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Accelerating the Pony.ai AV Sensor Data Processing Pipeline

Here’s how Pony.ai, which develops autonomous driving systems for robotaxis and trucks, uses GPU technology to develop a highly efficient data processing pipeline.

Just as humans rely on eyes to see, autonomous vehicles use sensors to gather information. These sensors collect a massive amount of data, which requires efficient onboard data processing for the vehicle to react quickly to situations on the road. This capability is crucial to autonomous vehicle safety and critical to making the virtual driver smarter.

With the need for redundant and diverse sensors and computing systems, it is challenging to design and optimize the processing pipeline. In this post, we present the evolution of Pony.ai’s on-vehicle sensor data processing pipeline.

Pony.ai’s sensor setup includes multiple cameras, LiDARs, and radars. An upstream module synchronizes sensors, encapsulates the data into pieces of messages, and sends them to downstream modules that use them to segment, classify, and detect objects, and so on

Each type of sensor data might have multiple modules, and the user algorithm could be either traditional or neural-network-based.

Figure 1. Block diagram of Pony’s autonomous driving sensing system

This entire pipeline must run at the highest levels of efficiency. The safety of our passengers is our number one priority. The sensor data processing system affects safety in two aspects.

First, one of the deciding factors of safety is how fast the autonomous driving system deals with the sensor data. If the perception and localization algorithms get the sensor data with a delay of hundreds of milliseconds, then the decision made by the vehicle would be too late.

Second, the whole HW/SW system must be reliable for long-term success. Consumers will never want to buy or take a ride on an autonomous vehicle that starts having problems several months after being manufactured. This is critical in the mass-production stage.

Processing sensor data efficiently

Easing the bottlenecks in the sensor processing pipeline required a multi-faceted approach, taking into account the sensor, GPU architecture, and GPU memory.

From sensor to GPU

When Pony.ai was founded, our original sensor setup consisted of off-the-shelf components. We used USB- and ethernet-based models for the cameras, which were directly connected to the on-vehicle computer, and the CPU was responsible for reading data from the USB/Ethernet interface.

Figure 2. Block diagram showing the pipeline from the camera, to the CPU, to the GPU.

Transferring camera data over ethernet/USB provides higher latency but consumes CPU cycles.

While this functioned well, there was a fundamental issue with the design. The USB and the Ethernet-camera interfaces (GigE-camera) were CPU-consuming. With more and higher resolution cameras added, the CPUs quickly became overwhelmed and incapable of performing all the I/O operations. It was difficult for this design to be scalable while maintaining sufficiently low latency.

We solved the problem by adding an FPGA-based sensor gateway for cameras and LiDARs.

Figure 3. FPGA as a sensor gateway (sensors showing only the camera)

Adding an FPGA as a sensor gateway offloads the CPU I/O cost, but DMA over PCIe has higher bandwidth and lower latency, so it could support more cameras.

FPGA handles the camera trigger and synchronization logic to provide better sensor fusion. When one or more camera data packets are ready, a DMA transfer is triggered to copy the data from the FPGA to the main memory through the PCIe bus. The DMA engine does this on the FPGA, and the CPU is freed up. It not only opens the CPU’s I/O resources but also reduces data transfer latency, yielding a more scalable sensor setup.

Because the camera data is used by many neural network models running on GPUs, it still must be copied to GPU memory after it has been transferred from FPGA to CPU through DMA. So, a CUDA HostToDevice memory copy is required somewhere, which takes ~1.5 ms for a single frame of an FHD camera image.

However, we wanted to further reduce this latency. Ideally, the camera data should be directly transferred into the GPU memory without being routed through the CPU.

Figure 4. Same block diagram showing the camera/FPGA/CPU/GPU pipeline, but using RDMA to communicate between FPGA and GPU.

Using FPGA-GPU RDMA, we added a PCIe switch for best performance. This solution eliminated the CPU-GPU data copy. We also integrated NVIDIA GPU Direct RDMA technology into Xilinx’s XDMA driver, for ~6 GB/s FPGA-GPU bandwidth on PCIe Gen3 x8.

We achieved this goal by using the NVIDIA GPU Direct RDMA. GPU Direct RDMA enables us to preallocate a chunk of CUDA memory accessible to PCIe peers through the PCIe BARs (Base Address Register, which defines linear windows of PCIe address space).

