RE: Redo latch contention

From: Deshpande, Kirti <kirti.deshpande_at_verizon.com>
Date: Sat, 16 Jun 2001 09:52:13 -0700
Message-ID: <F001.0032C76F.20010616093030@fatcity.com>

        >1. Is it OK to set log_simultaneous_copies higher than 2*CPU. What are

        > the golden rules. I have seen some authers not mentioning this.

Yes, it is okay to set it higher than this. It won't degrade the performance, but don't expect any considerable gain in the performance either. On some combination of Oracle version & platform, the max value can be as high as 8 times the number of CPUs. But as Mogens says if there are no waits for 'latch free' event you should forget about messing with this. Unless there is a chance that you are suffering from a compulsive tuning disorder ;-)

         
        > 2. Why this parameter is missing from Oracle 8i?? Has Oracle
changed the
        > algorithm?? What is the new strategy to handle redo latch

contention??
It is now an unsupported parameter in 8i, and it defaults to 2 times the number of CPUs. (The max according to Oracle). Oracle thinks that it is the appropriate value in most cases and there is no reason (most of the times) to adjust this parameter. There have been some enhancements, from 8.0, in how LGWR dealt with redo log buffer (with its own 3 secs timer) but I don't think there is any new strategy for handling redo latch contention. For some reason, sometimes Oracle is rather late in updating their documentation.

Regards,

• Kirti Deshpande Verizon Information Services http://www.superpages.com

> -----Original Message-----
> From: Mogens Nørgaard [SMTP:mno_at_MiracleAS.dk]
> Sent: Saturday, June 16, 2001 10:17 AM
> To: Multiple recipients of list ORACLE-L
> Subject: Re: Redo latch contention
>
> First of all, if you don't see cumulated waits for the 'latch free' event
> in
> either v$system_event or v$session_event (for a specific session/job)
> there
> is absolutely no need to do anything about these ratios. It's about the
> only
> two latches mentioned in the reference books and it's about the only two
> latches that never really have a problem :).
>
> My guess is that this effort (increasing log_simultaneous_copies and
> log_small_entry_max_size) hasn't increased performance on your system
> whatsoever.
>
> If - only if - you have latch contention/waits in the system (high
> percentage of time_waited in the above-mentioned
> v$system_event/v$session_event, you should ignore all those calculations
> with gets and misses and insted just look in v$latch like this:
>
> select name, sleeps from v$latch order by 2;
>
> to find the latches with highest contention. But again: If your system or
> session is not really waiting for latches, why bother?
>
> Rajesh Dayal wrote:
>
> > Hi All,
> > I had some situation of Redo Allocation and copy latch
> > contention as stated in following output.....
> >
> > SQL> SELECT substr(NAME,1,18) NAME, GETS,MISSES, IMMEDIATE_GETS,
> > IMMEDIATE_MISSES
> > FROM V$LATCH WHERE NAME LIKE '%redo%'
> > /
> >
> > NAME GETS MISSES IMMEDIATE_GETS IMMEDIATE_MISSES
> >
> > ------------------ ---------- ---------- -------------- ----------------
> >
> > redo allocation 74878 16 0 0
> >
> > redo copy 114 100 53756 232
> >
> > redo writing 30219 1 0 0
> >
> > 3 rows selected.
> >
> > Realizing small contention on redo allocation latch, I increased
> > the value of "log_small_entry_max_size" from 80 to 90.
> > But this would definitely overload (already suffering) redo copy
> > latches, so I increased the value of log_simultaneous_copies from 2 to
> > 6.
> > This sorted out redo latch contention, but somewhere in FM it's
> > mentioned that value of log_simultaneous_copies shouldn't be more than
> > (2 * #_of_CPUs). Again I know that the CPU is "not" heavily used so far.
> > So...
> >
> > 1. Is it OK to set log_simultaneous_copies higher than 2*CPU. What are
> > the golden rules. I have seen some authers not mentioning this.
> >
> > 2. Why this parameter is missing from Oracle 8i?? Has Oracle changed the
> > algorithm?? What is the new strategy to handle redo latch contention??
> >
> > Interestingly, in Oracle 8i (Oracle 8.1.6) Tuning Manuals they
> > still talk of these parameters(which are made obsolete)...
> >
> > Appreciate your inputs ;-)
> >
> > Cheers,
> > Rajesh
> > --
> > Please see the official ORACLE-L FAQ: http://www.orafaq.com
> > --
> > Author: Rajesh Dayal
> > INET: Rajesh_at_ohitelecom.com
> >
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> --
> Venlig hilsen
>
> Mogens Nørgaard
>
> Technical Director
> Miracle A/S, Denmark
> Web: http://MiracleAS.dk
> Mobile: +45 2527 7100
>

--
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--
Author: Deshpande, Kirti
  INET: kirti.deshpande_at_verizon.com

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Received on Sat Jun 16 2001 - 11:52:13 CDT