Jonathan,
OK, what am I missing? All of the PX sessions have to
access the segment header in order to get blocks off
the free lists so can't that result in buffer busy
waits on the segment header unless it was created with
more than one free list group?
- Anita
- Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
wrote:
>
> Not only that, PX slaves do direct reads
> anyway, bypassing the buffer cache.
>
> It is possible, though, that the flush
> that has to take place before the PX
> scan can read back is sufficient to
> cause other processes to wait for
> write complete waits, and I guess that
> you could also get extra rbs header
> waits as the flushed and scanned blocks
> are rolled back by the PX slaves for
> read consistency.
>
>
> Excessive serial tablescans, on the other
> hand can easily cause significant buffer
> busy waits.
>
> Jonathan Lewis
>
> Host to The Co-Operative Oracle Users' FAQ
> http://www.jlcomp.demon.co.uk/faq/ind_faq.html
>
> Author of:
> Practical Oracle 8i: Building Efficient Databases
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>
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>
>
> -----Original Message-----
> To: Multiple recipients of list ORACLE-L
> <ORACLE-L_at_fatcity.com>
> Date: 21 August 2001 18:32
>
>
> |On Tuesday 21 August 2001 08:21, Johnson
> Poovathummoottil wrote:
> |> Bufer busy waits can also be caused by parallel
> query
> |> servers trying to read the same buffer block at
> the
> |> same time.
> |
> |Just off the top of my head, and without giving it
> a lot
> |of thought, this doesn't sound right.
> |
> |PQS divvy up the blocks to read, and should not be
> trying
> |to access the same block.
> |
> |Got any documentation or evidence to back that up?
> |
> |Jared
>
>
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> Author: Jonathan Lewis
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Received on Thu Aug 23 2001 - 07:22:11 CDT
