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: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED]]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: [EMAIL PROTECTED] > > [mailto:[EMAIL PROTECTED]]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. > > > ---------------------------(end of broadcast)--------------------------- > TIP 3: if posting/reading through Usenet, please send an appropriate > subscribe-nomail command to [EMAIL PROTECTED] so that your > message can get through to the mailing list cleanly > ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
