Mingyu
Kim
Techniques for faster AI inference throughput with OpenVINO on Intel GPUs
Authors: Mingyu Kim, Vladimir Paramuzov, Nico Galoppo
Intel’s newest GPUs, such as Intel® Data Center GPU Flex Series, and Intel® Arc™ GPU, introduce a range of new hardware features that benefit AI workloads. Starting with the 2022.3 release, OpenVINO™ can take advantage of two newly introduced hardware features: XMX (Xe Matrix Extension) and parallel stream execution. This article explains what those features are and how you can check whether they are enabled in your environment. We also show how to benefit from them with OpenVINO, and the performance impact of doing so.
What is XMX (Xe Matrix Extension)?
XMX is a hardware acceleration for matrix multiplication on the newest Intel™ GPUs. Given the same number of Xe Cores, XMX technology provides 4-8x more multiplication capacity at the same precision [1]. OpenVINO, powered by OneDNN, can take advantage of XMX hardware by accelerating int8 and fp16 inference. It brings performance gains in compute-intensive deep learning primitives such as convolution and matrix multiplication.
Under the hood, XMX is a well-known hardware architecture called a systolic array. Systolic arrays increase computational capacity without increasing memory (or register) access. The magic happens by pipelining multiple computations with a single data access, as opposed to the traditional fetch-compute-store pipeline. It is implemented by connecting multiple computation nodes in series. Data is fed into the front, goes through several steps of multiplication-add, and finally is stored back to memory.
How to check whether you have XMX?
You can check whether your GPU hardware (and software stack) supports XMX with OpenVINO™’s hello_query_device sample. When you run the sample application, it lists all detected inference devices along with its properties. You can check for XMX support by looking at the OPTIMIZATION_CAPABILITIES property and checking for the GPU_HW_MATMUL value.
In the listing below you can see that our system has two GPU devices for inference, and only GPU.1 has XMX support.
As mentioned, XMX provides a way to get significantly more compute capacity on a GPU. The next feature doesn’t provide more capacity, but it allows ways to use that capacity more efficiently.
What is parallel execution of multiple streams?
Another improvement of Intel®’s discrete GPUs is to process multiple compute streams in parallel. Certain deep learning inference workloads are too small to fill all hardware compute resources of a given GPU. In such a case it is beneficial to run multiple compute streams (or inference requests) in parallel, such that the GPU hardware has more work to process at any given point in time. With parallel execution of multiple streams, Intel GPUs can increase hardware efficiency.
How to check for parallel execution support?
As of the OpenVINO 2022.3 release, there is only an indirect way to query how many streams your GPU can process in parallel. In the next release it will be possible to query the range of streams using the ov::range_for_streams property query and the hello_query_device_sample. Meanwhile, one can use the benchmark_app to report the default number of streams (NUM_STREAMS). If the GPU does not support parallel stream execution, NUM_STREAMS will be 2. If the GPU does support it, NUM_STREAMS will be larger than 2. The benchmark_app log below shows that GPU.1 supports 4-stream parallel execution.
However, it depends on application usage
Parallel stream execution can bring significant performance benefit, but only when used appropriately by the application. It will bring good performance gain if the application can run multiple independent inference requests in parallel, whether from single process or multiple processes. On the other hand, if there is no opportunity for parallel execution of multiple inference requests, then there is no gain to be had from multi-stream hardware execution.
Demonstration of performance tuning through benchmark_app
DISCLAIMER: The performance may vary depending on the system and usage.
OpenVINO benchmark_app is a very handy tool to analyze performance in various conditions. Here we’ll show the performance trend for an Intel® discrete GPU with XMX and four parallel hardware execution streams.
The performance was measured on a pre-production version of the Intel® Arc™ A770 Limited Edition GPU with 16 GiB of memory. The host system is a 12th Gen Intel(R) Core(TM) i9-12900K with 64GiB of RAM (4 DDR4-2667 modules) running Ubuntu OS 20.04.5 LTS with Linux kernel 5.15.47.
