Beowulf Questions

Eugen Leitl eugen at leitl.org
Sat Jan 4 14:20:50 EST 2003


On Sat, 4 Jan 2003, Mark Hahn wrote:

> there simply is no coming breakthrough that will make all networking
> fast, low-latency, cheap, ubiquitous and low-power.  and grid
> (in the grand sense) really does require *all* those properties.

I'm not quite sure. The only hard limit on latency is relativistic (in
vacuum, 1 ns = 0.3 m; 10 ns = 3 m, 100 ns = 30 m; 1 us = 3 km; 10 us = 30
km, 100 us = 300 km). Right now, commercial networks based on GBit fiber
Ethernet backbones exist, delivering sub-ms latency to end consumers. 10
GBit fiber Ethernet will be starting to displace GBit Ethernet in that
niche. At 10 GBps fiber acts as a FIFO, containing ~50 bit/m (50 kBit/km)
of fiber allowing (admittedly, there is no impetus for developing
cut-through WAN transmission technology) almost purely photonically
switched networks where routing latency is negligible in regards to
relativistic latency. That assumes that the fiber(s) is unloaded, of
course, as store-and forward will suddenly result in lousy latency. This 
can't happen on a true crossbar-switched LAN.

This clearly can't compete with dedicated ultralocal interconnects like
Myrinet & Co, but it indicates GBit based clusters need not to be located
physically close.



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