RE: cpu average load

I disagree that this advice is difficult to implement in practice, = because I
implement it in practice frequently. Optimizing Oracle Performance = pp170-173
illustrates a very simple case that applies nicely to much more = complicated
Oracle cases. Ironically, I helped diagnose another real-life case of = this
type just 15 minutes ago.

The Oracle c statistic includes both the utime and stime components of = the
getrusage return value. (You can *prove* that by intercepting your = getrusage
calls and manipulating their return data.) Therefore, the "e" time = that's
not accounted for by any "c" (usr mode CPU plus sys mode CPU consumed by = the
Oracle kernel) time or "ela" time (mostly time spent asleep awaiting a syscall return) is going to be a reasonably reliable measure (+/-10%) of time spent preempted.

What helps my confidence significantly is that over and over again, I = see
that unaccounted-for time computed in the manner I've described reliably measures the amount of time that a process spends preempted.

It works. Really.
=20

Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
* Nullius in verba *

Upcoming events:
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• SQL Optimization 101: 12/13 Atlanta
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• Visit www.hotsos.com for schedule details...

-----Original Message-----
From: oracle-l-bounce_at_freelists.org =
[mailto:oracle-l-bounce_at_freelists.org]
On Behalf Of Gogala, Mladen
Sent: Tuesday, November 30, 2004 9:19 AM To: 'cary.millsap_at_hotsos.com'; oracle-l_at_freelists.org Subject: RE: cpu average load

This was a very precise and correct answer, but unfortunately rather = hard to
Implement in practice. Process preemption or context switching, as it is sometimes called, happens while CPU is in kernel mode. Kernel mode is normally spent only on 2 major things:=20 1) context switching
2) paging
CPU being in kernel mode means that OS it taking care of its own health = and
Happiness and not working on the user request. Commands like ps -lp = <PID> or
top will give you precise amount of kernel mode spent by your process. = If
that amount is significant, it warrants further investigation by using = truss
or strace. BTW, whenever a system service is requested, a context change happens. It includes things like read, write, stat and alike. The = mechanism=20
for calling system services is based on "switch mode to kernel" trap. = System
service number is the 1st argument for the trap handler.

--
Mladen Gogala
Ext. 121

-----Original Message-----
From: Cary Millsap [mailto:cary.millsap_at_hotsos.com]=20
Sent: Tuesday, November 30, 2004 9:59 AM
To: oracle-l_at_freelists.org
Subject: RE: cpu average load

The amount of response time that process preemptions are costing your
performance is measured as the amount of response time in an extended =
SQL
trace file that is not accounted for by the sum of your file's c values =
at
recursive depth zero, plus the sum of your file's ela values.



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