Larson, Tim (Corporate) wrote:
> I was wondering if anyone has disk benchmarks for the Q840AV vs other
> 68k Macs under NetBSD.  Benchmarks for "real life" uses that include
> disk access would also be great.
> 
> Yes, I'm curious about the DMA SCSI support.  My Quadra has a fast
> (24x) CD-ROM and I'm wondering if I'm going to see much benefit.

As far as I know, DMA support for the Q840AV isn't in NetBSD yet. I 
submitted a patch to enable basic DMA SCSI support a while ago but even 
with this patch my initial testing showed that it wasn't a tremendous 
speed increase. It was just an incremental step in getting DMA working.

However, the lack of speed in my patch wasn't due to the DMA engine on 
the AV being slow. It had to do with two factors:
1. I couldn't get the general NCR SCSI code to get the devices in 
synchronous transfer mode. Async mode is much slower, so I think if we 
could get this working it might be a big performance win.
2. My code doesn't fully utilize all the speedups offered by the DMA 
engine. Theoretically, we should be able to have the n+0 segment of the 
transfer going and the n+1 segment queued and ready to go. Then, when 
the n+0 segment is done transferring, we take an interrupt and while the 
interrupt is loading the n+2 segment the DMA engine is operating on the 
n+1 segment. This would make the transfer look like one non-stop read or 
write to the NCR SCSI chip and require the least interrupt overhead 
possible available on the platform. Unfortunately, I ran out of time to 
get that going, so my patch basically just loads one physically 
contiguous block at a time and uses the SCSI chip interrupt to do each 
trasnfer. This is not optimal. Though, I suspect adding this will be a 
smaller gain compared to getting sync transfers working. Though, 
reloading the DMA should be much lower interrupt overhead compared to 
running through the SCSI state machine on each NCR interrupt.

So perhaps in the future, you'll notice a big speed increase but I think 
non-AV quadras have the fastest SCSI available on netbsd-mac68k at the 
moment.

-- 
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  Michael Zucca - mrz5149@acm.org
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  "I'm too old to use Emacs." -- Rod MacDonald
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