\documentclass{JINST} \usepackage[utf8]{inputenc} \usepackage{lineno} \usepackage{ifthen} \usepackage{caption} \usepackage{subcaption} \usepackage{textcomp} \usepackage{booktabs} \usepackage{floatrow} \newfloatcommand{capbtabbox}{table}[][\FBwidth] \newboolean{draft} \setboolean{draft}{true} \newcommand{\figref}[1]{Figure~\ref{#1}} \title{A high-throughput readout architecture based on PCI-Express Gen3 and DirectGMA technology} \author{ L.~Rota$^a$, M.~Vogelgesang$^a$, L.E.~Ardila Perez$^a$, M.~Caselle$^a$, S.~Chilingaryan$^a$, T.~Dritschler$^a$, N.~Zilio$^a$, A.~Kopmann$^a$, M.~Balzer$^a$, M.~Weber$^a$\\ \llap{$^a$}Institute for Data Processing and Electronics,\\ Karlsruhe Institute of Technology (KIT),\\ Herrmann-von-Helmholtz-Platz 1, Karlsruhe, Germany \\ E-mail: \email{lorenzo.rota@kit.edu}, \email{matthias.vogelgesang@kit.edu} } \abstract{% Modern physics experiments have reached multi-GB/s data rates. Fast data links and high performance computing stages are required for continuous data acquisition and processing. Because of their intrinsic parallelism and computational power, GPUs emerged as an ideal solution to process this data in high performance computing applications. In this paper we present a high- throughput platform based on direct FPGA-GPU communication. The architecture consists of a Direct Memory Access (DMA) engine compatible with the Xilinx PCI-Express core, a Linux driver for register access, and high- level software to manage direct memory transfers using AMD's DirectGMA technology. Measurements with a Gen3\,x8 link show a throughput of 6.4~GB/s for transfers to GPU memory and 6.6~GB/s to system memory. We also assessed the possibility of using the architecture in low latency systems: preliminary measurements show a round-trip latency as low as 1 \textmu s for data transfers to system memory, while the additional latency introduced by OpenCL scheduling is the current limitation for GPU based systems. Our implementation is suitable for real-time DAQ system applications ranging from photon science and medical imaging to High Energy Physics (HEP) systems. } \keywords{FPGA; GPU; PCI-Express; OpenCL; DirectGMA} \begin{document} \ifdraft \setpagewiselinenumbers \linenumbers \fi \section{Introduction} GPU computing has become the main driving force for high performance computing due to an unprecedented parallelism and a low cost-benefit factor. GPU acceleration has found its way into numerous applications, ranging from simulation to image processing. The data rates of bio-imaging or beam-monitoring experiments running in current generation photon science facilities have reached tens of GB/s~\cite{ufo_camera, caselle}. In a typical scenario, data are acquired by back-end readout systems and then transmitted in short bursts or continuously streamed to a computing stage. In order to collect data over long observation times, the readout architecture and the computing stages must be able to sustain high data rates. Recent years have also seen an increasing interest in GPU-based systems for High Energy Physics (HEP) (\emph{e.g.} ATLAS~\cite{atlas_gpu}, ALICE~\cite{alice_gpu}, Mu3e~\cite{mu3e_gpu}, PANDA~\cite{panda_gpu}) and photon science experiments. In time-deterministic applications, such as Low/High-level trigger systems, latency becomes the most stringent requirement. Due to its high bandwidth and modularity, PCIe is the commercial \emph{de facto} standard for connecting high-throughput peripherals such as GPUs or solid state disks. Moreover, optical PCIe networks have been demonstrated a decade ago~\cite{optical_pcie}, opening the possibility of using PCIe as a communication link over long distances. Several solutions for direct FPGA-GPU communication based on PCIe and NVIDIA's proprietary GPUdirect technology are reported in the literature. In the implementation of Bittner and Ruf the GPU acts as master during an FPGA-to-GPU read data transfer \cite{bittner}. This solution limits the reported bandwidth and latency to 514 MB/s and 40~\textmu s, respectively. When the FPGA is used as a master, a higher throughput can be achieved. An example of this approach is the \emph{FPGA\textsuperscript{2}} framework by Thoma et~al.\cite{thoma}, which reaches 2454 MB/s using a PCIe 2.0 8x data link. Lonardo et~al.