Oracle-L: RE: CPU WAIT I/O statistic

From: DENNIS WILLIAMS <DWILLIAMS_at_LIFETOUCH.COM>
Date: Fri, 11 Oct 2002 07:13:46 -0800
Message-ID: <F001.004E6B65.20021011071346@fatcity.com>

Pablo
What does your I/O look like while %wio is high? Specifically, I would suggest:

Look at the disk subsystem I/O, is it experiencing a high rate of I/O?
Run STATSPACK and see which tablespaces are "hot".
Are a lot of table scans being executed? Dennis Williams DBA Lifetouch, Inc. dwilliams_at_lifetouch.com

-----Original Message-----
Sent: Thursday, October 10, 2002 11:13 PM To: Multiple recipients of list ORACLE-L

First a minor correction: sar -u has %wio and not sar -q.

Now, %wio reports the % of the time the CPU was idle while processes, that otherwise would have run, waited for the outstanding I/O requests to complete.

I believe the next few sentences in the book shed more light on %wio and attempt to simplify it further...

Kirti

-----Original Message-----
Sent: Thursday, October 10, 2002 4:19 PM To: Multiple recipients of list ORACLE-L

Dennis:

Thanks for answering, what do you mean by, or may be what do you think Gaja means by:

"He points out that the Solaris sar -q command has a "%wio" column, a measure of processes that are currently using the CPU, but are waiting for I/O requests to be serviced and hence are not making prudent use of the CPU"

How can the processes be using the CPIU if they are waiting for some I/O requests?

What I'm trying to say is that that can't consume CPU cicles if they are waiting (SLEEPING).

Why does sar shows that these CPU cicles are used in waiting for I/O? Who's using them?

TIA

Pablo - I posted the following paragraph yesterday:

3) I looked in Oracle Performance Tuning 101 to see what Gaja has to say.
He points out that the Solaris sar -q command has a "%wio" column, a measure
of processes that are currently using the CPU, but are waiting for I/O
requests to be serviced and hence are not making prudent use of the CPU. He
further says that %sys and %wio should be less than 10-15% and if it is
consistently higher you need to get to the bottom of it, and usually it is a
application causing the problem. No details on how to get to the bottom.

Dennis Williams
DBA
Lifetouch, Inc.
dwilliams_at_lifetouch.com

-----Original Message-----
Sent: Thursday, October 10, 2002 3:16 PM To: Multiple recipients of list ORACLE-L

Hi list

Can anyone explain me what exactly does the WAIT I/O column of the sar -u output mean?

Does it represent the % of CPU used by the kernel processes to perform I/O?

As far as I know the waiting processes do no wait actively when they ask for an I/O. right? The OS uses the SLEEP and WAKEUP primitives.

So, Which process is using this CPU? (The WAIT I/O%)

Or does this WAIT I/O have to be taken as if the CPU were idle?

Please shed some light on this.
Thanks

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Author: =?iso-8859-1?q?Pablo=20Rodriguez?= INET: p_rodri99_at_yahoo.es

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Author: DENNIS WILLIAMS
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