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From: mankin@gateway.mitre.ORG
Newsgroups: comp.unix.wizards
Subject: Kernel Timings with MicrovaxII
Message-ID: <10646@brl-adm.ARPA>
Date: Thu, 3-Dec-87 11:14:14 EST
Article-I.D.: brl-adm.10646
Posted: Thu Dec  3 11:14:14 1987
Date-Received: Sun, 6-Dec-87 17:11:34 EST
Sender: news@brl-adm.ARPA
Lines: 34


Hi, folks,

I am working on IP-level performance in 4.3 BSD on a
MicrovaxII.  The CPU is a 630-Q-A, since we bought the system
over 2 years ago.  The operating system is pure Berkeley 4.3.

My question is about the real-time clock resolution.  The
hardware manual does not cover the 630, but analogy to the 750 
(and a good letter to this list by Chris Torek back in August)
got me hoping it would be possible to read times at a resolution
of microseconds from the  630's Interval Count Register.
I didn't see anything to contradict this in the 4.3 source code.
Alas, experiment and our DEC representative have at last revealed
that I will never read times of less than 10 milliseconds from
this ICR.

Now, in the latest Proceedings of the ACM Symposium on Operating
Systems Principles (SOSP-11), there are at least two articles where 
a MicrovaxII was used to do performance timings.  Jeff Mogul et al
includes figures like 1.77 milliseconds for the kernel to process a 
received IP packet.  Liskov et al give times such as that required for
a minimal system call (200 microseconds).  (By the way, their articles
are very juicy and interesting).  Do you get such timings only by doing 
thousands of identical operations and measuring the aggregate time?  

I've found out from DEC about a programmable real-time
clock for the MicrovaxII, the KWV-11-C.  Its cost is quite low, but
they do not have a driver for it outside an expensive package
called LabStar.  Could anyone point me to a UNIX driver, or to the good
techniques other than aggregate measurements for using the MicrovaxII
for kernel timings?

		AM