RE: CBC Latch contention in 10g

  • From: "Cary Millsap" <cary.millsap@xxxxxxxxxx>
  • To: "'Oracle-L (E-mail)'" <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 19 Oct 2004 15:41:59 -0500

I believe Steve Adams once told me that the shareable latch feature is =
not
implemented on HP-UX.


Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
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-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx =
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Smiley John - IL
Sent: Tuesday, October 19, 2004 3:31 PM
To: 'Oracle-L (E-mail)'
Subject: CBC Latch contention in 10g

I'm seeing large numbers of CBC latch waits when many sessions (20-40) =
run
the same SELECT statement simultaneously (I can peek at v$session_wait =
and
catch this many sessions waiting on latch free: cache buffers chains for =
the
same sql_id).

CBC latch children have been sharable in read mode since 9i.  I didn't =
see
any DML taking place at the same time that would cause these latches to =
be
taken in exclusive mode, so I'm wondering why they're blocking on the =
CBC
latches.  I've examined the CBC latch children with the most sleeps and
examined the blocks on their hash chains and found exactly what I =
expected
to find: hot blocks on some of the tables referenced in the queries.  =
But if
the latches are truly sharable for CR buffer gets, this shouldn't cause =
them
to pile up on the latch.  Assuming delayed block cleanout isn't the =
problem,
what might explain this?  I've searched MetaLink for latch related bugs =
in
10.1.0.2.  No dice.=20

John Smiley
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