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

Debi,

A log file sync event usually indicates that the application is probably committing too often, and LGWR cannot keep pace with it. You might be experiencing contention or I/O issues, on the disks where the redo log files are placed. Moving your redo log files away from RAID 5 is a step in the right direction. Else, try reducing the number of commits in the application. If thats not the problem, then you probably have a large value for log buffer.

My experience is that the top 5 events can be misleading. Sometimes, you resolve an event at a lower level, and the top ones resolve on their own.

Regards
Raj

                                                                                                                          
                    "Deshpande, Kirti"                                                                                    
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                    November 26, 2002                                                                                     
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So what is discussed in this paper is outdated already....uh! www.sun.com/blueprints/0101/SunOracle.pdf Gosh, time flies ;) Pages 13-14 talk about Oracle Redo Logs.

As a first attempt, I would consider reducing the number of log members
(from 20 to 4, or even 3) than removing them altogether. This will be of
some help right away. But monitor further and decide if more Groups are needed to help archiver process.

Do not change multiple things at the same time.

Good Luck,

• Kirti

-----Original Message-----
Sent: Tuesday, November 26, 2002 12:00 PM 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

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