RE: LGWR using lots of CPU time, low CPU usage

These are out of left field, but have bit me in the past. Take with a big ol' grain of salt.

Is the application issuing a 'COMMIT' after each 'SELECT'? This will cause a commit entry to be written to the log buffer and the buffer to be flushed. It could explain why LGWR is consuming time and not much usage. I also believe that this causes all of the process memory slots to be examined for pending commits.

Has someone changed the priority of the LGWR process?

Other issues (# of members, size, etc) have already been addressed accurately.

-----Original Message-----
Sent: Tuesday, November 26, 2002 11:00 AM To: Multiple recipients of list ORACLE-L

We are on 9.2.0.2, Solaris 8 on Sunfire 3800 with 16 GB memory and 128 MB on a hardware-controlled, mirrored RAID5 StorEdge T-3 Array.

Periodically throughout the day the LGWR background process clocks 20+ minutes of CPU time while actual CPU usage is quite low. I ran a statspack report and for a 45-minute period that included the slow LGWR process.

The top 5 timed events in my 45-minute report are:

CPU time 1,295 60.41
db file sequential read 392,516 341 15.91 db file scattered read 70,245 168 7.85
log file sync 26,916 133 6.22
library cache pin 22 59 2.76

(Now that the top 5 is "timed" events, 3 spots almost always include CPU
and the db file reads, so I only get two other events, usually log file sync, sometimes enqueue or latch free.)

Statspack also shows the log file parallel write had 28,589 timeouts in that 45 minute period--rather typical for us.

I have session_cached_cursors set to 150.

I am considering the following:

1. Removing my own redo log duplexing (mirroring) since redo logs are on the mirrored, hardware-controlled RAID5 disk array. (I know, I know) My sysadmin talked to the sun engineer yesterday and he said this is "old school" thinking that redo logs should not be on RAID5. He said because the RAID controller caches to memory all IO requests from the CPUs, all physical writes to disk are done behind the scenes
   (known as writebehind). He says the system is NOT waiting for IO.
   
2. Increasing redo log size (again). For the most part, log switches average 2.5 per day, although there were 20 times in the last month of 3-7 switches in a half hour. My logs are about 100 MB in 2 groups of 20 members each.
3. Upping the session_cached_cursors to ? (in response to the library cache pin event).

Or is there a better option I'm overlooking?

I would appreciate some advise on the best approach to resolve the slow LGWR process, especially your thoughts on option 1.

Thanks,
Debi
Deborah Lorraine, DBA
University of California, Davis
dlorraine_at_ucdavis.edu

-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Deborah Lorraine
  INET: debil_at_ucdavis.edu

Fat City Network Services    -- 858-538-5051 http://www.fatcity.com
San Diego, California        -- Mailing list and web hosting services
---------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L


(or the name of mailing list you want to be removed from).  You may

also send the HELP command for other information (like subscribing).
-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Fink, Dan
  INET: Dan.Fink_at_mdx.com

Fat City Network Services    -- 858-538-5051 http://www.fatcity.com
San Diego, California        -- Mailing list and web hosting services
---------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L


(or the name of mailing list you want to be removed from).  You may

also send the HELP command for other information (like subscribing).