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Date:      Sun, 8 Oct 1995 10:26:19 +1000
From:      Bruce Evans <bde@zeta.org.au>
To:        bde@zeta.org.au, se@zpr.uni-koeln.de
Cc:        hackers@freebsd.org
Subject:   Re: VLB Disk Controllers
Message-ID:  <199510080026.KAA08418@godzilla.zeta.org.au>

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>Well, the machine is more severely affected by a PIO
>device, than the CPU load indicates. The CPU must be 
>able to respond to data becoming available within a
>very short time, or data may be lost. (Hmm, maybe the
>larger buffers of current IDE drives make this a non-
>issue ???)

PIO is more bursty.  Consider transferring 64K at 2MB/sec.
That's 32 ms (more than 3 clock ticks) during which
softclock() won't run, so timeout functions won't get
called.  Most interrupts had better have higher priority
than PIO devices, and 2 hoggish PIO devices are likely to
block each other for too long.  This could be handled by
time slicing the interrupt handlers.  Data loss is only
a serious problem for volatile input.  Input from drives
is nonvolatile and anyway a mere 64K cache would probably
be large enough for input at a few MB/sec (it's serial
chips with 1 byte fifos at that lose data :-).

>Anyway, just a data point:

>  http://www-itg.lbl.gov/ISS/performance.ps

>contains SCSI controller performance data, and they 
>get 24.5MB/s simultaneously reading from 6 drives 
>connected to 3 NCR 53c810 controllers in pairs.

>The system is reported to be 70% idle under that load 
>(it's a Pentium 100 with Triton chip set).

EIDE certainly can't compete with that.  At best it will
achieve 15MB/sec throughput on a Pentium 1e6 :-).

Bruce



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