Re: DMA or not DMA...

[pubpc1@library.ucla.edu]

dempson@actrix.gen.nz (David Empson) wrote,
in response to my reply:
>[Discussion with Nathan on merits of DMA]
>> 
>> Since you don't believe me when I said that I noticed no difference,
>> I opened up the GS and ran some tests, with DMA off and flipping the
>> switch of the Apple HS SCSI card to turn DMA on (powering down each
>> time first).  I did this with 13.75MHz/16K cache acceleration and
>> without.  Here are the results of the shift-boot times measured from
>> the moment the "Welcome to the IIgs" screen popped up to the time
>> the Finder appeared.  My copy of System 6.0.1 was installed after the
>> volume had been used for a while, so there was fragmentation.
>> 
>> 
>> Time to boot (sec)      with DMA on     with DMA off
>> -------------------------------------------------------------------
>> @13.75MHz/16K cache             11              12
>> @non-Zip speed                  19              21
>
>I just ran the same test on my system (System 6.0.1 with _lots_ of stuff
>installed, but I was shift-booting).  You weren't precise about what you
>mean by "the time the Finder appeared" - I have timed from the "Welcome
>to IIgs" logo to the appearance of the menu bar, as subsequent times
>will vary substantially depending on your icon files, partitions, etc.
>
>I have an 8 MHz/16K ZIP in a ROM 3 IIgs with 5MB of RAM, and an Apple HS
>SCSI card.  The hard drive is an 850 MB Quantum Trailblazer (about two
>years old; 3200 RPM if Chinook SCSI Utilities is making an accurate
>measurement, and it has a reasonably large cache).
>
>The numbers are as follows:
>
>Time to boot (sec)      with DMA on     with DMA off         Ratio
>-------------------------------------------------------------------
>@8MHz/16K cache                 17              25           1:1.47
>@non-Zip speed                  26              36           1:1.38
>
>The right side of the "Ratio" column is the "DMA off" time divided by
>the "DMA on" time, thus booting takes 47% longer with DMA disabled and
>ZIP enabled, 38% longer with both disabled.
>
>As you can see, there is a substantial difference in speed with DMA
>disabled.  I would definitely not want to have to turn off DMA for
>normal operations.
>
>I don't understand how your system has such small values for all the
>counts, unless you have close to a minimum install of the system
>software.  The small difference between the values is probably due to
>the hard drive being relatively slow.

My timing trials were also from the "Welcome" logo's appearance
to the menu bar's appearance.

Let me venture a guess as to what is happening.  Acceleration
helps _a lot_.  Nate guaged 25% drop with DMA off in his 9/32
Zip.  Being a little slower at 8/16, you guaged a 47% drop.
Mitchell didn't guage much of a difference at all at 15/64.
And I got a bit more than a 5% difference at 13.75/16.

My hypothesis is that as acceleration increases, DMA makes
less difference, all other things being equal.

However, you note that in your results DMA makes _more_ evident
differences when the Zip is _on_.  This I do not understand.
My tests are also on a ROM 3, by the way.  Since you also have
a huge difference with the Zip off, one explanation may be
fragmentation.  As you stated, DMA makes the most difference
when data are in large chunks.  Could it be that the difference
comes from the fact that your hard drive is less fragmented than
mine?  Also, apparently the Quantum ELS (based on these tests)
is faster than the Quantum Trailblazer, since you seem to be
slower in every test.

My numbers were verified by repeating the test in each config.
three times.  I also powered down during each change in
configuration.  My Zip is set with all delays off and CPS
follow off and the Apple HS SCSI's slot is set to fast.

-Scott G.