture, and judiciously apply lightweight architectural augmentations to support the systolic execution model, which is proven efficient for convolutions .
While it is well-known that SIMD and systolic array share a high architectural similarity in both computation and memory structure, as exemplified in the recent TC in Nvidia’s GPUs , SMA is different from TC in two key aspects. First, SMA temporally integrates the systolic array and SIMD architecture while TC does so spatially, leading to area waste. Second, SMA employs a SIMD-friendly dataflow, which achieves high data reuse as a systolic array while maximizing the memory access efficiency as a GPU. In contrast, TC’s data-flow suffers from low data reuse.
The critical challenge in SMA architecture is to achieve a high execution efficiency in the systolic mode while keeping the runtime reconfigurability overhead low. We leverage GPU’s massive parallelism and propose a new fine-grained synchronization scheme, which provides building blocks for controlling the systolic array with little overhead. SMA eliminates most of the expensive register file and shared memory accesses and significantly improves the matrix multiplication efficiency than the SIMD-only mode. SMA completely preserves the programmability of the GPU, enabling efficient hybrid DNN workload acceleration.
The contribution of our work is as follows:
We quantify the execution inefficiencies of running DNNs with irregular operations on a variety of existing hardware platforms and identify their execution bottlenecks.
We consider various systolic array dataflows and identify a SIMD-friendly one that balances memory access efficiency and data reuse to facilitate its integration on GPUs.
We propose SMA, an architecture that temporally integrates SIMD and systolic execution model with little reconfiguration overhead by exploiting the architectural similarity.
We evaluate the performance and efficiency of SMA, and demonstrate its flexibility for emerging end-to-end applications with diverse DNN models and algorithmic choices.
Ii Analysis of Existing DNN Accelerators
We analyze the performance of executing the emerging hybrid DNN models on existing hardware accelerators, i.e., TPU  and TensorCore (TC) . They are the two commercially available DNN accelerators with highly optimized software stacks. However, our experimental results identify that the GEMM-incompatible computation in the hybrid models is too computational-demanding to run on the hardware accelerators even coupled with a general-purpose CPU.
Ii-a DNN Hardware Accelerators
Both TPU and TC accelerate the GEMM operation, which is the dominant operation in the commonly used models, such as convolution neural network  (CNN), multi-layer perceptron (MLP), and RNN/LSTM .
The TPU uses a weight stationary systolic array , whose size is in TPU-v1. The systolic array has a significant degree of data reuse, which leads to high-performance and energy-efficient execution of GEMM. In contrast, TC is still SIMD architecture and has a limited degree of data reuse. According to the reverse-engineered work , it executes the GEMM operation in the dot-product fashion and supports a GEMM operation, much smaller than the TPU.
We compare the GEMM performance between TPU and TC in Fig. 1. We use a cloud TPU-v2, which has a total of eight cores. We only use one core that has a systolic array with peak TFLOPS. The GPU is a Tesla V100, which has 15.7 FP32 and 125 TC peak TFLOPS . Owing to their different peak FLOPS, we use the FLOPS efficiency (achieved FLOPS divided by the peak FLOPS) as the metric for a fair comparison. With a large enough matrix, the TPU achieves almost 100% FLOPS efficiency, while the TC achieves less than 60% efficiency. As previously explained, the TC calculates the GEMM with multiple parallel dot-product operations while the TPU does it in the systolic fashion. As a result, a tenor core only supports a small GEMM operation with limited data reuse and high register bandwidth consumption, which leads to its low FLOPS efficiency.
Ii-B Hybrid Models
We now compare the performance of the two accelerators on hybrid models. Those models, which are the results of fast-evolving DL algorithms, can have operations that cannot be executed through the GEMM, and therefore present significant challenges for the existing fixed-function accelerators.
Fig. 2 shows two such hybrid DNN models, i.e., Mask R-CNN  and DeepLab . Both models target the semantic segmentation task, which aims to classify all the pixels in the input image and is more complicated than the image classification. As such, the state-of-the-art models rely on CNN models for feature extraction, but also introduce additional operations to improve the segmentation accuracy. Both models in Fig. 2 have GEMM-compatible CONV and FC layers. But Mask R-CNN has RoIAlign, a bi-linear interpolation that requires many reshape operations, and RegionProposal, a control-flow intensive non-max suppression (NMS) algorithm. Those operations are challenging to support with only GEMM operation. Similarly, DeepLab has the ArgMax and CRF (conditional random field ) that are GEMM-incompatible.
