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RE: question about cpu usage

From: Nelson, Allan <>
Date: Fri, 22 Oct 2004 19:41:22 -0500
Message-ID: <>

Response time for interactive users is non-linear with respect to cpu utilization. The curve looks like a hockey stick with the puck striking surface pointing up. If you are past the knee of the curve your interactive users are receiving unpredictable response times. If you are at 90% utilization and you have 4 CPU's or less you are probably driving your interactive users mad. Query response time for them will swing wildly between "normal" to 10 to 100 times normal. Your batch jobs and your total throughput on the other hand might be doing pretty well. You can get Cary Millsap and Jeff Holt's Optimizing Oracle Performance from O'Reilly. You can go to and register for a =66ree acount and get some white papers and an excel spreadsheet that will show you where the knee on the curve is relative to the number of CPU's.

BTW, number of cpu's is the only factor that affects the knee of the curve and for most affordable configurations and normal mixes of OLTP vs batch you will find the you really want to stay less than 85% and for small machines with 4 or less cpus you are best of to stay below 70%.


-----Original Message-----
[] On Behalf Of sol beach Sent: Friday, October 22, 2004 4:11 PM
Cc: Gogala, Mladen; Subject: Re: question about cpu usage

It is not as simple as you might initially think it is. Consider that in reality, saying a CPU is 50% busy/idle is somewhat nonsensical. Either is it doing useful work (at the request of the OS) [100% busy] or it is idle [0% busy]. At any & EVERY point in time the CPU is either 100% or 100% idle; nothing in between.

Now over a period of time it could be busy half the time (50% busy & 50% idle).

What happens if 10 different folks concurrently as the CPU to do work =66or them=3F The answer is that 9 folks go into the CPU-wait queue.

As system system statistics display the reality the CPU is approaching 100% busy, users may or may not experience a degredation in response time.

It all depends on the charateristics of the work load. It might even depend upon the law of large number.

HTH & YMMV! On Fri, 22 Oct 2004 20:57:15 +0000, <> wrote:
> ok i will refine the question.
> Assuming I am at a constant rate X of CPU usage, will there be a=20
> declining returns as CPU usage increases to X + n for a constant=20
> period of time.


> Assume CPU usage holds steady. CPU u sage is CPU usage. It shouldnt=20
> matter what its doing in terms of performance.

> -------------- Original message --------------
> > Question asked in such generality really doesn't make much sense and

> > can only have one
> > answer: it depends. Mostly, it depends on what is CPU doing. Well=20
> > optimized queries will typically have a short burst or two of=20
> > intense CPU activity and then will finish. Using
> > 100% of CPU power is, unfortunately, also characteristic for "well
> > queries which
> > can perform a gazillion logical block gets with no phyisical disk
reads. An
> > example of
> > such query is the following:


> --

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Received on Fri Oct 22 2004 - 19:49:06 CDT

Original text of this message