User Guide


CPU/FPGA Interaction View

Use the CPU/FPGA Interaction viewpoint to assess FPGA performance of executed kernels, overall time for memory transfers between the CPU and FPGA, and how well a workload is balanced between the CPU and FPGA.
To interpret the performance data provided in the CPU/FPGA Interaction viewpoint, you may follow the steps below:

Define a Performance Baseline

Start with exploring the
window that provides general information on your application execution. Key areas for optimization include application execution time, tasks with high CPU or FPGA time, and kernel execution time.
Use the Elapsed Time value as a baseline for comparison of versions before and after optimization.

Assess FPGA Utilization

Look at the
FPGA Top Compute Tasks
list on the
window for a list of kernels running on the FPGA.
FPGA top compute tasks
Switch to the
window and use the
Computing Task Purpose / Source Computing Task (FPGA)
grouping to view the hotspots for kernels.
You can click a task from the
FPGA Top Compute Tasks
list to be taken to that task on the
Review the
FPGA Utilization
timeline, which shows how many kernels and transfers are executing at the same time on the FPGA.
FPGA Utilization

Review Memory Transfers

Look at the
Data Transferred
column on the
window or the
Computing Queue
rows on the
window to view the FPGA kernels and memory transfers.

Determine Workload Impact

Context Switch Time
metric on the
window shows the amount of time the CPU spent in context switches. Switch to the
window and hover over the timeline to view the reason for the context switch. In some cases, CPU context switches may represent CPU waits for the FPGA. Look at the
FPGA Utilization
line to identify times when the CPU may have been waiting on the FPGA and vice versa. For instance, when there is no FPGA activity, but CPU activity is high, it is likely that the FPGA is waiting for the CPU to complete a preparation step.

Review FPGA Device Metrics

Switch to the
window to analyze
Global Bandwidth
metrics and see how efficiently your kernels run on the FPGA device.
Analyze the
Idle %
metrics values to understand the percentage of cycles when there were no valid work-items executing or stalling the memory or channel instruction. The
Activity %
metric shows the percentage of cycles a predicated channel or memory instruction is enabled.

Analyze Channel Depth

In the Bottom-up window, locate the
Maximum Channel Depth
information for selected instances. If required, adjust the channel depth for your needs.
FPGA channel depth
If the channel is full all the time, the write side of the channel is working faster than the read side, and the channel will be stalling in the write kernel. If the channel is mostly empty, the read side is likely to be stalling, and if the channel is bigger than 32 bits deep, you can reduce it in size without a performance hit.

Analyze Loops

Analyze the occupancy for profiled loops:
FPGA loop profiling

Analyze Source of the Host Application Part

Double-click the function you want to optimize to view its related source code file in the Source/Assembly window. You can open the code editor directly from the
Intel® VTune™
and edit your code (for example, minimizing the number of calls to the hotspot function).

Analyze Source of the Kernel Running on an FPGA Device

Double-click the kernel to see FPGA device metrics per the kernel source lines. Use the Source view to see what channels and memories cause most stalls and how much data they transfer.

Product and Performance Information


Performance varies by use, configuration and other factors. Learn more at