Performance of Generated Systems

A framework is only useful if it reduces the effort required by developers to achieve the desired performance objective. Trying to gauge the relative effort is difficult since the authors implemented both the framework and the running example. On PCIE-based platforms we were able to reduce the time required to search for a fixed set of strings in a large corpus by an order of magnitude after integrating hardware acceleration using Connectal. Performance improvements on the Zynq-based platforms was even greater due to the relative processing power of the ARM CPU and scaled with the number of bus master interfaced used for DMA. In the Connectal framework, developing these applications took very little time.

Performance of Portals

The current implementation of HW/SW textbf{portal} transfers 32 bits per FPGA clock cycle. Our example designs run at 100MHz to 250MHz, depending on the complexity of the design and the speed grade of the FPGA used. Due to their intended use, the important performance metric of Portals is latency. These values are given in Figure~ref{Fig:PortalLatency}.

begin{figure}

centering begin{tabular}{|c|c|c|c|c|c|c|c|c|}

hline
& rt{KC705} & rt{VC707} & rt{ZYBO} & rt{Zedboard} & rt{ZC702} & rt{ZC706} & rt{Parallel} & rt{Mini-ITX} \

hline HW $rightarrow$ SW & 3 & 3 & X & 0.80 & 0.80 & 0.65 & X & 0.65 \ hline SW $rightarrow$ HW & 5 & 5 & X & 1.50 & 1.50 & 1.10 & X & 1.10 \ hline

end{tabular} caption{Latency ($mu$s) of communication through portals on supported

platformslabel{Fig:PortalLatency}}

end{figure}

The Xilinx KC705 and VC707 boards connect to x86 CPUs and system memory via PCIe gen1. The default FPGA clock for those boards is 125MHz. The other platforms use AXI to connect the programmable logic to the quad-core ARM Cortex A9 and system memory. The ZYBO, Zedboard and ZC702 use a slower speed grade part on which our designs run at 100MHz. The ZC706 and Mini-ITX use a faster part on which many of our designs run at 200MHz. The lower latency measured on the ZC706 reflects the higher clock speed of the latency performance test.

Performance of Reads/Writes of System Memory

For high bandwidth transfers, we assume the developer will have the application hardware read or write system memory directly. Direct access to memory enables transfers with longer bursts, reducing memory bus protocol overhead. The framework supports transfer widths of 32 to 128 bits per cycle, depending on the interconnect used.

Our goal in the design of the library components used to read and write system memory is to ensure that a developer’s application can use all bandwidth available to the FPGA when accessing system memory. DMA Bandwidth on supported platforms is listed in Figureref{Fig:DmaBandwidth}.

begin{figure}

centering begin{tabular}{|c|c|c|c|c|c|c|c|c|}

hline
& rt{KC705} & rt{VC707} & rt{ZYBO} & rt{Zedboard} & rt{ZC702} & rt{ZC706} & rt{Parallel} & rt{Mini-ITX} \

hline Read & 1.4 & 1.4 & X & 0.8 & 0.8 & 1.6 & X & 1.6 \ hline Write & 1.4 & 1.4 & X & 0.8 & 0.8 & 1.6 & X & 1.6 \ hline

end{tabular} caption{Maximum bandwidth (GB/s) between FPGA and host memory using

Connectal RTL libraries on supported platformslabel{Fig:DmaBandwidth}}

end{figure}

On PCIe systems, Connectal currently supports 8 lane PCIe gen1. We’ve measured 1.4 gigabytes per second for both reads and writes. Maximum throughput of 8 lane PCIe gen1 is 1.8GB/s, taking into account 1 header transaction per 8 data transactions, where 8 is the maximum number of data transactions per request supported by our server’s chipset. The current version of the test needs some more tuning in order to reach the full bandwidth available. In addition, we are in the process of updating to 8 lane PCIe gen2 using newer Xilinx cores.

Zynq systems have four high performance ports for accessing system memory. Connectal enables an accelerator to use all four. In our experiments, we have been able to achieve 3.6x higher bandwidth using 4 ports than using 1 port.