Some questions about A/UX

[Alexis Rosen]

In article <19528 at apple.Apple.COM> phil at Apple.COM (Phil Ronzone) writes:
>In article <941 at ccnysci.UUCP> alexis at ccnysci.UUCP (Alexis Rosen) writes:
>>9) Lastly, I have asked in the past if anyone knew of a SCSI DMA board for
>>   the Mac II/IIx. Phil Ronzone was kind enough to spend a few moments with
>>   me explaining that DMA wouldn't be a big win. Nevertheless, all of my
>>   calculations indicate otherwise. What am I missing, if anything?
>
>Killing this misinformation on Mac II and SCSI hopefully for the last time ...
>
>Assume you are running a typical one user I/O load of 40 to 80 1K blocks
>a second. When the Apple HD80 presents the data requested, then A/UX
>"yanks" the 1K, 4 bytes at a time, in a very tight loop. There is hardware
>assist to make for very quick "yanking". How quick? 3.657 bytes per
>microsecond. Or, 280 microseconds to "yank" the 1K block.
>
>NOTE THAT: Both a DMA chip and the 68020 would both begin the "yanking"
>of bytes at the same time. They would both finish around the same time.
>
>A DMA CHIP CAN DO ABSOLUTELY NOTHING (REPEAT NOTHING NOTHING NOTHING ...)
>TO MAKE THIS HAPPEN FASTER THAN HAVING THE 68020 TRANSFER THE BYTES. IS
>THE UNIVERSE NOW AWARE OF THIS!!! :-) :-) <- tongue-in-cheek screaming.
>[etc.]
>SUMMARY - A DMA chip on the Mac II can NOT increase I/O throughput. It can
>free up more I/O cycles, although only 4% (predicted) / 8% (measured) for
>typical heavy UNIX I/O loads. DMA buys the most in reducing interrupt
>timing sensitivity, and in support the "very large data transfers" peripherals
>such as LaserWriter II SC.

First of all, thanks to Phil for speaking out on this. My previous comment
about him was sincere; I do appreciate the time he's willing to spend answering
these questions.

That said, I still have some questions. The Mac can transfer a 1KByte block in
280 usecs. That's fine, but it's not the whole story. If it were, it could do
about 3.5 MBytes/sec. In fact, it can do less than one tenth of that speed. So
what causes that discrepancy? My guess (uneducated, so please correct me if I'm
wrong) is that it's the overhead for transferring that block. Can the Mac
transfer 10 or 100 blocks in 2800 or 28000 microseconds? I don't think so.

I don't know what the overhead for DMA is, but it seems to be a lot less. The
Golden Triangle folks say they are getting about 1 MByte/sec from their board,
using CDC Wrens. Since the Wrens are capable of just over 1 MByte/sec, G.T.
might be able to do even better with a faster controller (then again, maybe
not- I didn't ask).

As I think my numbers show, (read my original posting for them) there is a BIG
difference between 300KB/sec and 1MB/sec, even for single-user stuff. Maybe the
question is whether there is something about the Mac that makes these faster
transfers difficult (or impossible). I can't think of anything though. The very
thought seems silly.

So again, what am I missing?

p.s.- I am leaving for a week Monday, that's why I won't be answering anything
that needs a response until around Election Day. Sorry.

----
Alexis Rosen                       alexis at dasys1.UUCP  or  alexis at ccnysci.UUCP
Writing from                       {allegra,philabs,cmcl2}!phri\
The Big Electric Cat                                       uunet!dasys1!alexis
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