[Beowulf] Adding Latency to a Cluster Environment

Bogdan Costescu bogdan.costescu at iwr.uni-heidelberg.de
Sat Feb 14 18:12:38 EST 2004


On Sat, 14 Feb 2004, Mark Hahn wrote:

> hack the driver to artificially pre/append a constant number of
> bytes to each message.

I thought of this as well, but I dsmissed it because:

- if the higher level protocol uses fragmentation and checksums, I
think that it's pretty hard for the driver to mess with the messages.
- a side effect might be faster filling up of some FIFO buffers on the 
receiver side, which might influence in unexpected ways the latency 
that we want to measure. Another side effect might be on the switch 
(assuming a network that uses switches) where data might be kept 
longer in buffers or peak bandwidth might be reached for short times, 
but enough to make a difference...
- for networks that offer a very low latency, simulating a large 
latency might require adding a big lot of junk data, many times larger 
than the original message.

-- 
Bogdan Costescu

IWR - Interdisziplinaeres Zentrum fuer Wissenschaftliches Rechnen
Universitaet Heidelberg, INF 368, D-69120 Heidelberg, GERMANY
Telephone: +49 6221 54 8869, Telefax: +49 6221 54 8868
E-mail: Bogdan.Costescu at IWR.Uni-Heidelberg.De

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