\ achieved low latencies with their NaNet design, an FPGA-based PCIe network interface card~\cite{lonardo2015nanet}. The Gbe link however limits the latency performance of the system to a few tens of \textmu s. If only the FPGA-to-GPU latency is considered, the measured values span between 1~\textmu s and 6~\textmu s, depending on the datagram size. Nieto et~al.\ presented a system based on a PXIexpress data link that makes use of four PCIe 1.0 links~\cite{nieto2015high}. Their system, as limited by the interconnect, achieves an average throughput of 870 MB/s with 1 KB block transfers. In order to achieve the best performance in terms of latency and bandwidth, we developed a high-performance DMA engine based on Xilinx's PCIe 3.0 Core. To process the data, we encapsulated the DMA setup and memory mapping in a plugin for our scalable GPU processing framework~\cite{vogelgesang2012ufo}. This framework allows for an easy construction of streamed data processing on heterogeneous multi-GPU systems. However, the framework is based on OpenCL and cannot be used with GPUDirect technology, which is targeted on CUDA only. To overcome this limitation we have evaluated AMD's DirectGMA technology which allows us to integrate direct PCIe data transfers with applications developed using OpenCL language. In this paper we present the hardware/software interface and report the throughput performance of our architecture together with preliminary measurements about DirectGMA's applicability in low-latency applications. %% LR: this part -> OK \section{Architecture} As shown in \figref{fig:trad-vs-dgpu} (a), traditional FPGA-GPU systems route data through system main memory by copying data from the FPGA into intermediate buffers and then finally into the GPU's main memory. Thus, the total throughput and latency of the system is limited by the main memory bandwidth. NVIDIA's GPUDirect and AMD's DirectGMA technologies allow direct communication between GPUs and auxiliary devices over PCIe. By combining this technology with DMA data transfers as shown in \figref{fig:trad-vs-dgpu} (b), the overall latency of the system is reduced and total throughput increased. Moreover, the CPU and main system memory are relieved from processing because they are not directly involved in the data transfer anymore. \begin{figure}[t] \centering \includegraphics[width=1.0\textwidth]{figures/transf} \caption{% In a traditional DMA architecture (a), data are first written to the main system memory and then sent to the GPUs for final processing. By using GPUDirect/DirectGMA technology (b), the DMA engine has direct access to the GPU's internal memory. } \label{fig:trad-vs-dgpu} \end{figure} %% LR: this part -> Text:OK, Figure: must be updated \subsection{DMA engine implementation on the FPGA} We have developed a DMA engine that minimizes resource utilization while maintaining the flexibility of a Scatter-Gather memory policy~\cite{rota2015dma}. The main blocks are shown in \figref{fig:fpga-arch}. The engine is compatible with the Xilinx PCIe 2.0/3.0 IP-Core~\cite{xilinxgen3} for Xilinx FPGA families 6 and 7. DMA data transfers between main system memory and GPU memory are supported. Two FIFOs,operating at 250 MHz and a data width of 256 bits, act as user-friendly interfaces with the custom logic at an input bandwidth of 7.45 GB/s. The user logic and the DMA engine are configured by the host system through PIO registers. The resource utilization on a Virtex 7 device is reported in Table~\ref{table:utilization}. \begin{figure}[t] \begin{floatrow} \ffigbox{% \includegraphics[width=0.45\textwidth]{figures/fpga-arch} }{% \caption{Block diagram of the FPGA architecture}% \label{fig:fpga-arch} } \capbtabbox{% \begin{tabular}{@{}llll@{}} \toprule Resource & Utilization & (\%) \\ \midrule LUT & 5331 & (1.23) \\ LUTRAM & 56 & (0.03) \\ FF & 5437 & (0.63) \\ BRAM & 21 & (1.39) \\ \bottomrule \end{tabular} }{% \caption{Resource utilization on a xc7vx690t-ffg1761 device.