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Home -> Community -> Mailing Lists -> Oracle-L -> RE: CPU Capacity Planning
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
-----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.
-- Please see the official ORACLE-L FAQ: http://www.orafaq.net -- Author: Boris Dali INET: boris_dali_at_yahoo.ca 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: ListGuru_at_fatcity.com (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: cary.millsap_at_hotsos.com 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: ListGuru_at_fatcity.com (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).Received on Sun Dec 07 2003 - 02:29:26 CST
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