It also provides a series of kernel-space APIs for the third-party device driver to obtain the GPU memory physical address. These APIs facilitate the DMA engines in third-party devices to send and read data directly to and from the GPU memory just like it sends and reads data to and from the main memory.

The GPU Direct RDMA reduces the latency by eliminating the CPU-to-GPU copy and achieves the highest bandwidth ~6 GB/s under PCIe Gen3 x8, which has a theoretical limit of 8 GB/s.

Scaling across GPUs

Due to the increasing compute workload, we needed more than one GPU. With more and more GPUs added to the system, communication between GPUs might also become a bottleneck. Going through the CPU through a staging buffer increases the CPU cost and limit the overall bandwidth.

Figure 5. GPU-GPU communication by PCIe switch

We added a PCIe switch that provides the best possible peer-to-peer transfer performance. The peer-to-peer communication can reach PCIe line speed in our measurement, thus providing much better scaling across multiple GPUs.

Offloading computing to dedicated hardware

We also offloaded tasks that previously ran on CUDA cores to dedicated hardware to accelerate sensor data processing.

For example, when encoding an FHD camera image into a JPEG string, the NvJPEG library takes ~4ms on a single CPU thread with an RTX5000 GPU. The NvJPEG might consume CPU and GPU resources because some of its phases, like Huffman encoding, might be purely on the CPU.

Figure 6. Block diagram showing data flow with JPEG encoding using the NvJPEG library on GPU.

JPEG encoding on GPU is resource-consuming for both GPU and CPU. NvJPEG GPU encoding still has some phases being calculated on CPU (Huffman encoding and decoding). One CPU thread plus the NVIDIA RTX 5000 takes ~4ms to encode one FHD image into JPEG. This also interferes with other running GPU tasks.

We adopted the NVIDIA Video Codec for on-vehicle use to relieve the CPU and GPU (CUDA part) from doing image encoding and decoding. This codec uses encoders in a dedicated portion of the GPU. It is part of the GPU, but it does not conflict with other CUDA resources used for running kernels and deep learning models.

We have also been migrating our image compression format from JPEG to HEVC (H.265), by using the dedicated hardware video encoder on NVIDIA GPUs. We achieved an improvement in the encoding speed and freed up both CPU and GPU resources for other tasks.

It takes ~3 ms to encode an FHD image fully on GPU without hurting its CUDA performance. The performance is measured in I-frame-only mode, which ensures consistent quality and compression artifacts across frames.

Figure 7. Block diagram showing data flow with HEVC encoding, which avoids consuming CUDA cores or the CPU.

NVIDIA Video Codec uses encoders in a dedicated partition of the GPU chip that does not consume CUDA cores or CPU. NVENC supports H264/H265. Encoding one FHD image to HEVC takes ~3 ms, so the GPU and CPU are freed to do other tasks. We used I-frame-only mode to ensure that each frame has the same quality and same type of artifacts.

On-GPU data flow

Another critical topic is the efficiency of sending the camera frames as messages to downstream modules.

We use Google’s protobuf to define a message. Take the CameraFrame message as an example. Camera specs and attributes are primitive types in the message. The real payload—camera data—must be defined as a bytes field in the main system memory, due to the limitation of protobuf.

Figure 8. Example of a CameraFrame message

The message in the following code example is a proto. Due to the limitation of protobuf, data has to be in main memory.

message CameraFrame {
  optional string device_name = 1;
  optional int32 width = 2;
  optional int32 height = 3;
  optional int32 pixel_format = 4;
  optional bytes data = 5;
};

CUDA H2D copy is required to make the camera data available to DL models.

We use a publisher-subscriber model with zero-copy message passing among modules to share information. Many subscriber modules of this CameraFrame message use the camera data to make deep learning inferences.

In the original design, when such a module received the message, it would have had to call a CUDA HostToDevice memory copy to transfer the camera data to the GPU before the inference.

Figure 9. Block diagram showing a Publisher-Subscriber model that the camera module sends a CameraFrame message to multiple consumer modules. Each consumer module needs to do a CPU-to-GPU memory copy.

Every module has to do an H2D copy for the same data, which is time-consuming! The following code example shows the zero-copy message passing on the CPU but not on the GPU.

message CameraFrame {
  optional string device_name = 1;
  optional int32 width = 2;
  optional int32 height = 3;
  optional int32 pixel_format = 4;
  optional bytes data = 5;
};

Every module must do the CUDA HostToDevice copy, which is redundant and resource-consuming. Although the zero-copy message passing framework works well on the CPU, it involves a lot of CPU-GPU data copy.