Fig. 3 shows the performance comparison and breakdown on TPU and GPU. The TPU executes Mask R-CNN about 75% slower than the GPU. A closer observation reveals that the GPU is slower than the TPU on the GEMM-compatible kernels (i.e., CONV and FC), but far out-performs the TPU on RoIAlign and RegionProposal. We examine the TPU-version source code for performance debugging and find that it converts the control-flow intensive NMS operation in RegionProposal to multiple dataflow-based GEMM operations, and converts RoIAlign operation to multiple average pooling operations for which TPU has hardware support. As such, the TPU can execute Mask R-CNN without relying on the CPU, i.e., with no data transferring overhead. However, the improper mapping causes severe performance degradation.
The DeepLab runs much slower on the TPU than the GPU owing to its infeasibility to support the important CRF operation. As such, the TPU transfers the data to the CPU for executing CRF, and we separate the CRF time from the overall execution time. The TPU has higher performance () than the GPU for the GEMM-compatible kernels, but the data transferring overhead is of its GEMM operation, leading to an overall slowdown compared to the GPU. Also, the performance of CRF is worse on the CPU (with one core).
The results show that over-specialization can severely degrade the performance for incompatible operations so that it is crucial to provide general-purpose programmability for emerging hybrid DNN models. However, the approach of relying on general-purpose cores can cause significant data movement overhead and also fails to exploit the computation resources inside the accelerator.
Iii Simultaneous Multi-mode Architecture
To balance the efficiency and flexibility, we propose the simultaneous multi-mode architecture. SMA integrates a GPU-like SIMD execution mode and a systolic execution mode, and temporally switches between the two modes. The SIMD mode efficiently executes GEMM-incompatible operations, and the specialized mode accelerates GEMM operations. We describe the design principles behind SMA and highlight its key novelty over TC, another architecture instance that can switch between generic SIMD execution and DNN acceleration.
Iii-a Temporal Integration
The first design principle in SMA is the temporal integration between the general-purpose mode and specialized mode. In contrast, the TC adopts the spatial integration methodology, which leads to the overhead in both the area and performance.
As explained previously, each TC has multiple dot-product units for executing matrix multiplication, and each dot-product unit has 4 MAC units . The SIMD units in GPU have the same computation ability but are not used when TC is active, which is essentially area wastage. Meanwhile, the TC also requires an adder tree for result reduction, which incurs additional area overhead. Spatial integration of the two architectures leads to area inefficiency and resource wastage because the computation of DNN models is usually layer-by-layer where only one architecture is used when performing the computation for one layer. In contrast, SMA is built on top of the existing SIMD execution units and aims for the maximal sharing of hardware structures (i.e., improved area efficiency).
The spatial integration of TC also leads to its highly decoupled execution model : the SIMD units load data to register file and the TC relies on an explicit synchronization to receive the data. In addition, the TC only supports a fixed shape (i.e., ) of matrix multiplication and does not expose the opportunity of SIMD-accelerator collaboration to more aggressively hiding the data loading latency. This decoupled execution model has inherent performance inefficiency. In contrast, the temporal integration in SMA enables such collaboration by imposing zero switching overhead between SIMD and accelerator mode.
Iii-B Choice of Dataflow
SMA starts with a SIMD substrate and adds another systolic mode for DNN acceleration because systolic array exploits data reuse and outperforms the dot-product-based TC (Fig. 1). However, the SIMD and systolic array favor distinct memory access patterns, which in turn expose a fundamental trade-off between memory access efficiency and data-reuse that we must reconcile when integrating the two modes.
The SIMD architecture of GPU favors a coalesced memory access, which is supported by the cache system . However, systolic array incurs memory accesses that could not be coalesced. Fig. 4 (left) shows the weight-stationary dataflow in the TPU. Loading matrix and writing output matrix requires accessing different rows of the respective matrix as the same cycle. Thus, directly implementing the systolic execution model on the SIMD hardware would lead to unsupported memory behaviors. While shared memory (scratchpad) in GPUs supports uncoalesced memory accesses via banking, it has a limited number of banks and thus does not scale well to large (or multiple) systolic arrays.
