Re: Script to compute random page cost

Hi again-

I bounced these numbers off of Ray Ontko here at our shop, and he pointed
out that random page cost is measured in multiples of a sequential page
fetch. It seems almost impossible that a random fetch would be less
expensive than a sequential fetch, yet we all seem to be getting results <
1. I can't see anything obviously wrong with the script, but something very
odd is going.

-Nick

> -----Original Message-----
> From: pgsql-hackers-owner(at)postgresql(dot)org
> [mailto:pgsql-hackers-owner(at)postgresql(dot)org]On Behalf Of Nick Fankhauser
> Sent: Monday, September 09, 2002 11:25 AM
> To: Bruce Momjian; PostgreSQL-development
> Cc: Ray Ontko
> Subject: Re: [HACKERS] Script to compute random page cost
>
>
> Bruce-
>
> With the change in the script that I mentioned to you off-list (which I
> believe just pointed it at our "real world" data), I got the following
> results with 6 successive runs on each of our two development platforms:
>
> (We're running PGSQL 7.2.1 on Debian Linux 2.4)
>
> System 1:
> 1.2 GHz Athlon Processor, 512MB RAM, Database on IDE hard drive
> random_page_cost = 0.857143
> random_page_cost = 0.809524
> random_page_cost = 0.809524
> random_page_cost = 0.809524
> random_page_cost = 0.857143
> random_page_cost = 0.884615
>
> System 2:
> Dual 1.2Ghz Athlon MP Processors, SMP enabled, 1 GB RAM, Database on Ultra
> SCSI RAID 5 with Hardware controller.
> random_page_cost = 0.894737
> random_page_cost = 0.842105
> random_page_cost = 0.894737
> random_page_cost = 0.894737
> random_page_cost = 0.842105
> random_page_cost = 0.894737
>
>
> I was surprised that the SCSI RAID drive is generally slower than IDE, but
> the values are in line with the results that others have been getting.
>
> -Nick
>
> > -----Original Message-----
> > From: pgsql-hackers-owner(at)postgresql(dot)org
> > [mailto:pgsql-hackers-owner(at)postgresql(dot)org]On Behalf Of Bruce Momjian
> > Sent: Monday, September 09, 2002 1:14 AM
> > To: PostgreSQL-development
> > Subject: Re: [HACKERS] Script to compute random page cost
> >
> >
> >
> > OK, turns out that the loop for sequential scan ran fewer times and was
> > skewing the numbers. I have a new version at:
> >
> > ftp://candle.pha.pa.us/pub/postgresql/randcost
> >
> > I get _much_ lower numbers now for random_page_cost.
>
>
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