Re: FSM search modes

Kevin Grittner wrote:
> Tom Lane <tgl@sss.pgh.pa.us> wrote: 
>  
> > (Hm, so we might want to make the probability depend on
> > max_connections?)
>  
> Without doing rigorous math on it, I'd guess that to prevent
> contention among n connections you'd want the probably of resetting
> the sweep to be about 1 / (n * 2).  That would mean you'd advance to
> the nth page about 60.6% of the time without resetting the sweep.  For
> less contention, 1 / (n * 4) would let you get to the nth page about
> 77.9% of the time.
>  
> > Maybe what we want is some bias against inserting in the last half
> > or quarter of the table, or some such rule, rather than necessarily
> > heading for the start of the relation.
>  
> I think it would make sense to just start using this once you get into
> the last half or quarter of the free pages.  If you go with the last
> quarter, then you might want to use a higher probability than I
> suggested above, although that would tend to leave you with contention
> when all the free space was in the last quarter.  I'd be inclined to
> use something like the above probability and start using it at 50%.

Two things that might not have been mentioned:  First, only reset if you
are given a page in the last 1/4 of the table;  that way, if there is no
free space in the last 1/4 of the table, you will not be resetting.  A
second idea is to heavily bias against using the last table page with
data in it; that should help bias toward empty pages on the end of the
table.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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