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Home -> Community -> Mailing Lists -> Oracle-L -> Re: cache buffer chains latch
My current understanding is:
a) For normal buffer gets,
the latch is acquired,
the chain is scanned
the buffer is pinned
the latch is released
the buffer is used
--
the latch is acquired
the pin is dropped
(although the pin may be held
for the duration of the SQL or pl/sql
call if Oracle expects to revisit the buffer)
b) Consistent gets - examination
the latch is acquired
the chain is scanned
if the buffer can be found it read
the latch is dropped
Iin case (b), Oracle can make use of a
shared read latch - where the underlying
CPU supports it.
But I may be wrong - especially about (b), and I haven't identified all the cases where an examination is legal.
Question 2 - the best answers come from Steve Adams. But in summary, think library cache latch children - you may need to validate several objects in the library cache at once, and they could be covered by different child latches.
Latches have a level#, from 0 to 9, and there is a precedence of latch acquisition across levels that is designed to stop latch acquisition deadlocks (again see S.A.).
The holding of multiple latches also explains the presence of the column named (something like) waits_holding in v$latch. You have waited for this latch whilst holding another latch.
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk
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who can answer the questions, but the
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Hi All,
My system is suffering in cache buffer chain,I found the reason.
My general questions is
1)
When the process holding the latch how long the latch will be held ,until
the hase chain is read or
the process goes to the particular block and return the rows(output) ?
2)
When the situation will occur for the process to hold two latches(different
or same) simultaneously ?
Please reply.
Syed.
--
Please see the official ORACLE-L FAQ: http://www.orafaq.net
--
Author: Jonathan Lewis
INET: jonathan_at_jlcomp.demon.co.uk
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