Figure 10. Zero-copy publisher-subscriber message passing with GPU support

We used the protobuf codegen plugin to enable the data fields in the GPU memory. The following code example shows the zero-copy message passing on the GPU. The GPUData field is in GPU memory.

message CameraFrame {
  optional string device_name = 1;
  optional int32 width = 2;
  optional int32 height = 3;
  optional int32 pixel_format = 4;
  optional GpuData data = 5;
};

We solved this issue by adding a new type of data, the GpuData field, into the protobuf code generator through protobuf’s plug-in API. GpuData supports the standard resize operation just like the CPU memory bytes field. However, its physical data storage is on-GPU.

When the subscriber modules receive the message, they can retrieve the GPU data pointer for direct use. Thus, we achieved full zero-copy throughout the entire pipeline.

Improving GPU memory allocation

When we call the resize function of the GpuData proto, it calls CUDA cudaMalloc. When the GpuData proto message is destroyed, it calls cudaFree.

These two API operations are not cheap because they must modify the GPU’s memory map. Each call could take ~0.1 ms.

Because this proto message is extensively used while the cameras are producing data non-stop, we should optimize the alloc/free cost of the GPU proto message.

We implemented a fixed slot-size GPU memory to address this problem. The idea is simple: We maintain a stack of preallocated GPU memory slots that match our desired camera data frame buffer size. Every time alloc is called, we take one slot from the stack. Every time free is called, the slot is returned to the pool. The alloc/free time is near zero by re-using the GPU memory.

Figure 11. GPU memory pool supporting fixed allocation size only

camera_frame.mutable_gpu_data()->Resize(size);
ptr = pool->Alloc();
camera_frame destructs
pool->Free(ptr);

What if we want to support cameras with different resolutions? Using this fixed-size memory pool, we must always allocate the largest possible size or initialize multiple memory pools with varying slot sizes. Either reduces efficiency.

New features in CUDA 11.2 solved this issue. It officially supports cudaMemPool, which can be preallocated and later used for cudaMalloc and free. Compared with our previous implementation, it helps any allocation size. This greatly improves the flexibility at a tiny performance cost (~2us each allocation).

Figure 12. GPU memory pool supporting dynamic allocation size

camera_frame.mutable_gpu_data()->Resize(size);
pool->cudaMallocFromPoolAsync(&ptr, pool, ...);
camera_frame destructs
pool->cudaFreeAsync(ptr);

In both methods, the resize call falls back to conventional cudaMalloc and free when a memory pool overflows.

Cleaner data flow in YUV color space

We have achieved a highly efficient data flow with all the preceding optimizations of the hardware design and system software architecture. The next step is to optimize the data format itself.

Our system used to process camera data in RGB color space. However, the ISP output of our cameras is in the YUV color space, and a conversion from YUV to RGB is performed on GPU, which takes ~0.3 ms. Also, some perception components do not need color information. Feeding RGB color pixels to them is wasteful.

Figure 13. Using the YUV format eliminates the colorspace conversion

For these reasons, we migrated from the RGB camera frames to the YUV frames. We chose to use the YUV420 pixel format because human vision is not as sensitive to chrominance information as to luminance information.

By adopting the YUV420 pixel format, we saved half of the GPU memory consumption. This also enabled us to send only the Y channel to the perception components, which do not require chrominance information, saving two-thirds of the GPU memory consumption compared to RGB.

Processing lidar data on-GPU

Besides camera data, we also process lidar data, which is more sparse, mostly on-GPU. While considering different types of Lidars, it is more difficult to process. We have taken several optimizations when processing lidar data:

  • Because lidar scan data contains a lot of physical information, we use the GPU-friendly Structure of Array instead of Array of Structures to describe the point cloud, making the GPU memory access pattern more coalesced instead of being scattered.
  • When some fields must be exchanged between CPU and GPU, we keep them in page-locked memory to accelerate the transfer.
  • The NVIDIA CUB library is extensively used in our processing pipeline, specifically the scan/select operations.

Figure 14. Block diagram showing pipeline from lidar sensor, to point cloud processing on GPU.

Lidar data processing pipeline on GPU produces the following results:

  • A GPU-friendly structure-of-array data layout.
  • Page-locked memory to accelerate the CPU-GPU transfer.
  • NVIDIA CUB library fast routines that are ~58% faster in filtering.

