Optimizing Capacitance and Switching Activity to Reduce Dynamic Power - Computer Architecture Techniques for Power-Efficiency - page 83

Disabled for 6-issue and 4-issue

Disabled for 4-issue

Branch
Prediction
Unit

Right Cluster
Functional Units

Right Arbiter 0
Right Arbiter 1
Right Arbiter 2
Right Arbiter 3

Right
Register
File

4 Integer Units
2 FP Units
2 Memory Ports

Register
Rename
Unit

Fetch
Unit

Performance
Monitors &
Controls

Data Cache

Left Cluster
Functional Units

Left Arbiter 0

Instruction
Cache

Left
Register
File

Left Arbiter 1
Left Arbiter 2
Left Arbiter 3

4 Integer Units
2 FP Units
2 Memory Ports

FIGURE 4.16: Adjusting the width of an 8-issue machine to 6- or 4-issue. Adapted from [ 19 ].

6-issue or 4-issue when the application cannot take advantage of the additional width. They
model their target processor after an 8-issue Alpha 21264 [ 134 ], comprising two 4-issue clusters
(Figure 4.16). To switch the processor to 6-issue, one-half of one of the clusters is disabled. To
switch to the 4-issue, one whole cluster is disabled.
To disable half or a whole cluster, the appropriate functional units are clock gated. In
addition to disabling functional units, part of the instruction queue hardware is also disabled,
thus realizing additional power benefits. This is different from IQ resizing (described previ-
ously), in that the instruction window size does not change. What is disabled in the IQ are the
scheduling units that pick ready instructions for execution (called arbiters in Alpha parlance).
All in all, this technique can save up to 20% (10%) power from the execution units, 35% (17%)
from the instruction queue, and 12% (6%) in total, in the 4-issue (6-issue) low-power mode.
Again, one of the main contributions of this work is the technique to adjust the issue
width to the needs of the program. Decisions are made at the end of a sampling window
assuming that the behavior of the program in the last window is a good indicator for the next. 7
A set of rules decides whether to enable or disable the two low-power modes, the 4-issue and
the 6-issue mode. Enabling a mode sets the width to 4 or 6, but disabling it jumps back to
normal operation, which is the full 8-issue mode. The rules compare the issue IPC (for both

7 This assumption is put to test by Bahar and Manne and is shown to hold up quite nicely [ 19 ].