The TC chooses to address the problem by favoring coalesced memory accesses to maximally utilize its GPU’s memory subsystems. More specifically, TC uses a set of dot-product units to implement GEMM . In that way, all the memory accesses (matrix , , and ) are coalesced in TC. However, this approach leads to low data reuse and therefore poor GEMM performance as evident in Fig. 1.
To strike a balance of memory access efficiency and data reuse, we analyze different systolic array dataflow proposed in prior work and identify a SIMD-friendly dataflow called semi-broadcasted weight-stationary . The dataflow, shown in Fig. 4 (right), is similar to the TPU’s data. However, instead of passing a matrix element from top to bottom in the original design, each element now is broadcasted to all the PEs in the same column. Every cycle, all the PEs in the same column get the same element from matrix , perform a MAC operation, and send the data to the corresponding PEs on the right.
This dataflow is more SIMD-friendly and enables the seamless integration on a SIMD substrate. Specifically, each element in the matrix and is reused times in the array, which is the same as the weight-stationary systolic execution model and is better than the TC. Meanwhile, accesses to matrix and are coalesced, and only accesses to matrix are uncoalesced.
Iv SMA Implementation
This section describes the architectural modification to the baseline GPU to implement the SMA. The architectural design challenge is that SMA temporally integrates the SIMD mode and the systolic mode, and temporally reconfigures itself at runtime. Thus, we must minimize the reconfiguration overhead with minimal hardware augmentation. We then explain the instruction control and SIMD-systolic interaction. The temporal integration is feasible owing to our SIMD-friendly systolic dataflow that has high enough architectural similarity compared to the baseline GPU. As such, our design can reuse as many existing resources (i.e., computation, memory, and control) and architectural features as possible.
|CUDA Core/SM||64 FP32 units||3 SMA unit|
|Tensor Core/SM||4 (256 FP16 units)|
|Shared Memory/SM||32 banks||32 banks (8 for all SMA units)|
|Configurable up to 96KBÂ||Configurable up to 96 KB|
|Register File/SM||256 KB||256 KB|
Iv-a SMA Unit Design
The heart of our design is a set of SMA units, with each being a systolic array in the specialized mode and reconfigured to conventional SIMD lanes in the general-purpose mode.
We use the latest Volta architecture  in Tbl. I as our baseline GPU architecture, which has 80 cores (or streaming multiprocessor, SM). Each SM has 64 CUDA cores (i.e., 64 FP32 unit) and 4 TCs (i.e., 256 FP16 units in total). It also has up to 96 KB shared memory that has 32 banks and each bank provides a 32-bit value. The register file can provide vector-like access and its size is 256 KB in an SM.
SMA reuses the same computation resources in each SM (64 CUDA cores and 4 TCs, equivalent to 128 FP32 units in total), and provides three SMA units per SM. Each SMA unit is a FP32 systolic array. Fig. 5 shows the microarchitecture of one SMA unit. The SMA unit is implemented on top of the baseline SM architecture in the GPU with two key architectural augmentations to support the semi-broadcasted weight-stationary data-flow (Sec. III-B).
First, we repurpose the existing operand collector as a local buffer for storing the stationary weights of each PE. Second, we add additional wires to support broadcasting elements in matrix and communicating partial sums within the array. Overall, the layout of the systolic array could be done with minimal routing changes to the physical layout of existing SIMD units as the bottom of Fig. 5(C) shows.
Certain NVIDIA GPUs support the precision conversion between FP32 and FP16 . For example, two FP16 MAC units can be grouped to one FP32 MAC unit. If the baseline GPU supports this precision conversion, our SMA can also exploit it, leading to a FP16 systolic array instead of the current FP32 systolic array. Similarly, our SMA unit can also be built from other data types such as INT8.
Iv-B Instruction Control
We present our asynchronous instruction based control mechanism that can be seamlessly integrated into the existing SIMD pipeline and enable the simultaneous presence of two distinctive modes. Under the hood, SMA uses GPU’s rich memory resources, abundant parallelism, and fine-grained synchronization to maximize the performance.
