DMF vs. DH timing results

[stevens at hsi.UUCP]

There have been some questions on the net concerning the DMA versus
programmed i/o mode of the asynchronous ports of the DMF32, plus
I have been wondering what the DMF buys you over the DH, so I ran
the following timing tests.

The tests consisted of continual output (cd /sys/vaxuba; cat *.c)
to 7 terminals (CIT-101e's) on an 11/750 running 4.2.
I monitored this on the console with a "vmstat +5".  This is where
the %sys column came from.  I averaged 20 of the vmstat's numbers
together - I don't know how accurate the vmstat us/sy/id percentages
are?  I then ran a smaller output test (cd /sys/vaxuba; time cat *.h)
to see what the user, system and clock times were, again on 7 terminals
simultaneously.  That is where the system time and clock time columns
came from (the 7 values, averaged).  (All of the user times were 0.0.)

				"vmstat"	       "time cat *.h"
	mode			  %sys		sys time (sec)	clock time
	-------------------	-------		--------------	----------
	DEC DMF32, DMA		 64.6 %		     7.14	    1:29
	Emulex CS21/H (DH)	 67.2 %		     7.17	    1:29
	DEC DMF32, prog i/o	 70.1 %		     7.54	    1:29

Note that 7 terminals, each with about 7-8 seconds of system time over
89 minutes, corresponds quite well to about 65% of the system (the vmstat
number).

>From these numbers, the DMF in DMA mode is barely better than the DH,
but the difference between prog. i/o and DMA isn't that great.
Note that these tests (output only) aren't the best.  I'd guess that a
good DMF driver that can dynamically switch between programmed i/o and
DMA modes would probably be the best for real world tty i/o, but it
probably won't be that much better than the DH.  If you're interested in
some comparisons of the DH and the DZ, take a look at the "Emulex
Controller Handbook" (free from Emulex) as they have some output only
tests run under VMS.  (This book has some better information on the DEC
bus'es and controllers than DEC's Peripheral Handbooks.)

	Richard Stevens
	Health Systems International, New Haven, CT
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