Re: LRU latches

Acutally the foreground will do the immediate get on the copy latch. If we can't get a copy that way, we will wait on one.

Anjo.

kyle hailey wrote:

> For the log buffer latch questions see:
> http://www.geocities.com/oraperf/redo/redo_alloc_latch.html
> I've pasted the page below. The LRU stuff is for the buffer cache,
> maybe someone else wants to address that - or maybe I could write it
> up later.
>
> Log Buffer Latching:
>
> Normally only one process writes to the log buffer in the SGA at a
> time. The redo allocation latch assures that only one user can write
> to the log buffer at a time. A user aquires the redo allocation latch
> writes to the redo log buffer then releases the redo allocation latch.
> In order reduce the contention on the redo allocation latch we need to
> reduce the amount of time that users hold the redo allocation latch.
> This is accomplished by using the redo allocation latch not to write
> to the log buffer but just to allocatate the necessary space in the
> log buffer for the process. After the process has allocated the
> necessary space it can use a redo copy latch to write into the log
> buffer. There can be multiple redo copy latches.
>
> The redo copy latches allows process to concurrently write into the
> log buffer. The number of redo copy latches is determined by the
> parameter LOG_SIMULTANEOUS_COPIES.
>
> Even with LOG_SIMULTANEOUS_COPIES set to greater than 0, the
> processeses will continue to write to the log buffer using the redo
> allocation latch if their log entry size is smaller than the parameter
> LOG_SMALL_ENTRY_MAX_SIZE.
>
> So both LOG_SIMULTANEOUS_COPIES and LOG_SMALL_ENTRY_MAX_SIZE need to
> be set to alleviate contention on the redo allocation latch.
>
> The algorithm is the following (early V7 but idea is the same 7+ and
> v8)
>
> if ( log entry size > LOG_SMALL_ENTRY_MAX_SIZE ) and
> ( LOG_SIMULTANEOUS_COPIES > 0 ) then
> get the redo allocation latch
> allocate the memory
> get a redo copy latch
> release the redo allocation latch
> write the redo entry
> release the redo copy latch
> else
> gets the redo allocation latch
> writes into the log buffer
> release redo allocation latch
> fi
>
> From Graham Wood:
> Q: Why would there be both immediate requests and willing-to-wait
> requests for a
> copy latch - I see both of them in my v$sysstat output.
> A: Foreground processes do immediate requests, log writer willing to
> wait (must
> get all the latches before it can write).
>
> From Richard Exley
> You may be interested to know that at Oracle 8 the redo copy latches
> (parent
> and children) become shareable. This means that, on platforms that
> support
> it, more than one process can hold the same latch in share mode at the
> same
> time.
> In the new scheme, the foregrounds get the copy latch in share mode,
> and LGWR
> gets all the latches in exclusive mode before it writes. This, I
> imagine,
> will remove the need for multiple copy latches on these platforms, in
> fact
> setting log_simultaneous_copies=1 should improve performance.
> Shareable latch functionality is only available on some platforms.
> For
> example, from the code it appears that shareable latches are available
> on
> Solaris but only for ultra sparc processors.
> Anyone had any experience working with this? Comments are welcome.
>
> note1:
> Redo latch order of obtaining:
> Before 7.3 the redo allocation latch was gotten first
> and then
> the redo copy latch (if it was configured). In 7.3
> that was
> changed to first get the copy latch and then the
> allocation
> latch.
> This reduces contention on the allocation latch should
> the process be unable to get the copy latch.
>
> note2: average log entry size can be caculated by
> log entry size = ( redo size / redo entries )
> note3: Alpha (both VMS and Unix) ports ignore
> log_simultaneous_copies.
> Copy latches are not used.
>
> References:
> LOG_SIMULTANEOUS_COPIES
> LOG_SMALL_ENTRY_MAX_SIZE
> Questions or comments ? Send a note to oraperf_at_yahoo.com
>
> Best
> Kyle Hailey
>
> On Sat, 24 Feb 2001 17:03:21 GMT, "Thomas Tomello" <tom_at_tatsoft.com>
> wrote:
>
> >Could someone explain LRU latches and the relationship of these to the
> >LOG_SIMULTANEOUS_COPIES and LOG_SMALL_ENTRY_MAX_SIZE parms? Thanks...
> >
  Received on Wed Feb 28 2001 - 07:17:57 CST