}% \label{table:utilization} } \end{floatrow} \end{figure} The physical addresses of the host's memory buffers are stored into an internal memory and are dynamically updated by the driver or user, allowing highly efficient zero-copy data transfers. The maximum size associated with each address is 2 GB. %% LR: -----------------> OK \subsection{OpenCL management on host side} \label{sec:host} \begin{figure}[b] \centering \includegraphics[width=0.75\textwidth]{figures/opencl-setup} \caption{The FPGA writes to GPU memory by mapping the physical address of a GPU buffer and initating DMA transfers. Signalling happens in reverse order by mapping the FPGA control registers into the address space of the GPU.} \label{fig:opencl-setup} \end{figure} %% Description of figure On the host side, AMD's DirectGMA technology, an implementation of the bus-addressable memory extension for OpenCL 1.1 and later, is used to write from the FPGA to GPU memory and from the GPU to the FPGA's control registers. \figref{fig:opencl-setup} illustrates the main mode of operation: to write into the GPU, the physical bus addresses of the GPU buffers are determined with a call to \texttt{clEnqueue\-Make\-Buffers\-Resident\-AMD} and set by the host CPU in a control register of the FPGA (1). The FPGA then writes data blocks autonomously in DMA fashion (2). To signal events to the FPGA (4), the control registers can be mapped into the GPU's address space passing a special AMD-specific flag and passing the physical BAR address of the FPGA configuration memory to the \texttt{cl\-Create\-Buffer} function. From the GPU, this memory is seen transparently as regular GPU memory and can be written accordingly (3). In our setup, trigger registers are used to notify the FPGA on successful or failed evaluation of the data. Using the \texttt{cl\-Enqueue\-Copy\-Buffer} function call it is possible to write entire memory regions in DMA fashion to the FPGA. In this case, the GPU acts as bus master and pushes data to the FPGA. %% Double Buffering strategy. Due to hardware restrictions with AMD W9100 FirePro cards, the largest possible GPU buffer sizes are about 95 MB. However, larger transfers can be achieved by using a double buffering mechanism: data are copied from the buffer exposed to the FPGA into a different location in GPU memory. To verify that we can keep up with the incoming data throughput using this strategy, we measured the data throughput within a GPU by copying data from a smaller sized buffer representing the DMA buffer to a larger destination buffer. At a block size of about 384 KB the throughput surpasses the maximum possible PCIe bandwidth. Block transfers larger than 5 MB saturate the bandwidth at 40 GB/s. Double buffering is therefore a viable solution for very large data transfers, where throughput performance is favoured over latency. For data sizes less than 95 MB, we can determine all addresses before the actual transfers thus keeping the CPU out of the transfer loop. %% Ufo Framework To process the data, we encapsulated the DMA setup and memory mapping in a plugin for our scalable GPU processing framework~\cite{vogelgesang2012ufo}. This framework allows for an easy construction of streamed data processing on heterogeneous multi-GPU systems. For example, to read data from the FPGA, decode from its specific data format and run a Fourier transform on the GPU as well as writing back the results to disk, one can run the following on the command line: \begin{verbatim} ufo-launch direct-gma ! decode ! fft ! write filename=out.raw \end{verbatim} The framework takes care of scheduling the tasks and distributing the data items to one or more GPUs. High throughput is achieved by the combination of fine- and coarse-grained data parallelism, \emph{i.e.} processing a single data item on a GPU using thousands of threads and by splitting the data stream and feeding individual data items to separate GPUs. None of this requires any user intervention and is solely determined by the framework in an automatized fashion. A complementary application programming interface allows users to develop custom applications in C or high-level languages such as Python. %% -------------------------------------------------------------------------- \section{Results} \begin{table}[b] \centering \small \caption{Setups used for throughput and latency measurements.