We have reduced the entire pipeline latency by ~4 ms in the critical path with all these optimizations.

Overall timeline

With all these optimizations, we can view our system tracing using our in-house timeline visualization tool.

Figure 15. Overall timeline from sensor data to DL inference

The overall timeline shows the broad idea of how intensively we rely on our GPU today. Though the two GPUs are used for ~80% of the time, the workload of GPU0 and GPU1 are not ideally balanced. For GPU 0, it is heavily used throughout the perception module iteration. For GPU 1, it has more idle gaps in the middle of the iteration.

In the future, we will focus on further improving GPU efficiency.

Production-readiness

In the early days of development, the FPGA enabled us to easily experiment with our ideas in hardware-based sensor data processing. As our sensor-data processing unit becomes increasingly mature, we have been looking into the possibility of using a system-on-a-chip (SoC) to deliver a compact, reliable, and production-ready sensor data processor.

We found that the automotive-grade NVIDIA DRIVE Orin SoC perfectly meets our requirements. It is ASIL-rated, making it a great fit to run on production vehicles. 

Migrating from FPGA to NVIDIA DRIVE Orin

In the early days of development, the FPGA enabled us to easily experiment with our ideas in hardware-based sensor data processing.

As our sensor-data processing unit becomes increasingly mature, we have been looking into the possibility of using a system-on-a-chip (SoC) to deliver a compact, reliable, and production-ready sensor data processor.

We found that the automotive-grade NVIDIA DRIVE Orin SoC perfectly meets our requirements. It is ASIL-rated, making it a great fit to run on production vehicles. Despite its compact size and low cost, it can connect to a wide spectrum of automotive-grade sensors and efficiently process large-scale sensor data.

We are going to use NVIDIA Orin to handle all the sensor signal processing, synchronization, packet collection, as well as camera frame encoding. We estimated that this design, combined with other architectural optimization, will save ~70% total BOM cost.

Figure 16. Using NVIDIA DRIVE Orin SoC as the new sensor gateway

Orin SoC replaces FPGA as a sensor gateway to support 10+ cameras and lidars, which are automotive-grade, for a ~70% cost reduction.

In collaboration with NVIDIA, we ensured that all communication among the Orin-CPU-GPU components goes through the PCIe bus with DMA support through NvStreams.

  • For compute-intensive DL work, the NVIDIA Orin SoC uses NvStream to transfer sensor data to the discrete GPU to process.
  • For non-GPU work, the NVIDIA Orin SoC uses NvStream to transfer data to the host CPU to process.

A level 2/3 compute platform application

Figure 17. Block diagram showing a L2/L3 system camera pipeline achieved by removing the X86 CPU.

The gap between L4 and L2/L3 could be narrowed as Orin SoC provides up to 250 TOPS DL performance. Models are executed at a similar performance as NVIDIA RTX 5000, so models used in L4 scenarios can be pruned to run on Orin at a reasonable speed. The flexible architecture design can share resources between the L4 and L2/L3 solutions.

An excellent benefit of this design is that it has the potential to be used as an L2/L3 compute platform.

NVIDIA Orin provides 254 trillion operations per second of compute power that could potentially handle a similar workload as the RTX5000 discrete GPU being used on our current level 4 autonomous vehicle compute platform. However, it requires multiple optimizations to fully unleash the potential of the NVIDIA Orin SoC, such as:

  • Structural sparse network
  • DLA cores
  • Scaling across multiple NVIDIA Orin SoCs

Conclusion

The evolution of Pony’s sensor data processing pipeline has demonstrated our systematic approach toward high-efficiency data processing pipeline and enhanced system reliability, which helps achieve higher safety goals. The simple ideas behind this approach are:

  • Make the data flow simple and smooth. Data should be transferred directly to the location where it will get consumed, in the format that minimizes conversion overhead.
  • Use dedicated hardware for computation-intensive tasks and save the general-purpose computing resources for other tasks.
  • Resource sharing between level 4 and level 2 systems improves reliability and saves engineering costs.

This approach cannot be achieved by software or hardware alone, but by a joint effort of software and hardware co-design. We believe that this is crucial to satisfy the fast-growing computing needs with the production expectations.

Acknowledgments

This post includes years of evolution in Pony’s sensor data pipeline. The credit belongs to multiple teams and engineers who have continuously contributed to developing this highly efficient sensor data pipeline.

Source:: NVIDIA