Boris,
If you mean that some people on your system execute Bus.Tx1, some others
execute Bus.Tx2, and some others (maybe with some overlap) execute
Bus.Tx3, then my answer to your question is:
No, I would strongly encourage you *not* to do this!
It was exercises like this that first led me to discover the fact that
there's no such thing as a "system" in the sense that most people use
the term (that is, as a big mishmash of different transactions, in which
averages have any real meaning).
Combining your three CDFs will hurt you in the way described in "Why
understanding distribution is important" on pp238-239 of the Optimizing
Oracle Performance book. Here's another example: Imagine the following
"system"...
avg. avg.
runs/day sec/run who uses it
Tx1 10,000 1 Group A
Tx2 1,000 10 Group B
Tx3 100 100 Group C
So, what's this "system's" average response time? A naïve
"mathematician" might think it's the weighted average of all the
response times: (10000*1 + 1000*10 + 100*100) / (10000+1000+100) = 2.7
sec. But what use is this figure? Nobody's response time is "ever"
really 2.7 sec. <footnote>I say "ever" here because it's of course
possible that a program whose "avg. sec/run" is 1 (or even 10) will
occasionally have a true response time of 2.7.</footnote>
If you're *anybody* actually using the system, the number "2.7 sec/run"
is just stupid! The 2.7s figure is especially ludicrous if you're a
member of Group B or C, because your average response time is either
really 3.7x that number (B) or 37x that number (C)! The mathematical
explanation for the stupid-looking-ness is that, no matter what you're
doing, this 2.7 number is an average influenced by stuff that you're
*not* doing.
There is no such thing as an "average user" (any more than there's an
American family with 2.3 children); in this example, there are only
members of Groups A, B, and C. What if you're a member of two groups
simultaneously (e.g., you run different transaction types in the same
day)? It's the same problem, because your expectation of, for example,
Tx1 response time is completely different from your expectation of Tx3
response time. Clumping response times from Tx1 and Tx2 into one average
makes no sense even then. Expecting *anything* you do to take 2.7
sec/run is going to leave you unfulfilled.
The number 2.7 has no useful meaning here. Certainly, if you have some
kind of service level agreement (SLA) wired to the number 2.7 sec/run in
this case, then I would say you have a SLA that's worse than having no
SLA at all.
Maybe an easier analogy is this... How would you respond to the
question, "Using the global air transportation system, how long does it
take to fly someplace?" One way to answer would be to compute a weighted
average of all flight durations recorded by IAPA for the past 12 months.
Imagine that the worldwide average is 2.7 hours. How much good does this
do someone who really wants to know how long it takes to get from
Chicago to Sydney? How 'bout Chicago to Detroit? No, it's fundamentally
the wrong way to respond. The right way to respond to "How long does it
take to fly someplace?" begins with asking the question "From where to
where?"
The "problem with averages" comes when the statistics you're trying to
average don't come from a single well-behaved distribution. See
pp236-254 of "Optimizing Oracle Performance" for a more complete
explanation of what I mean by this.
Virtually every computer system used by participants on this list has
two or more transaction types whose performance characteristics do not
come from a single well-behaved statistical distribution. On these
systems, it is impossible to come up with a single number (an average)
that will present a useful description of your "system." And I mean
"impossible" in the strictest, most carefully considered sense.
Bottom line: Do not attempt to combine your Bus.Tx# data in the way you
describe.
Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
Upcoming events:
- Performance Diagnosis 101: 12/16 Detroit, 1/27 Atlanta
- SQL Optimization 101: 12/8 Dallas, 2/16 Dallas
- Hotsos Symposium 2004: March 7-10 Dallas
- Visit www.hotsos.com for schedule details...
-----Original Message-----
Boris Dali
Sent: Saturday, December 06, 2003 11:59 AM
To: Multiple recipients of list ORACLE-L
Let's say I have 3 business transactions (consisting
of numerous Oracle transactions each) and I know total
service time for each (from c readings off sql traces
for the length of the bus.tx). Doing queuing theory
exercise I can also get CDF(r max) for each. Let's say
Bus.Tx1 - CPU time=5s CDF(r)=97%
Bus.Tx2 - CPU time=8s CDF(r)=95%
Bus.Tx3 - CPU time=10s CDF(r)=90%
How can I combine these three together and make any
conclusions as to what the overall CDF(r) would be for
the whole system consisting of the above 3 business
transactions? Is this doable?
Thanks,
Boris Dali.
______________________________________________________________________
Post your free ad now! http://personals.yahoo.ca
--
Please see the official ORACLE-L FAQ: http://www.orafaq.net
--
Author: Boris Dali
INET: [EMAIL PROTECTED]
Fat City Network Services -- 858-538-5051 http://www.fatcity.com
San Diego, California -- Mailing list and web hosting services
---------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L
(or the name of mailing list you want to be removed from). You may
also send the HELP command for other information (like subscribing).
--
Please see the official ORACLE-L FAQ: http://www.orafaq.net
--
Author: Cary Millsap
INET: [EMAIL PROTECTED]
Fat City Network Services -- 858-538-5051 http://www.fatcity.com
San Diego, California -- Mailing list and web hosting services
---------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L
(or the name of mailing list you want to be removed from). You may
also send the HELP command for other information (like subscribing).