} \label{table:setups} \tabcolsep=0.11cm \begin{tabular}{@{}llll@{}} \toprule & Setup 1 (workstation) & Setup 2 (embedded)\\ \midrule CPU & Intel Xeon E5-1630 & Intel Atom D525 \\ Chipset & Intel C612 & Intel ICH9R Express \\ GPU & AMD FirePro W9100 & AMD FirePro W9100 \\ PCIe slot: System memory & x8 Gen3 & x4 Gen1 \\ PCIe slot: FPGA \& GPU & x8 Gen3 (different RC) & x8 Gen3 (same RC) \\ \bottomrule \end{tabular} \end{table} We carried out performance measurements on two different setups in order to evaluate two different ways of connecting PCIe devices: same PCIe Root Complex (RC) and different RC. In addition, we also want to verify that low-power embedded systems can drive DMA transfers if both the FPGA board and the GPU are connected using fast PCIe switch. The setups are described in table~\ref{table:setups}. In both setups, a Xilinx VC709 evaluation board was used. In Setup 1, the FPGA board and the GPU were plugged into a PCIe 3.0 slot, but they were connected to different RC. In Setup 2, a low-end Supermicro X7SPA- HF-D525 system was connected to a Netstor NA255A external PCIe enclosure. As opposed to Setup 1, both the FPGA board and the GPU were connected to the same RC through a x16 Gen3 PCIe link, available within the Netstor box. In case of FPGA-to-CPU data transfers, the software implementation is the one described in~\cite{rota2015dma}. \subsection{Throughput} \begin{figure}[t] \includegraphics[width=0.85\textwidth]{figures/throughput} \caption{% Measured throughput for data transfers from FPGA to main memory (CPU) and from FPGA to the global GPU memory (GPU) using Setup 1. } \label{fig:throughput} \end{figure} In order to evaluate the maximum performance of the DMA engine, measurements of pure data throughput were carried out using Setup 1. The results are shown in \figref{fig:throughput} for transfers to the system's main memory as well as to the global memory. For FPGA-to-GPU data transfers bigger than 95 MB, the double buffering mechanism was used. As one can see, in both cases the write performance is primarily limited by the PCIe bus. Up until 2 MB data transfer size, the throughput to the GPU is slowly approaching 100 MB/s. From there on, the throughput increases up to 6.4 GB/s at about 1 GB data size. The CPU throughput saturates earlier at a maximum throughput of 6.6 GB/s. The slope and maximum performance depend on the different implementation of the handshaking sequence between DMA engine and the hosts. With Setup 2, the PCIe 1.0 link limits the throughput to system main memory to around 700 MB/s. However, transfers to GPU memory yielded the same results as Setup 1, demonstrating that a high throughput can be achieved even if a slow embedded PC is used to drive the communication. \subsection{Latency} \begin{figure}[t] \centering \begin{subfigure}[b]{.49\textwidth} \centering \includegraphics[width=\textwidth]{figures/latency-cpu} \label{fig:latency-cpu} \vspace{-0.4\baselineskip} \caption{System memory transfer latency} \end{subfigure} \begin{subfigure}[b]{.49\textwidth} \includegraphics[width=\textwidth]{figures/latency-gpu} \label{fig:latency-gpu} \vspace{-0.4\baselineskip} \caption{GPU memory transfer latency} \end{subfigure} \caption{% Measured round-trip latency for data transfers to system main memory (a) and GPU memory (b). } \label{fig:latency} \end{figure} We conducted the following test in order to measure the latency introduced by the DMA engine: 1) the host starts a DMA transfer by issuing the \emph{start\_dma} command, 2) the DMA engine transmits data into the system main memory, 3) when all the data has been transferred, the DMA engine notifies the host that new data is present by writing into a specific address in the system main memory, 4) the host acknowledges that data has been received by issuing the \emph{stop\_dma} command. A counter on the FPGA measures the time interval between the \emph{start\_dma} and \emph{stop\_dma} commands with a resolution of 4 ns, therefore measuring the round-trip latency of the system. The correct ordering of the packets is guaranteed by the PCIe protocol. The measured round-trip latencies for data transfers to system main memory and GPU memory are shown in \figref{fig:latency}. With Setup 1 and system memory, latencies as low as 1.1 \textmu s can be achieved for a packet size of 1024 B. Higher latencies and a dependency on size measured with Setup 2 are caused by the slower PCIe x4 1.0 link connecting the FPGA board to the system main memory. The same test was performed