Power Protocol: Reducing Power Dissipation on Off-Chip Data Buses
Authors:
Kohinoor Basu, Alok Choudhary, Jay Pisharath (Northwestern University),
and Mahmut Kandemir (Pennsylvania State University)
Abstract:
Power consumption is becoming increasingly important for both embedded and high-performance systems. Off-chip data buses
can be a major power consumer. In this paper, we present a strategy called ``power protocol'' that tries to
reduce the dynamic power dissipation on off-chip data buses. To accomplish this, our strategy reduces the number of
bus lines that need to be activated for data transfer by
employing a small cache (called ``value cache'') at each side of the off-chip data bus. These value caches keep
track of the data values
that have recently been transmitted over the bus. The entries in these caches are constructed in such a way that the contents of
both the value caches are the same all the time. When a data value needs to be transmitted over the bus, we first check
whether it is in the value cache of the sender. If it is, we transmit only the index of the data
(i.e., its value cache address) instead of the actual data value and, the other side (receiver) can determine the data value by
using this index and its value cache. Our experimental results using a set of fifteen benchmark codes
from embedded systems domain show that power protocol is very effective in practice, and reduces the bit switching activity
on the data bus by as much as 70.7% (with a value cache of 128 entries).
We also present results from an implementation that combines our strategy with 1-to-2 encoding, a popular bus
encoding strategy for low power. Our results indicate that this combined optimization strategy reduces bit switching
activity by 67.8% on the average (across all benchmarks). These reductions
in bit switching activity lead to more than 7% reduction
on overall system energy on the average for a value cache of 256 entries.
We also study the sensitivity of our savings to the value cache capacity and
data cache capacity.
Web Site
http://www.ece.northwestern.edu/cpdc/PACT/PACT.html