We propose a new instruction LSMA (Load, Store and Multiply-accumulate) for the systolic mode. The instruction executes the operation in Eq. 1 and requires four register operands: the addresses of the first element in matrix and , one element value in matrix , and the height of matrix . The instruction executes asynchronously with respect to other SIMD instructions, minimizing the interference to the existing pipeline control logic. The threads need to issue an explicit synchronization to access the systolic computation results.
Once LSMA instruction is issued, a dedicated systolic controller in Fig. 5 is responsible for controlling the array with two main roles. First, it has an active mask for controlling the idle or active status of individual PE. Second, it has multiple address generation units for feeding the data to the array that has two different kinds of memory accesses (Sec. III-B). For an SMA unit, we assign 8 shared memory banks for loading matrix with uncoalesced accesses and one register file (RF) bank for storing matrix with coalesced accesses. In the baseline GPU, a RF bank provides 32 values (32-bit) for a warp, which is enough for the SMA unit that reads 8 values in a cycle. The three SMA units can be combined as an array to coordinate their memory accesses. Fig. 5(C) shows the register/shared memory access for the three SMA units.
TC has inherent inefficiency owing to the strictly synchronous semantics and fixed matrix shape (Sec. III-B). In contrast, our instruction design overcomes this inefficiency by adopting the asynchronous semantics and a flexible shape (), which enables the more fine-grained SIMD-systolic collaboration as we describe in the next subsection.
Iv-C Algorithm Mapping
We describe the algorithm mapping and optimization in SMA, most of which run at the software level and leverage the SIMD-systolic collaboration. We also present the new SMA-specific warp synchronization primitive and scheduler.
We implement the GEMM of and adopt common parallelization techniques such as partition, tiling, and double buffering as shown in Fig. 6. We divide the computation by the output matrix on the two-dimensional grid of thread blocks (TBs). This partition avoids the inter-TB communication and each TB is responsible for calculating a sub-matrix of , which is stored in the register file for faster access. Owing to the constraints of register file capacity, we choose the sub-matrix size of .
For increased data-locality, and are divided into tiles of and with the size of . Different tiles work in the double-buffer fashion: in each iteration, each TB uses and to update the sub-matrix . Since each core has an weight-stationary systolic array, we divide the tile into 16 sub-tiles to run on the systolic array. As such, each systolic array operation computes () and (). To maximize the concurrency, we use 64 warps (i.e. 2048 threads) per TB, which are divided into two sets for double buffer. The two sets work alternatively between loading tiles with the SIMD mode and computing the tile with the systolic mode via the LSMA instruction. The warps in the two sets are synchronized through CUDA’s recently added fine-grained sync primitive cooperative groups .
The challenge of running the double-buffered algorithm on the GPU lies in the architecture’s throughput-oriented design, which leads to its greedy-then-oldest (GTO) warp scheduler. The scheduler tries to issue the same set of warps over and over to maximize the throughput, which may cause starvation in the double-buffered warps. To overcome such a challenge, we add a SMA-specific scheduler that works in the round-robin fashion. The new scheduler works only in the systolic mode and does not affect the original scheduler.
In this section, we perform comprehensive performance and energy efficiency evaluation of SMA in different scenarios. For regular DNN models, we compare SMA with its baseline SIMD architecture and demonstrate its efficiency and flexibility for supporting both the regular and hybrid models. In the end, we evaluate the SMA’s dynamic resource scheduling capability in the context of autonomous driving application that contains both DNN and traditional algorithms.
V-a Simulation Methodology
For performance simulation of SMA, we modify GPGPU-Sim 4.0  and add the systolic mode in the baseline SIMD architecture. We use GPUWattch  and CACTI  for energy estimation. We use regular and hybrid models in Tbl. II.
For the GPU-based GEMM implementation, we use NVIDIA’s open-sourced and highly optimized CUTLASS library . In specific, we use the tiling size of and modify it to use the systolic mode as detailed in Sec. IV-C. The convolution layer in CNN models is converted to GEMM through the img2col .
SMA has little area overhead over the baseline GPU architecture due to the reuse of existing structures. The only significant extra logic is the systolic controller, which has 256B (bytes) storage (B for and B for ) and little extra logic. Modern GPU has 256KB register file, 128KB shared memory, and various computations per SM. Therefore, we estimate the overhead is less than 0.1%.
