KURAS@BCVAX3.BITNET (03/19/86)
I would like to add one final note to the discussion about whether the Mac can
really communicate at high speed. (The bulk of the discussion occurred about
two weeks ago.)
I pointed out that the effective communication speed depended not upon the
ability of the serial driver to send data out the port quickly, but upon the
program's ability to get text to the screen quickly. The program will almost al-
ways be slower, especially at speeds above 2400B, but the speed of a serial port
is a given. In the case of file transfers, the slowdown from nominal baud is
caused by data redundancy for error checking.
Another slowdown I did not mention is associated with multi-user systems. The
more users are on a system, especially if they are running at high speed, the
slower everything will be, including file transfers. For example, when transfer-
ring a file, you may get a packet of a few blocks, then have to wait while some
other user does some CPU intensive processing like compilation, so a lot of your
time is spent waiting for the CPU, not communicating. The same problem occurs
with text reception, but is easier to diagnose, since text appears on the screen
as it is received.
If the system is rather full, and its users are running at high speed, things
seem to go even slower. Consider this scenario: ten users are on a system, with
nine running at 1200 baud and one running at 9600 baud. The 9600 baud user
spends less time with I/O, since his terminal sends and receives the info,
faster, and more time processing, so the CPU polls his terminal more often. The
9600 baud user gets more done in less time than the others because his terminal
is busy with I/O less of the time. As more users jump to 9600 baud, the slow-
down is magnified until, when enough users are logged in, the system hangs. It
has no time to do anything but poll each terminal because each user is sending
data to the CPU faster than it can process it.
So don't complain about the Mac being slow. Maybe your system is just too
full to work any faster.
Regards,
Pat Kuras
Boston College
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