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RE: SPIN_COUNT - 8i obsolete? What now?

From: Denham Eva <EvaD_at_TFMC.co.za>
Date: Thu, 07 Nov 2002 21:43:24 -0800
Message-ID: <F001.004FE8C9.20021107214324@fatcity.com>


Thanks Cary and all for the imput. Now I atleast have something to work with.
Many Thanks
Denham

-----Original Message-----
Sent: Thursday, November 07, 2002 6:42 PM To: Multiple recipients of list ORACLE-L

Thanks Rachel.

For Denham: Changing SPIN_COUNT from its default value is usually a Very Bad Thing. Your sleeps are caused by lots of competition for one or more latches. The best way to stop those sleeps from impacting response time is to cut out the unnecessary competition. Because of how we're all taught to tune SQL (by eliminating physical I/Os and then quitting), almost every system in the world suffers from unnecessarily excessive numbers of latch acquisition attempts.

If the competition is for the "shared pool" or a "library cache" latch, then your application parses too much. For more details, see "Scaling applications to massive user counts" at www.hotsos.com/catalog. If the competition is for a "cache buffers chains" or "cache buffers lru chain" latch, then your application reads too many blocks from the buffer cache. For more details, see "Why you should focus on LIOs instead of PIOs" at www.hotsos.com/catalog.

Yes, increasing SPIN_COUNT can reduce the number of sleeps, but it does so by wasting more user-mode CPU doing an operation called a "busy wait." See the LIO/PIO paper mentioned earlier for details about what increasing SPIN_COUNT does to Oracle's latch acquisition algorithm.

Rachel's right: Instead of worrying whether there are too many sleeps on your system, decide whether the business benefit of improving the performance of one or more specific business functions will be worth the effort. Direct performance improvement decisions by what's important to the BUSINESS, not by what some V$ table says might be wrong with your system.

Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com

Upcoming events:

- Hotsos Clinic, Dec 9-11 Honolulu
- 2003 Hotsos Symposium on OracleR System Performance, Feb 9-12 Dallas
- Jonathan Lewis' Optimising Oracle, Nov 19-21 Dallas


-----Original Message-----
Carmichael
Sent: Thursday, November 07, 2002 9:09 AM To: Multiple recipients of list ORACLE-L

Denham,

Sorry, I didn't mean to make you wince! It's just a case of seeing way too many DBAs spending way too much time tuning things that really aren't a problem.

Cary Millsap talks about this often. Find the business problem and tune that. It's possible that something that shows up as "very slow" when you look at the overall system is actually something that runs rarely and tuning it will give you no noticeable improvement.

If the log switches are happening that frequently, you might look into resizing them. I'd first look at what is going on during office hours that's causing them to fill so fast.

Rachel


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Author: Denham Eva
  INET: EvaD_at_TFMC.co.za

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Received on Thu Nov 07 2002 - 23:43:24 CST

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