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Home -> Community -> Mailing Lists -> Oracle-L -> RE: question about cpu usage
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 hotsos.com 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%.
Allan
-----Original Message-----
=46rom: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of sol beach
Sent: Friday, October 22, 2004 4:11 PM
To: ryan_gaffuri_at_comcast.net
Cc: Gogala, Mladen; oracle-l_at_freelists.org
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, ryan_gaffuri_at_comcast.net
<ryan_gaffuri_at_comcast.net> 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.
>=20
>=20 >=20
> > 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
cached"
> > queries which
> > can perform a gazillion logical block gets with no phyisical disk
reads. An
> > example of
> > such query is the following:
>=20
-- http://www.freelists.org/webpage/oracle-l ___________________________________________________________________________= ___ This email is intended solely for the person or entity to which it is = addressed and may contain confidential and/or privileged information. = Copying, forwarding or distributing this message by persons or entities = other than the addressee is prohibited. If you have received this email in = error, please contact the sender immediately and delete the material from = any computer. This email may have been monitored for policy compliance. = [021216] -- http://www.freelists.org/webpage/oracle-lReceived on Fri Oct 22 2004 - 19:49:06 CDT
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