Path: utzoo!attcan!uunet!aplcen!samsung!munnari.oz.au!uniwa!DIALix!bernie
From: bernie@DIALix.UUCP (Bernd Felsche)
Newsgroups: comp.sys.amiga.hardware
Subject: Re:  GVP Trade-in
Summary: SCSI-2 is fast
Keywords: SCSI, GVP
Message-ID: <552@DIALix.UUCP>
Date: 28 Aug 90 12:54:13 GMT
References: <589@oregon.oacis.org> <38CP09P@dri.com> <02048.002057@thiger.UUCP>
Reply-To: bernie@DIALix.oz.au (Bernd Felsche)
Organization: DIALix Services, Perth Western Australia
Lines: 58

In article <02048.002057@thiger.UUCP> skraw@thiger.UUCP (Stephan von Krawczynski) writes:
[previous ref deleted]
>1. GVP does not DMA to amiga-memory.
>2. why is DMA so important for you? it is generally slower than the
>processor-method (lets call it this way). you win nothing because you
>have a whole lot of DMA going on already inside the system and processor's
>running into heavy troubles sometimes, e.g. harddisk-performance is
>very low while using overscan-graphics (just to mention an example).

DMA is an advantage because the CPU can be doing other (real
processing) things.  DMA hardware is designed to do one thing...
move data from one bit of RAM to another.  Just like other custom
hardware, it frees up the CPU for general prupose number crunching,
allowing greater throughput and better response in a multi-tasking 
environment.

>
[other ref deleted]
>
>well, how about "transfer rates up to 4MB/SEC synchronous" (gvp). in fact
>i have never understood this one. what do they mean? 4MBytes/sec?
>i have never seen a controller/hd-combination reaching this.

4MB/SEC is the peak transfer rate on most SCSI-2 controllers which
are not bound by arbitrary design constraints.  Whether or not GVP
can get the data into the Amiga's RAM that quickly is another
matter. At the Zorro II bus speed of about 3.5 MHz, this would
consume more than 50% of the available bus time... although the time
to transfer will be a lot shorter than via polled (CPU) transfer.

>4MBits/sec = 512kBytes/sec. seems to be more like the truth, but is an
>absolutely ridiculous value (ALF3 makes over 200kBytes/sec more in a
>standard amiga - NO processor-card installed)

I know of controller/hd combinations which have _sustained_
throughput rates of over 2MB/sec (not Amigas, workstations), with
the hard disk able to provide up to 2.8MB/sec in bursts.

By designing for 4MB/sec, GVP is doing the right thing, because the
bottleneck is the hard disk (at the moment). You can also connect
more than one drive without "choking" on the bus, especially during
overlapping seeks and transfers.

Another consideration is the speed of the filesystem.  AmigaDOS
isn't exactly renowned for filesystem performance, even with FFS.
It involves significant CPU cycles to convert the raw SCSI data into
something which DOS presents to the user (FFS is a _lot_ better than
the original FS).  When you measure the average transfer speed, the
filesystem (and trackdisk.device) get in the way and give you a
distorted view of what the hardware _can_ do under ideal
conditions.

I didn't intend this article to become a comp.arch dissertation, so
I won't go any deeper into the subject at this stage.

bernie
(Real computers have flashing lights, front panel switches, 8K RAM  and
hard-sectored floppies - anybody recognize this?)