CBC Latch contention in 10g
- From: Smiley John - IL <SMILEYJ@xxxxxxxx>
- To: "'Oracle-L (E-mail)'" <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 19 Oct 2004 15:31:24 -0500
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.
John Smiley
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