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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 1042
SEC. 12.4
PERFORMANCE
1011
12.4.1 Why Are Operating Systems Slow?
Before talking about optimization techniques, it is worth pointing out that the
slowness of many operating systems is to a large extent self-inflicted. For example,
older operating systems, such as MS-DOS and UNIX Version 7, booted within a
few seconds. Modern UNIX systems and Windows 8 can take several minutes to
boot, despite running on hardware that is 1000 times faster.
The reason is that they
are doing much more, wanted or not.
A case in point.
Plug and play makes it
somewhat easier to install a new hardware device, but the price paid is that on
every
boot, the operating system has to go out and inspect all the hardware to see if
there is anything new out there. This bus scan takes time.
An alternative (and, in the authors’ opinion, better) approach would be to scrap
plug-and-play altogether and have an icon on the screen labeled ‘‘Install new hard-
ware.’’ Upon installing a newhardware device, the user would click on it to start
the bus scan, instead of doing it on every boot. The designers of current systems
were well aware of this option, of course.
They rejected it, basically, because they
assumed that the users were too stupid to be able to do this correctly (although they
would word it more kindly).
This is only one example, but there are many more
where the desire to make the system ‘‘user-friendly’’ (or ‘‘idiot-proof,’’ depending
on your linguistic preferences) slows the system down all the time for everyone.
Probably the biggest single thing system designers can do to improve per-
formance is to be much more selective about adding new features. The question to
ask is not whether some users like it, but whether it is worth the inevitable price in
code size, speed, complexity, and reliability.
Only if the advantages clearly out-
weigh the drawbacks should it be included. Programmers have a tendency to as-
sume that code size and bug count will be 0 and speed will be infinite. Experience
shows this view to be a wee bit optimistic.
Another factor that plays a role is product marketing. By the time version 4 or
5 of some product has hit the market, probably all the features that are actually use-
ful have been included and most of the people who need the product already have
it. To keep sales going, many manufacturers nevertheless continue to produce a
steady stream of new versions, with more features, just so they can sell their exist-
ing customers upgrades. Adding new features just for the sake of adding new fea-
tures may help sales but rarely helps performance.
12.4.2 What Should Be Optimized?
As a general rule, the first version of the system should be as straightforward
as possible. The only optimizations should be things that are so obviously going to
be a problem that they are unavoidable. Having a block cache for the file system is
such an example. Once the system is up and running, careful measurements
should be made to see where the time is
really
going. Based on these numbers,
optimizations should be made where they will help most.
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