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Re: Some Performance Advice Needed

From: William Yu <wyu(at)talisys(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Some Performance Advice Needed
Date: 2004-12-23 20:49:09
Message-ID: cqfb26$1s9j$ (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Alex wrote:

> Hi,
> i recently run pgbench against different servers and got some results I 
> dont quite understand.
> A) EV1: Dual Xenon, 2GHz, 1GB Memory, SCSI 10Krpm, RHE3
> B) Dual Pentium3 1.4ghz (Blade), SCSI Disk 10Krmp, 1GB Memory, Redhat 8
> C) P4 3.2GHz, IDE 7.2Krpm, 1GBMem, Fedora Core2
> Runnig PGbench reported
> A) 220 tps
> B) 240 tps
> C) 510 tps
> Running hdparm reported
> A) 920mb/s   (SCSI 10k)
> B) 270mb/s   (SCSI 10k)
> C) 1750mb/s  (IDE  7.2k)
> What I dont quite understand is why a P3.2 is twice as fast as a Dual 
> Xenon with SCSI disks, A dual Xenon 2GHz is not faster than a dual P3 
> 1.4Ghz, and the hdparm results also dont make much sense.

A few things to clear up about the P3/P4/Xeons.

Xeons are P4s. Hence, a P4 2ghz will run the same speed as a Xeon 2ghz 
assuming all other variables are the same. Of course they aren't because 
your P4 is probably running unregistered memory, uses either a 533mhz or 
800mhz FSB compared to the Xeon's shared 400mhz amongs 2 CPUs, running a 
faster non-smp kernel. Add all those variables up and it's definitely 
possible for a P4 3.2ghz to run twice as fast as a Dual Xeon 2ghz on a 
single-thread benchmark. (The corollary here is that in a multi-thread 
benchmark, the 2X Xeon can only hope to equal your P4 3.2.)

P3s are faster than P4s at the same clock rate. By a lot. It's not 
really that surprising that a P3 1.4 is faster than a P4/Xeon 2.0. I've 
seen results like this many times over a wide range of applications.

The only variable that is throwing off your comparisons are the hard 
drives. IDE drives have write caching on by default -- SCSI drives have 
it off. Use: hdparm -W0 /dev/hda to turn it off on the P4 system and 
rerun the tests then.

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