Home » RDBMS Server » Performance Tuning » DB time in AWR Report - (CPU time,I/O and waits)? (Oracle 10.2.0.4)
DB time in AWR Report - (CPU time,I/O and waits)? [message #559996] Mon, 09 July 2012 01:42 Go to next message
orapratap
Messages: 113
Registered: November 2011
Location: Canada
Senior Member
Hello

I understand that when data is read from the disk, I/O is done
And When computations are done then CPU is used

Then where the following equation fits?

DB Time = sum of database CPU time + waits

Is I/O considered as a part of CPU time?

Does this equation changes with SAN, OS caching?

Thanks and Regards
Orapratap
Re: DB time in AWR Report - (CPU time,I/O and waits)? [message #560001 is a reply to message #559996] Mon, 09 July 2012 02:21 Go to previous messageGo to next message
Michel Cadot
Messages: 60063
Registered: March 2007
Location: Nanterre, France, http://...
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Account Moderator
Database 2 Day + Performance Tuning Guide
Chapter 2 Oracle Database Performance Method
Section Time Model Statistics

Regards
Michel

Re: DB time in AWR Report - (CPU time,I/O and waits)? [message #560036 is a reply to message #560001] Tue, 10 July 2012 00:17 Go to previous messageGo to next message
orapratap
Messages: 113
Registered: November 2011
Location: Canada
Senior Member
Hello Michel

Thanks for your reply and the link mentioned in it

I am reading the topics covered in the link. However there isn't a clue regarding "if cpu is used while doing I/O"

We always refer the terms "system is CPU bound" or "system is I/O bound" implying these are 2 different things and Mostly meaning cpu is used as a part of I/O operation

Also you will see in OEM - Top Activity page, sql statements sre categorized as different wait class - 1) CPU Usage 2) System I/O

This seems to me as CPU usage and I/O are 2 different things and if those are diffrent does the CPU Time in AWR covers I/O operation time in it?

Thanks and Regards
Orapratap
Re: DB time in AWR Report - (CPU time,I/O and waits)? [message #560039 is a reply to message #560036] Tue, 10 July 2012 01:04 Go to previous message
Michel Cadot
Messages: 60063
Registered: March 2007
Location: Nanterre, France, http://...
Senior Member
Account Moderator
Quote:
However there isn't a clue regarding "if cpu is used while doing I/O"


Yes and no. IT is more an OS question than an Oracle one.
IO is:
1/ Prepare the io -> cpu
2/ Ask the io controller to execute the io -> cpu
3/ wait for the controller to answer -> wait (if synchroneous io)
4/ Receive the io acknowledge and possibly (if read) fill the caller buffer -> cpu
(I simplify because you also have cpu due to your context swicth off and on, possibly cpu due to other interruptions...)

So CPU measure includes this cpu that is part of io, and IO measure only includes the "step" 3.

Regards
Michel
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