Performance Comparison Between AI Engine/PL and AI Engine/NoC Interfaces - 2021.2 English

Versal ACAP AI Engine Programming Environment User Guide (UG1076)

Document ID
UG1076
ft:locale
English (United States)
Release Date
2021-12-17
Version
2021.2 English

The AI Engine array interface consists of the PL and NoC interface tiles. The AI Engine array interface tiles manage the two following high performance interfaces.

  • AI Engine to PL
  • AI Engine to NoC

The following image shows the AI Engine array interface structure.

Figure 1. AI Engine Array Interface Topology

One AI Engine to PL interface tile contains eight streams from the PL to the AI Engine and six streams from the AI Engine to the PL. The following table shows one AI Engine to PL interface tile capacity.

Table 1. AI Engine Array Interface to PL Interface Bandwidth Performance
Connection Type Number of Connections Data Width (bits) Clock Domain Bandwidth per Connection (GB/s) Aggregate Bandwidth (GB/s)
PL to AI Engine array interface 8 64 PL

(500 MHz)

4 32
AI Engine array interface to PL 6 64 PL

(500 MHz)

4 24
Note: All bandwidth calculations in this section assume a nominal 1 GHz AI Engine clock for a -1L speed grade device at VCCINT = 0.70V with the PL interface running at half the frequency of the AI Engine as an example.

The exact number of PL and NoC interface tiles is device-specific. For example, in the VC1902 device, there are 50 columns of AI Engine array interface tiles. However, only 39 array interface tiles are available to the PL interface. Therefore, the aggregate bandwidth for the PL interface is approximately:

  • 24 GB/s * 39 = 0.936 TB/s from AI Engine to PL
  • 32 GB/s * 39 =1.248 TB/s from PL to AI Engine

The number of array interface tiles available to the PL interface and total bandwidth of the AI Engine to PL interface for other devices and across different speed grades is specified in Versal AI Core Series Data Sheet: DC and AC Switching Characteristics (DS957).

GMIO uses DMA in the AI Engine to NoC interface tile. The DMA has two 32-bit incoming streams from the AI Engine and two 32-bit streams to the AI Engine. In addition, it has one 128-bit memory mapped AXI master interface to the NoC NMU. The performance of one AI Engine to NoC interface tile is shown in the following table.

Table 2. AI Engine to NoC Interface Tile Bandwidth Performance
Connection Type Number of connections Bandwidth per connection (GB/s) Aggregate Bandwidth (GB/s)
AI Engine to DMA 2 4 8
DMA to NoC 1 16 16
DMA to AI Engine 2 4 8
NoC to DMA 1 16 16

The exact number of AI Engine to NoC interface tiles is device-specific. For example, in the VC1902 device, there are 16 AI Engine to NoC interface tiles. So, the aggregate bandwidth for the NoC interface is approximately:

  • 8 GB/s * 16 = 128 GB/s from AI Engine to PL
  • 8 GB/s * 16 = 128 GB/s from PL to AI Engine

When accessing DDR memory, the integrated DDR memory controller (DDRMC) number in the platform limits the performance of DDR memory read and write. For example, if all four DDRMCs in a VC1902 device are fully used, the hard limit to access DDR memory is as follows.

  • 3200 Mb/s * 64 bit * 4 DDRMCs / 8 = 102.4 GB/s

The performance of GMIO accessing DDR memory through the NoC is further restricted by the NoC lane number in the horizontal and vertical NoC, inter NoC configurations, and QoS. Note that DDR memory read and write efficiency is largely affected by the access pattern and other overheads. For more information about the NoC, memory controller use, and performance numbers, see the Versal ACAP Programmable Network on Chip and Integrated Memory Controller LogiCORE IP Product Guide (PG313).

For a single connection from the AI Engine or to the AI Engine, both PLIO and GMIO have a hard bandwidth limit of 4 GB/s. Some advantages and disadvantages for choosing PLIO or GMIO are shown in the following table.

Table 3. Comparison of PLIO vs GMIO
PLIO GMIO
Advantages
  • Number of AI Engine to PL interface streams are larger, hence larger aggregate bandwidth
  • No interference between different stream connections
  • Supports packet switching
  • No PL resource required
  • No timing closure requirement
Disadvantages
  • Congestion risk if there are too many stream connections in a region of the device
  • Timing closure required for achieving best performance
  • Less GMIO ports available
  • Aggregate bandwidth is lower
  • Multiple GMIO ports competing for bandwidth