Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!unisoft!hoptoad!gnu
From: gnu@hoptoad.uucp (John Gilmore)
Newsgroups: comp.sys.mac
Subject: Re: Mac DMA
Message-ID: <4130@hoptoad.uucp>
Date: 26 Feb 88 22:27:55 GMT
References: <2365@mandrill.CWRU.Edu>
Organization: Nebula Consultants in San Francisco
Lines: 24

howarth@mandrill.CWRU.Edu (David J. Howarth) wrote:
> DMA on the Mac, if done correctly, could actually be very advantageous.
>   One thing to keep in mind, though, is that even a factor of
>   10 increase in performance at this low level will NOT result
>   in a user perceivable speedup of 10.  It will be faster from
>   the user's standpoint, but by how much is not easily quantified.

A friend has been doing some performance work on A/UX; it turns out
that the main problem is rotational latency and seek time -- as usual
with disks.  The contribution of software "DMA" seems to not be
significant.  Put a scope on the SCSI bus and watch where it spends its
time -- on a fast disk, maybe 18 ms waiting for the disk to turn and 2
ms moving data over the SCSI bus.  If you sped up the 2ms to .2ms,
you'd wait 18 ms instead of 20 -- a bit 10% (not 10x) speedup.

Since A/UX runs the ancient System V file system, it has to seek all
over to read a file, and as James Woods pointed out, this is the major
cause for the slow A/UX disk timings.

All I know is that even if they speed up the file system, 14-char
filenames are a major pain in the butt after spending 5 years on Suns.
Let's hope the ATT/Sun merge fixes this once and for all.
-- 
{pyramid,ptsfa,amdahl,sun,ihnp4}!hoptoad!gnu			  gnu@toad.com
		"Watch me change my world..." -- Liquid Theatre