Hi to all - apologies if I am cross-posting to the wrong group.  Linux
people tend to be very aware of Operating Systems, and C/C++ programmers
might have some clues to my problem below.

First, I am just a mechanical engineer in robotics - I never took OS and
have only a high-level understanding of the kernel. I am using GNU C++
under SunOs 5.1 on a Sparc 20.  

The problem:
The Sun is serially connected to a device (pan-tilt motor unit) that works
at 9600 baud.  My C program can write to and read from it, in ASCII chars
just fine.  The program writes chars to the device, then reads back a
3-char return prompt.  

Physcially connecting a 2-channel oscilloscope on the serial port's RX/TX
and GND revealed the following:  writing 4 chars and reading the 3
char return prompt took apx. 10 millseconds (ms).  The scope showed a 1.51
ms delay between the RX and TX signals.  10 ms makes sense since a char
with start/stop bits yields:

     7 chars * 10 bits/char / 9600 bit/s = 7.3 ms
     add the delay of 1.51 ms gives a grand total of 8.81 ms

This is close to the actual scope reading...

Enveloping the read/write statements in a gettimeofday() says
read/write takes 40 ms !!

My Comp Sci friends tell me it's because of context switching.
Sparc 20 benchmarks say that a context switch takes 0.2 msec
and my friends speculate about 2 ms/time-slice.  

Our Sparc20 is a multi-user networked machine.  But I still can't
believe that the read/write cycle takes 40 ms... more than 4 times
longer than the scope reading.  I don't have many processes running
(less than 10 I'd say).  Typically I am the only one running the machine
too.

Any clues?  Are my friends right?  Seems like an awfully long time to me!

Very grateful,

-paul

----------------------------------------------------------------------------
Paul Oh                
PhD Candidate
Columbia University
Dept of Mechanical Engineering
Center for Research in Intelligent Systems
http://www.*-*-*.com/ ~paul
----------------------------------------------------------------------------