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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 457
426
INPUT/OUTPUT
CHAP. 5
In order to measure the energy usage, Flinn and Satyanarayanan devised a soft-
ware tool called PowerScope. What it does is provide a power-usage profile of a
program. To use it, a computer must be hooked up to an external power supply
through a software-controlled digital multimeter. Using the multimeter, software is
able to read out the number of milliamperes coming in from the power supply and
thus determine the instantaneous power being consumed by the computer. What
PowerScope does is periodically sample the program counter and the power usage
and write these data to a file. After the program has terminated, the file is analyzed
to give the energy usage of each procedure. These measurements formed the basis
of their observations. Hardware energy-saving measures were also used and
formed the baseline against which the degraded performance was measured.
The first program measured was a video player.
In undegraded mode, it plays
30 frames/sec in full resolution and in color.
One form of degradation is to aban-
don the color information and display the video in black and white. Another form
of degradation is to reduce the frame rate, which leads to flicker and gives the
movie a jerky quality. Still another form of degradation is to reduce the number of
pixels in both directions, either by lowering the spatial resolution or making the
displayed image smaller. Measures of this type saved about 30% of the energy.
The second program was a speech recognizer.
It sampled the microphone to
construct a waveform. This waveform could either be analyzed on the notebook
computer or be sent over a radio link for analysis on a fixed computer. Doing this
saves CPU energy but uses energy for the radio.
Degradation was accomplished by
using a smaller vocabulary and a simpler acoustic model. The win here was about
35%.
The next example was a map viewer that fetched the map over the radio link.
Degradation consisted of either cropping the map to smaller dimensions or telling
the remote server to omit smaller roads, thus requiring fewer bits to be transmitted.
Again here a gain of about 35% was achieved.
The fourth experiment was with transmission of JPEG images to a Web brow-
ser. The JPEG standard allows various algorithms, trading image quality against
file size. Here the gain averaged only 9%.
Still, all in all, the experiments showed
that by accepting some quality degradation, the user can run longer on a given bat-
tery.
5.9 RESEARCH ON INPUT/OUTPUT
There is a fair amount of research on input/output. Some of it is focused on
specific devices, rather than I/O in general. Other work focuses on the entire I/O
infrastructure. For instance, the Streamline architecture aims to provide applica-
tion-tailored I/O that minimizes overhead due to copying, context switching, sig-
naling and poor use of the cache and TLB (DeBruijn et al., 2011). It builds on the
notion of Beltway Buffers, advanced circular buffers that are more efficient than
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