Finalize Kernel Boundaries - 2023.2 English

Vitis Unified Software Platform Documentation: Application Acceleration Development (UG1393)

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2023.2 English

As discussed earlier, performance can be improved by creating multiple instances of kernels (compute units). However, adding CUs has a cost in terms of I/O ports, bandwidth, and resources.

In the Vitis software platform flow, kernel ports have a maximum width of 512 bits (64 bytes) and have a fixed cost in terms of device resources. Most importantly, the targeted platform sets a limit on the maximum number of ports which can be used. Be mindful of these constraints and use these ports and their bandwidth optimally.

An alternative to scaling with multiple compute units is to scale by adding multiple engines within a kernel. This approach allows increasing performance in the same way as adding more CUs: multiple data sets are processed concurrently by the different engines within the kernel.

Placing multiple engines in the same kernel takes the fullest advantage of the bandwidth of the kernel’s I/O ports. If the datapath engine does not require the full width of the port, it can be more efficient to add additional engines in the kernel than to create multiple CUs with single engines in them.

Putting multiple engines in a kernel also reduces the number of ports and the number of transactions to global memory that require arbitration, improving the effective bandwidth.

On the other hand, this transformation requires coding explicit I/O multiplexing behavior in the kernel. This is a trade-off the developer needs to make.