On Fri, Jul 14, 2000 at 12:51:14PM +1000, Stephen McKay wrote:
> On Thursday, 13th July 2000, "Rodney W. Grimes" wrote:
> >>On Thu, 13 Jul 2000, Stephen McKay wrote:
> >> 
> >>>Does anyone here actually measure these latencies?  I know for a fact
> >>>that nothing I've ever done would or could be affected by extra latencies
> >>>that are as small as the ones we are discussing.  Does anybody at all
> >>>depend on the start-transmitting-before-DMA-completed feature we are
> >>>discussing?
> >> 
> >> I don't like the idea of removing that feature.  Perhaps it should be a
> >> sysctl or ifconfig option, but it should definitely remain available.
> >> Those minute latencies are critical to those of us who use MPI for
> >> complex parallel calculations.
> >
> >I have to agree here.  The store and forward adds an approximate
> >11uS (by theory under ideal conditions 1500bytes@132MB/s = 11uS,
> >practice actually makes this worse as typical PCI does something
> >less than 100MB/s or 15uS) to a 120uS packet time on the wire (again,
> >ideal, but here given that switches, and infact often cut-through
> >switches, are used for these types of things, ideal and practice
> >are very close.)
> >
> >I don't think these folks, nor myself, are wanting^H^H^H^H^H^H^Hilling
> >to give up 12.5%.
> OK.  It seems that repairing the feature, rather than disabling it is
> the most popular option.  Still, I am quite interested in finding anyone
> who actually measures these things, and is affected by them.  These very
> same people might be able to trace why we get the underruns in the first
> place.  I suspect an interaction between the ATA driver and VIA chipsets,
> because other than the network, that's all that is operating when I see
> the underruns.  And my Celeron with a ZX chipset is immune.

That theory is not correct, I have seen multiple Alpha machines reporting 
buffer underruns as well. No ATA disk in sight there..

Wilko Bulte             http://www.freebsd.org  "Do, or do not. There is no try"
[EMAIL PROTECTED]       http://www.nlfug.nl     Yoda - The Empire Strikes Back

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