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Re: Performance difference between 2 machines

From: Noons <wizofoz2k_at_yahoo.com.au>
Date: 21 Jun 2005 07:06:47 -0700
Message-ID: <1119362807.305474.43450@g44g2000cwa.googlegroups.com>


Etunimi Sukunimi apparently said,on my timestamp of 21/06/2005 7:20 PM:
>
> But wouldn't the trace file output suggest that the processors are
> waiting for something?

Yes, absolutely. Something.

>
> Remember that the sql-command given was "select count(1) from TABLE_A",
> that doesn't really need that much of processing power.

Well yes, but what is the NATURE of that wait? What you have from the Oracle trace is approx 200 secs of *Oracle* waiting for scattered read, but that is no confirmation whatsoever that there are no waits outside of Oracle or CPU is free.

Remember: the Oracle trace measures overall time elapsed from start of Oracle event to end of Oracle event. If that event does IO in the middle, then it is quite possible you may have time blow outs that have nothing to do with time waited in Oracle itself.

Unless you are using raw datafiles, in HP you will not be using aio nor direct io. That means everytime there is an IO request from Oracle, HPUX takes over and passes it to the file system layer, which then passes it to the device driver and the hardware, THEN sits in there twiddling its thumbs on a tight wait loop until the disk hardware returns. When that happens, the CPU must now go through the return path of the file system, then the OS, then back to Oracle. All the while copying 16K of buffered IO until it eventually makes its way back into the SGA.

You see this in sar for example as IOwait%, but that doesn't mean your CPUs aren't flat out: they are, copying 16k buffers and executing all that layer code. It just gets counted as IO wait, that is merely a convention of *n*x.

It doesn't come out of thin air, it comes out of processing cycles. Which are slower in the 9k/800 than in the P4.

Hence the difference?

But I still suspect there is something very wrong with your IO hardware. Can you run some tests with dd on both systems to gauge the non-db baseline IO speed? There is a very simple script I left in www.dizwell.com forum, maybe you could use that? It's here:

http://www.phpbbserver.com/phpbb/viewtopic.php?t=120&postdays=0&postorder=asc&start=15&mforum=dizwellforum

and you don't have to sign up to look at it. Just cut and paste, run that on both systems in a suitable mount point, then compare times. I'll bet you'll get some marked differences.

--
Cheers
Nuno Souto
in sunny Sydney, Australia
wizofoz2k_at_yahoo.com.au.nospam
Received on Tue Jun 21 2005 - 09:06:47 CDT

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