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I/O PERFORMANCE
How does I/O performance effect the real world?
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Often performance is not just the processor but the
I/O (input and output) systems. Computers do processing, but
often they must move data around and bring input in to the
computer, or give the user output. Sometimes this is not
very critical, other times it is what people are measuring
performance by.
Macs often have far better I/O performance than the PC
counterparts. Contributing to this performance (or lack
thereof) are the following -
- ISA Slots - older slots that are 16 bit and
slow relative to the rest of the computer - so
anything on these slots can slow the whole machine down
(dramatically). An example of this is PC's
Sound which still goes through ISA
architecture (even if it is on the motherboard) and in
some extreme conditions can tie up the computers I/O for
up to 40% of its time while only doing sound. (Macs do
not have this problem).
- PCI Slots - PCI slots are much faster than ISA, but
many PC-motherboards implement PCI so that they are a
multiples of the processors frequency. (Example - if a PC
has a 150mhz processor, than the nearest even multiple
for PCI would be 30mhz, instead of the 33mhz that is its
max., that is a 10% drop, but with syncronization issues
can be much worse). This can reduce PCI card performance
by 10 or 20% easy. Macs always run PCI at max. frequency,
and also split PCI sub systems and often have dedicated
bridge chips (so that PCI cards can talk to other PCI
cards without bothering the main computer).
- Archaic IRQ structure - depending on the structure of
IRQ's and DMA's and sharing of interrupts (and COM-ports)
you not only deal with compatibility and conflict issues,
but sometimes arbitration and performance issues as well.
- Slow Serial - many PC's still have older slower
serial controllers (called UART's). This keeps the serial
ports running at some slow speeds (like 56kbps). (Macs
serial ports can run at 2,000kbps)
- Slow Parallel - Parallel is slow especially compared
to other interfaces like SCSI or IDE. So hanging a disk
drive off of parallel is a way to halve (or more) your
performance. Also most older PC parallel ports are
unidirectional (they can only send information and not
recieve it back). (Macs use SCSI for disks, scanners, and
Networks for printers so this is not an issue)
- IDE - IDE (or EIDE) is how PC's usually access their
disk drives and CD-ROMs. IDE can run at a fraction the
speed of SCSI, and there are other issues. It seems that
all devices on an IDE controller have to be running the
same protocol. So if you have a CD-ROM and HardDrive
sharing the same controller, the lowest common
denominator protocol is used - which often means that
your HD is slowed down even more. Some people buy another
IDE controller to get around this problem. (Macs usually
use SCSI and so this is not an issue).
- Floppies are slow. So some people use a
Cache (a little bit of fast
RAM as a buffer to speed up reading and
writing to floppies). PC's can't use a write cache on
floppies because the floppy eject is not software
controlled (a user could eject the disk before the cache
is written causing loss of data, or swap a disk and the
cache would be out of sync, etc.).
There are other gotcha's. And there are many things I did
not mention like -
- Modems. Modems run the same speed on Macs as PC's,
but Macs have faster serial ports - so if you are likely
to have a problem with your machine not keeping up with
the modem, then it is likely to be on a PC.
- Scanners. Scanners are likely to use SCSI on a Mac
(fast) or serial or parallel on a PC. The real world
results are that the Mac flavor will usually blow the PC
flavor away - but that is more the type of port chosen.
- Tablets. Those fancy drawing tablets by artists -
often the Macs have high-speed serial ports, while PC's
often have to add special cards to use those tablets
(either serial or proprietary). The end results vary.
Conclusion
The I/O subsystems are just a part of computer
performance - but sometimes a significant part. There are
many "little" details in computer system design that effect
performance. PC's tend to be commodities - and in
commodities "cheap" is often much more important than
"fast".
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