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\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}

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\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}
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\end{document}
