>
> Someone asked earlier about how processor speeds vary and whatnot. A
The real drag on your system is head seek time.
An operating system that allocates on a sector by sector basis, as
Dos/Windows does, fragments files, and will slowly degrade in performance
unless disk optimization is used. Linux and Unix place a premium on
allocating files in blocks large enough to fit files. It also runs
optimisaztion things at night.
Next, we have disk transfer rates. SCSI helps. So does keeping your
swap disk separate from your other disks, so as to overlap seeks and
intersperse data from one drive with non-data-producing activity on other
drives. Windows uses a file, rather than a disk drive for that. Unix/Linux
use separate directories for them, and so are more able to keep those
things on separate drives. (Linux uses a separate partition for swap
space.) I have seen programs that do a lot of I/O run faster on a 486 DX
50, than an SGI Indy running at 100Mhz, but using a three year older SCSI
disk drive. Of course, programs with little I/O and heavy CPU utilization
would run at least twice as fast on the SGI! Even faster if there was
much floating point math.
Then we get to memory space. I have seen programs that run three
hours on a faster machine run in three minutes on a machine that had 50%
more memory, when memory size is a limit. It depends upon the way data is
handled by the program. If too much memory is expected, portions of the
program may be swapped in and out repeatedly. This is called thrashing.
A good operating system will switch between tasks or programs
whenever the program or task starts an I/O operation so as to overlap I/O
and execution. This seems to be one major advantage of the Unix/Linux
systems over DOS/Windows.
If your work load is suitable, the fastest way is to use multiple
processors with multiple disk drives. The graphics work for the Titanic
was done on paralleled DEC Alpha computers running Linux. Not all things
can be done this way.
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