Hardware Reference
In-Depth Information
microprocessors use hundreds of pins and multiple interconnect layers just for power and
ground. Second, power is dissipated as heat and must be removed.
Power And Energy: A Systems Perspective
How should a system architect or a user think about performance, power, and energy? From
the viewpoint of a system designer, there are three primary concerns.
First, what is the maximum power a processor ever requires? Meeting this demand can be
important to ensuring correct operation. For example, if a processor atempts to draw more
power than a power supply system can provide (by drawing more current than the system
can supply), the result is typically a voltage drop, which can cause the device to malfunction.
Modern processors can vary widely in power consumption with high peak currents; hence,
they provide voltage indexing methods that allow the processor to slow down and regulate
voltage within a wider margin. Obviously, doing so decreases performance.
Second, what is the sustained power consumption? This metric is widely called the thermal
design power (TDP), since it determines the cooling requirement. TDP is neither peak power,
which is often 1.5 times higher, nor is it the actual average power that will be consumed during
a given computation, which is likely to be lower still. A typical power supply for a system
is usually sized to exceed the TDP, and a cooling system is usually designed to match or ex-
ceed TDP. Failure to provide adequate cooling will allow the junction temperature in the pro-
cessor to exceed its maximum value, resulting in device failure and possibly permanent dam-
age. Modern processors provide two features to assist in managing heat, since the maxim-
um power (and hence heat and temperature rise) can exceed the long-term average speciied
by the TDP. First, as the thermal temperature approaches the junction temperature limit, cir-
cuitry reduces the clock rate, thereby reducing power. Should this technique not be successful,
a second thermal overload trip is activated to power down the chip.
The third factor that designers and users need to consider is energy and energy eiciency.
Recall that power is simply energy per unit time: 1 wat = 1 joule per second. Which metric is
the right one for comparing processors: energy or power? In general, energy is always a better
metric because it is tied to a specific task and the time required for that task. In particular, the
energy to execute a workload is equal to the average power times the execution time for the
workload.
Thus, if we want to know which of two processors is more efficient for a given task, we
should compare energy consumption (not power) for executing the task. For example, pro-
cessor A may have a 20% higher average power consumption than processor B, but if A ex-
ecutes the task in only 70% of the time needed by B, its energy consumption will be 1.2 × 0.7 =
0.84, which is clearly beter.
One might argue that in a large server or cloud, it is sufficient to consider average power,
since the workload is often assumed to be infinite, but this is misleading. If our cloud were
populated with processor Bs rather than As, then the cloud would do less work for the same
amount of energy expended. Using energy to compare the alternatives avoids this pitfall.
Whenever we have a fixed workload, whether for a warehouse-size cloud or a smartphone,
comparing energy will be the right way to compare processor alternatives, as the electricity
bill for the cloud and the batery lifetime for the smartphone are both determined by the en-
ergy consumed.
When is power consumption a useful measure? The primary legitimate use is as a con-
straint: for example, a chip might be limited to 100 wats. It can be used as a metric if the work-
load is fixed, but then it's just a variation of the true metric of energy per task.
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