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Input Devices/Power
Jeffrey James Valerio

performance per watt
In computing, performance per watt is a measure of
the energy efficiency of a particular computer
architecture or computer hardware. Literally, it
measures the rate of computation that can be
delivered by a computer for every watt of power
consumed.
System designers building parallel computers, such as
Google's hardware, pick CPUs based on their
performance per watt of power, because the cost of
powering the CPU outweighs the cost of the CPU itself.

FLOPS (Floating Point Operations
Per Second) per watt
FLOPS (Floating Point Operations Per Second) per
watt is a common measure. Like the FLOPS it is based
on, the metric is usually applied to scientific computing
and simulations involving many floating point
calculations.

Instructions per second (IPS) is a
measure of a computer's
processor speed.
The term is commonly used in association with a numeric value such as
thousand instructions per second (kIPS), million instructions per second
(MIPS), Giga instructions per second (GIPS), or million operations per second
(MOPS).

average CPU power (ACP),
The average CPU power (ACP), is a
scheme to characterize power
consumption of new central processing
units under "average" daily usage,
especially server processors, the rating
scheme is defined by Advanced Micro
Devices (AMD) for use in its line of
processors based on the K10
microarchitecture (Opteron 8300 and
2300 series processors)

thermal design power (TDP),
The thermal design power (TDP),
sometimes called thermal design point, is
the maximum amount of heat generated
by the CPU that the cooling system in a
computer is required to dissipate in typical
operation. Rather than specifying CPU's
real power dissipation, TDP serves as the
nominal value for designing CPU cooling
systems.

CPU power dissipation
Central processing unit power dissipation
or CPU power dissipation is the process in
which central processing units (CPUs)
consume electrical energy, and dissipate
this energy both by the action of the
switching devices contained in the CPU
(such as transistors or vacuum tubes) and
by the energy lost in the form of heat due
to the impedance of the electronic circuits.

CPU power dissipation
There are several factors contributing to the CPU power consumption; they
include dynamic power consumption, short-circuit power consumption, and
power loss due to transistor leakage currents:
Pcpu = Pdyn + Psc + Pleak
The dynamic power consumption originates from logic-gate activities in the
CPU. When logic gates toggle, energy is flowing as capacities inside the
logic gates are charged and discharged. The dynamic power consumed by
a CPU is approximately proportional to the CPU frequency, and to the
square of the CPU voltage:
P = CV2f
where C is capacitance, f is frequency, and V is voltage.

40 Watt ACP = 60 Watt TDP

Why haven't CPU clock
speeds increased in the
last 5 years?

Narrow pipelining
is easy to speed up

Narrow pipelining
is single threaded with only
serial codes.

"Moore's law" is the
observation that, over the
history of computing hardware,
the number of transistors in a
dense integrated circuit
doubles approximately every
two years.

Problems:
Heat (too much of it and too
hard to dissipate), power
consumption (too high), and
current leakage problems.

Power dissipation in CMOS technology

The first part (addend) of the equation accounts for
the dynamic power consumption on the chip (i.e. the
power consumption caused by charging and
discharging capacitive loads when transistors are
switched) that represents the useful work performed
by the chip. A is the activity factor meaning the
proportion of switching transistors in each cycle (since
not all transistors have to switch every clock cycle); C is
the capacitive load of the transistor; V is the voltage;
and f is the frequency.

If we observe the first term of the equation we can see
why power has being increasing only linearly while
frequency has been doing it logarithmically. The
reason is the quadratic dependence on the voltage.

Engineers have been able to continuously reduce this voltage
from 5V down to below 1V, which has helped them to control
dissipated power without losing performance. Unfortunately,
many factors are interdependent and engineers have to make
trade-offs constantly. For example, imagine we want to
decrease dynamic power consumption on a chip (consider only
first term of the equation) by reducing the supply voltage
initially fixed at 2V. If we are able to reduce it to 1.7V, it is only a
15% decrease in voltage but we get a significant 28% decrease
in power. However, reducing supply voltage has a side-effect
on the maximum frequency for the circuit and on the threshold
voltage of transistors (the voltage at which a transistor switches
on):

The problem is that while
transistors are getting smaller,
they're NOT getting faster.

Switching speed of transistors
depend on the strength of the
electric field.

Strength of the electric field
depends on the thickness of
the gate.

As transistors shrink, the area of
the gate decreases.

In the past, this means meant
that the gate of a transistor
could also be made thinner.

"The thinner the separation
between two conductive plates,
the stronger the electric field is
between them."

As the switching could be faster
transistors could be made
thinner without adding load
capacitance.

However, as of 45nm, the gate
dielectric is now approximately
0.9nm thick

However, as of 45nm, the gate
dielectric is now approximately
0.9nm thick--about the size of a single silicon-
dioxide molecule.

It is simply impossible to make
this thinner.

As transistors get smaller, the
wires connecting them get thinner.

Thinner wires mean higher
resistance and lower current.

Clock-gating inserts a clock-
enable before each state
element (register, latch, etc.)
such that the element is not
clocked if new data is not going
to be written.

Clock-gating inserts a clock-
enable

This saves a significant amount
of charge/discharge that would
be wasted writing the same bit
back to the cell.

This involves putting large, fat
transistors at the voltage source
for various sections of the chip
and powering those sections
off when not used.

How much does my laptop
consume?

Intel Core 2 Duo 2.0 GHz processor
2 GB RAM
32 GB solid state hard drive
13.3" 1280x800 LED backlit display
NVIDIA GeForce Go 8400M GS video
Windows Vista Ultimate

LCD brightness 7 (max) 20w
LCD brightness 6 19w
LCD brightness 5 18w
LCD brightness 0-4 17w

HDD idle 20w
HDD defragmenting 23w

CPU idle 20w
CPU running one prime95 torture test 50w
CPU running two prime95 torture tests 63w

GPU idle 20w
GPU running rthdribl 55w
GPU running ATITool 3D warmup 40w

DVD idle 20w
DVD spinning with disc inserted 25w
DVD copying 33w

CPU fan off 20w
CPU fan low 21w
CPU fan med/high 22w

1.Don't do anything with 3D graphics (gaming, etc)
2.Avoid using DVDs
3.Turn down the screen brightness 1 or 2 notches
4.Avoid CPU intensive web pages or programs

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