V-B DNN models
We evaluate the SMA’s efficiency advantages in terms of performance and area. In specific, we perform an iso-FLOP comparison on various data-flows including the SMA, TensorCore and TPU. We also perform an iso-area comparison to demonstrate the advantage of our temporal integration.
We first perform the iso-FLOP comparison for the SMA with broadcast weight stationary dataflow, TensorCore with the dot-product dataflow, and TPU with the weight stationary dataflow. Specifically, Fig. 7 left plane shows the square GEMM performance in the case of two SMA units (2-SMA) and four TensorCores (4-TC) per SM, which both have the same 256 FP16 units. The 2-SMA achieves 30% better performance improvement than 4-TC and over 90% FLOP efficiency (i.e., the ratio of theoretical peak performance) because it eliminates the RF bandwidth limitations. Fig. 7 right plane shows that the TPU dataflow is 20% - 40% slower than SMA dataflow because the former has a large amount of shared memory bank conflicts.
In the baseline architecture, SIMD units and TC are spatially integrated and the DNN models can only leverage one resource for acceleration. In contrast, SMA is based on the temporal integration and can use all computation resources. For the iso-area comparison, we estimate three SMA units (3-SMA) have the same area with one SIMD unit and two TC, which add up to the area of 384 FP16 units. Fig. 8 top planes compare performance in various cased on the regular and hybrid models. The 2-SMA performance is 22% faster than 4-TC owing to the more efficient dataflow. The temporal integration leads to 63% faster 3-SMA. We also compare the energy consumption between SMA and TC. As Fig. 8 (bottom) shows, 3-SMA (2-SMA) consumes 23% (12%) less energy than 4-TC on average, where the energy reduction mainly comes from the on-chip memory structures such as register file and shared memory.
In summary, the SMA outperforms the baseline GPU in both performance and energy efficiency owing to the following reasons. First, it reduces memory consumption by reusing input/result inside arrays. Under the same memory/register bandwidth, it performs better than the TC, also consumes less memory energy (including shared memory, cache, and RF) and dedicates more energy for the useful computation. Second, SMA adopts a complex control instruction which mitigates the overhead of instruction fetch/decode. Third, the systolic array requires less thread/warp-level parallelism because of the co-ordinated double buffering, which reduces the cache contention. The vanilla GPU requires many more threads/thread blocks per SM to hide the memory access latency.
V-C End-to-end DL Applications
We also evaluate SMA’s ability of dynamic resource allocation in the autonomous driving scenario which includes a mixed CNN and non-CNN algorithms. Prior study shows that it has three major algorithms: detection (DET), tracking (TRA), localization(LOC) . The tracking runs after the detection and they are both CNN-based. The localization runs independently and is not CNN based. We choose representative DeepLab , GOTURN , and ORB-SLAM  for them.
The Fig. 9 (left) shows their results in different platforms. Three algorithms can occupy the entire GPU so the frame latency equals the sum of each algorithm. The GPU exceeds the 100 ms single frame latency target owing to the slow CNN performance. The execution on SMA is similar but meets the latency target because of the faster CNN performance. The TC has a similar latency of SMA, but with DET and TRA running sequentially on the TC, and LOC running on the GPU in parallel. However, these results are based on running object detection and tracking on every frame. Prior work has suggested only running the detection every (e.g, 4) frames and relying on the tracking for every frame does not impact the final accuracy . This dynamic optimization creates uneven demand for CNN computation which SMA can accommodate to reduce the frame latency. Fig. 9 (right) shows with , SMA can reduce the frame latency by almost 50%.
Vi Related Work
There has been much recent work on designing ASIC accelerators for deep learning . Prior work also tried a programmable acceleration through FPGA and CGRA [24, 18]. Those work, together with general-purpose architecture and ASICs, represent different points in the trade-off curve between generality and efficiency. Our work integrates the two extreme points in the same architecture. Recent Volta GPU spatially integrates the TensorCore accelerator  while we temporally integrate the SIMD architecture with the systolic array.
We develop a simultaneous multi-mode architecture (SMA) with lightweight integration approach on GPU to achieve high programmability and energy-efficiency. Using the systolic array, SMA significantly improves the performance and reduce the energy. It also has the same characteristics as GPU and maintains the GPU’s programmability, configurability, and generality for fast-evolving DNN workloads.
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