Performance comparison with high-level performance hints
Even though all supported devices in OpenVINO™ offer low-level performance settings, utilizing them is not recommended outside of very few cases. The preferred way to configure performance in OpenVINO Runtime is using performance hints. This is a future-proof solution fully compatible with the automatic device selection inference mode and designed with portability in mind.
OpenVINO benchmark_app exposes the high-level performance hints with the performance hint option for easy configuration of best latency and throughput. In short, latency mode picks the optimal configuration for low latency with the cost of low throughput, and throughput mode picks the optimal configuration for high throughput with the cost of high latency.
The table below shows throughput for various combinations of execution configuration for resnet-50.
Throughput mode is achieving much higher FPS compared to latency mode because inference happens with higher batch size and parallel stream execution. You can also see that, in throughput mode, the throughput with fp16 is 5.4x higher than with fp32 due to the use of XMX.
In the experiments below we manually explore different configurations of the performance parameters for demonstration purposes; It is generally not recommended to tune manually. Once the optimal parameters are known, they can be applied in production.
Performance gain from XMX
Performance gain from XMX can be observed by comparing int8/fp16 against fp32 performance because OpenVINO does not provide an option to turn XMX off. Since fp32 computations are not executed by the XMX hardware pipe, but rather by the less efficient fetch-compute-store pipe, you can see that the performance gap between fp32 and fp16 is much larger than the expected factor of two.
We choose a batch size of 64 to demonstrate the best case performance gain. When the batch size is small, the performance difference is not always as prominent since the workload could become too small for the GPU.
As you can see from the execution log, fp16 runs ~5.49x faster than fp32. Int8 throughput is ~2.07x higher than fp16. The difference between fp16 and fp32 is due to fp16 acceleration from XMX while fp32 is not using XMX. The performance gain of int8 over fp16 is 2.07x because both are accelerated with XMX.
Performance gain from parallel stream execution
You can see from the log below that performance goes up as we have more streams up to 4. It is because the GPU can handle 4 streams in parallel.
Note that if the inference workload is large enough, more streams might not bring much or any performance gain. For example, when increasing the batch size, throughput may saturate earlier than at 4 streams.
How to take advantage the improvements in your application
For XMX, all you need to do is run your int8 or fp16 model with the OpenVINO™ Runtime version 2022.3 or above. If the model is fp32(single precision), it will not be accelerated by XMX. To quantize a model and create an OpenVINO int8 IR, please refer to Quantizing Models Post-training. To create an OpenVINO fp16 IR from a fp32 floating-point model, please refer to Compressing a Model to FP16 page.
For parallel stream execution, you can set throughput hint as described in Optimizing for Throughput. It will automatically set the number of parallel streams with best number.
Conclusion
In this article, we introduced two key features of Intel®’s discrete GPUs: XMX and parallel stream execution. Most int8/fp16 deep learning networks can benefit from the XMX engine with no additional configuration. When properly configured by the application, parallel stream execution can bring significant performance gains too!
[1] In the Xe-HPG architecture, the XMX delivers 256 INT8 ops per clock (DPAS), while the (non-systolic) Xe Core vector engine delivers 64 INT8 ops per clock – a 4x throughput increase [reference]. In the Xe-HPC architecture, the XMX systolic array depth has been increased to 8 and delivers 4096 FP16 ops per clock, while the (non-systolic) Xe Core vector engine delivers 512 FP16 ops per clock – a 8x throughput increase [reference].
Notices & Disclaimers
Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.
Performance results are based on testing as of dates shown in configurations and may not reflect all publicly available updates. See backup for configuration details. No product or component can be absolutely secure.
See backup for configuration details. For more complete information about performance and benchmark results, visit www.intel.com/benchmarks
© Intel Corporation. Intel, the Intel logo, and other Intel marks are trademarks of Intel Corporation or its subsidiaries. Other names and brands may be claimed as the property of others.