when transferring data inside GPU memory. Like in the previous case, the notification was written into main memory. This approach was used because a latency of 100 to 200 \textmu s introduced by OpenCL scheduling did not allow a precise measurement based only on FPGA-to-GPU communication. When the devices are connected to the same RC, as in Setup 2, a latency of 2 \textmu s is achieved and limited by the latency to system main memory, as seen in \figref{fig:latency} (a). On the contrary, if the FPGA board and the GPU are connected to a different RC as in Setup 1, the latency increases significantly with packet size. Connecting devices to the same RC is therefore necessary to achieve the best performance, as stated in the NVIDIA's GPUDirect documentation. It must be noted that, due to the measurement approach, data is trasferred to GPU memory while the completion is determined by the packet written into system memory. This desynchronizes the transfer in case PCIe buffers are used on receiver side: the packet destined to system memory may be written before all data has been processed in the PCIe receiving buffer and delivered to GPU memory. The low latencies below 1 kB may be attributed to this effect, which must be taken into account as it could potentially lead to data corruption. %%%%%%%%%%% As stated in the NVIDIA's GPUDirect documentation, the devices must share the same RC to achieve the best performance. \section{Conclusion and outlook} We developed a hardware and software solution that enables DMA transfers between FPGA-based readout systems and GPU computing clusters. The net throughput is primarily limited by the PCIe link, reaching 6.4 GB/s for a FPGA-to-GPU data transfer and 6.6 GB/s for a FPGA-to-CPU's main memory data transfer. The measurements on a low-end system based on an Intel Atom CPU showed no significant difference in throughput performance. Depending on the application and computing requirements, this result makes smaller acquisition system a cost-effective alternative to larger workstations. We measured a round-trip latency of 1 \textmu s when transferring data between the DMA engine and system memory. We also assessed the applicability of DirectGMA in low latency applications: preliminary results shows that latencies as low as 2 \textmu s can be achieved during data transfers to GPU memory. However, at the time of writing this paper, the latency introduced by OpenCL scheduling is in the range of hundreds of \textmu s. In order to lift this limitation and make our implementation useful in low-latency applications, we are currently optimizing the the GPU-DMA interfacing OpenCL code with the help of technical support by AMD. Moreover, measurements show that dedicated connecting hardware must be employed in low latency applications. In order to increase the total throughput, a custom FPGA evaluation board is currently under development. The board mounts a Virtex-7 chip and features two fully populated FMC connectors, a 119 Gb/s DDR memory interface and a PCIe x16 3.0 connection. Two PCIe x8 3.0 cores, instantiated on the board, will be mapped as a single x16 device by using an external PCIe switch. With two cores operating in parallel, we foresee an increase in the data throughput by a factor of two as demonstrated in~\cite{rota2015dma}. The proposed software solution allows seamless multi-GPU processing of the incoming data, due to the integration in our streamed computing framework. This allows straightforward integration with different DAQ systems and introduction of custom data processing algorithms. Support for NVIDIA's GPUDirect technology is also foreseen in the next months to lift the limitation of one specific GPU vendor and compare the performance of hardware by different vendors. Further improvements are expected by generalizing the transfer mechanism and include InfiniBand support besides the existing PCIe connection. Our goal is to develop a unique hybrid solution, based on commercial standards, that includes fast data transmission protocols and a high performance GPU computing framework. \acknowledgments This work was partially supported by the German-Russian BMBF funding programme, grant numbers 05K10CKB and 05K10VKE. \bibliographystyle{JHEP} \bibliography{literature} \end{document}