Fundamentals of Quantitative Design and Analysis - Computer Architecture: A Quantitative Approach - page 25

FIGURE 1.12 Energy savings for a server using an AMD Opteron microprocessor, 8 GB
of DRAM, and one ATA disk . At 1.8 GHz, the server can only handle up to two-thirds of the
workload without causing service level violations, and, at 1.0 GHz, it can only safely handle
one-third of the workload. ( Figure 5.11 in Barroso and Hölzle [2009].)
3. Design for typical case . Given that PMDs and laptops are often idle, memory and storage
offer low power modes to save energy. For example, DRAMs have a series of increasingly
lower power modes to extend batery life in PMDs and laptops, and there have been pro-
posals for disks that have a mode that spins at lower rates when idle to save power. Alas,
you cannot access DRAMs or disks in these modes, so you must return to fully active mode
to read or write, no mater how low the access rate. As mentioned above, microprocessors
for PCs have been designed instead for a more typical case of heavy use at high operating
temperatures, relying on on-chip temperature sensors to detect when activity should be re-
duced automatically to avoid overheating. This “emergency slowdown” allows manufac-
turers to design for a more typical case and then rely on this safety mechanism if someone
really does run programs that consume much more power than is typical.
4. Overclocking . Intel started offering Turbo mode in 2008, where the chip decides that it is safe
to run at a higher clock rate for a short time possibly on just a few cores until temperat-
ure starts to rise. For example, the 3.3 GHz Core i7 can run in short bursts for 3.6 GHz.
Indeed, the highest-performing microprocessors each year since 2008 in Figure 1.1 have
all offered temporary overclocking of about 10% over the nominal clock rate. For single
threaded code, these microprocessors can turn of all cores but one and run it at an even
higher clock rate. Note that while the operating system can turn of Turbo mode there is no
notiication once it is enabled, so the programmers may be surprised to see their programs
vary in performance due to room temperature!
Although dynamic power is traditionally thought of as the primary source of power dis-
sipation in CMOS, static power is becoming an important issue because leakage current lows
even when a transistor is of:
That is, static power is proportional to number of devices.