IEEE Consumer Electronics Magazine - May/June 2022 - 102
Special Section on ISVLSI-2020
Figure 9. Latency and off-chipmemory access. BWA: BusWidth Aware, SS:SmartShuttle. (a) VGG16: Energy and latency
efficiency. (b) BVGG16: Varying on-chip buffer sizes. (c) VGG16: Fully connected layers. (d) AlexNet: Energy and latency efficienc.
(e) BAlexNet: Varying on-chip buffer sizes. (f) AlexNet: Fully connected layers.
approach, we computed the energy consumption
using the FPGA design power reported by
the Vivado synthesis tool, the execution time,
and the number of bytes accessed from off-chip
memory logged using Xilinx APM IP.16 We used
70 pJ/bit off-chip memory access energy per
bit23 for the energy computations.
Figure 9(a) and (d) show the energy, off-chip
memory accesses, and latency efficiency achieved
using the BWA compared to the SS approach. We
observed that the changes in energy and latency
are proportional to the changes in memory
access. This observation confirms that off-chip
memory access dominates the energy consumption
of the CNN accelerators.
On-Chip Memory Size. Thesizeofthe on-chip
memory constrains the tile dimensions. Larger onchip
memory can accommodate bigger tiles, which
reduces the trips and number of off-chip memory
access transactions. Figure 9(b) and (e) shows offchipmemoryaccess
for VGG16and AlexNet, respectively.
Off-chip memory access reduces with
increasing on-chip memory size for both
102
approaches. However, the BWA approach performs
better than SS for all the on-chipmemory sizes.
Impact of Bus Width on B of FCLs. In FCLs,
the height and width of tiles and data are the
same. Tiles in FCL can be accessed from off-chip
memory using a single transaction, which results
in fewer transactions in FCLs compared to CLs.
Whereas in CLs, multiple transactions are
required to access different rows of the tiles.
Thus, the impact of bus width is less on B of
FCLs as compared to CLs. Figure 9(c) and (f)
shows the off-chip memory accesses of FCLs of
VGG16 and AlexNet, respectively, for 8 bits data
width. BWA reduces BFC
4% and BFC
VGG16 by 1%, 2%, 3%, and
AlexNet by 0.2%, 1%, 3%, and 6% on 32,
64, 128, and 256 bits wide buses, respectively.
CONCLUSION
The energy efficiency of CNN accelerators is
critical for their usage in battery-operated devices.
Majority of their energy results from a large
number of off-chip memory accesses. Due to the
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