From owner-freebsd-questions  Sun Jan 27 15:49:54 2002
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Date: Sun, 27 Jan 2002 23:50:43 +0000
To: TD790@aol.com
From: Neil McGann <neil@neilmcgann.co.uk>
Subject: Re: UDMA and 100Mbit NIC speed issues
Cc: freebsd-questions@freebsd.org
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At 17:04 27/01/02 -0500, you wrote:
>In a message dated 1/27/02 4:03:42 PM Eastern Standard Time,
>neil@neilmcgann.co.uk writes:
>
> > Realistically 100Mb/sec is flat out
> >  for 33MHz PCI and trying to get some bandwidth for the NIC may just 
> not be
> >  do-able.
>
>Its more like 400Mb/s, but highly dependent on what else is on the bus.
>100Mb/s is what you'll get with single quadword transactions, but any
>burstable bus-master will be able to do substantially better than that. You
>can do routing with dual full duplex 100Mb/s ethernets without seeing xmitter
>underruns, which requires 400Mb/s across the bus. A 3rd nic or disk
>transactions will cause problems beyond that.

Not on a single bus it isn't - 33Mhz, 32-bit width = 132Mbytes/sec 
theoretical max - less address cycles and general overhead. 66MHz and/or 
64-bit is obviously a different story - as are chip sets with multiple 
parallel PCI data paths.

If UDMA100 can really hit 100Mbytes/sec for extended periods then it's 
going to need serious control of burst sizes and latencies to allow much 
other concurrent throughput on that physical bus.

Neil


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