The actual act of putting something on the freelist
and/or taking it off I don't think is the cause of the
overhead. However, you do need to update the segment
header block which contains the freelist header
information, which means the segment header becomes a
'hot block'..
The only empiric evidence I have to support this is
that in places where I had lots of segment header
waits, tweaking the pctused gave a good reduction in
buffer busy waits in system event.
Other than that, I can't give any precise measurement
of "how much" impact this is.
hth
connor
- Greg Moore <sqlgreg_at_pacbell.net> wrote: > The
general theory about increasing PCTUSED is that
> blocks will be fuller,
> but they may go on and off the freelist more often
> and that will hurt DML
> performance.
>
> But in reality what is the overhead of this? Isn't
> the need to bring more
> blocks in and out of the data buffer greater than
> the overhead of taking
> such a block on and off the freelist?
>
> How the heck does a person measure what the overhead
> is of taking a block on
> and off the freelist? Furthermore, even if you have
> a measure, how do you
> compare the disadvantage from fuller blocks going on
> and off a freelist
> against the advantage of fuller blocks making full
> table scans faster?
>
> I see all this stuff in books, but it's just a lot
> of general statements
> that amount to so much garbage. It can sound fancy
> to talk about PCTUSED,
> and if you can do it in a book you can pose as an
> expert and fool most
> hiring managers into thinking you're an expert, but
> unless you can actually
> measure the gains and the losses from changing this
> parameter, what good is
> any of this talk?
>
> --
> Please see the official ORACLE-L FAQ:
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> --
> Author: Greg Moore
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Received on Thu Jun 07 2001 - 03:46:32 CDT
