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From: djg@nsc-pdc.UUCP (Derek Godfrey)
Newsgroups: net.unix-wizards
Subject: Re: Drive/Device driver performens
Message-ID: <482@nsc-pdc.UUCP>
Date: Wed, 18-Sep-85 11:26:03 EDT
Article-I.D.: nsc-pdc.482
Posted: Wed Sep 18 11:26:03 1985
Date-Received: Fri, 20-Sep-85 05:06:04 EDT
References: <160@daab.UUCP>
Organization: NSC Portland Development Center, Portland Oregon
Lines: 24

> Here you all have a real brain training problem.
> 
> Let's say that you have two different disk drives, one is a STDC whith
> DMA transfer to main memory and the other one is a SMD drive but with a very
> pick each byte from the controller one by one from a data-port instead of
> transfer it with DMA as in the STDC controller.
> 
> Now the problem: 
> ----------------
> How can you measure witch one is faster than the other one in real life?,
 
 First what do you mean by faster? smallest latency or greatest
 transfer rate - both of which can be found by specs. Or as I suspect
 greatest aggregate thougth-put on a running system.
	To get a fisrt order measuerment of I/O overhead:
	A) get an empty machine.
	B) write a cpu intensive program (A counting loop that
	   terminates).
	C) find out how long that program runs with no activity.
	D) run the loop and each of the disc tests together.
	E) the increases in time of execution now represents cpu cycles
	   (including DMA) not available to it due to the disc accesses.
 Note you are measuring an approximation for what happens in real life
 and does not model multiple requests at random or synchronised times.


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