[Fwd: Re: 32-port gigabit switch]

Richard Walsh rbw at ahpcrc.org
Fri Mar 7 12:12:35 EST 2003


On Fri, 7 Mar 2003 06:52:26, Eray Ozkuraln wrote:

>It depends on the application :)
>
>Actually throughput is more important than latency in many cases I believe. 
>And of course while it is not too difficult using common programming 
>techniques to optimize for high throughput high latency it would be much 
>harder to do so with low throughput low latency don't you think? :)

Right.  In the limit of a 1 byte message, the inverse of the latency
is the worst-case bandwidth for repeatedly sending 1 byte. On a GBE 
system with a latency of say 50 usecs your worst case bandwidth 
is 20 KB/sec :-(. This is mostly a hardware number.  If you add in
other contributors to the latency things get worse.  As message size 
shrinks latency eventually dominates the transfer time ... the larger
the latency the sooner this happens. Under the heading of "everything
is just another form of something else", the distinction between latency
and bandwidth gets muddy as latency grows relative to message size.
[Reminds of that mass <--> energy thing ... ;-) ... E = MC**2] 

On the other hand, if you can manage your message sizes, keep the 
latency piece a small percentage of the message transit time, and have 
good bandwidth you may not care what the latency is.  Pushing up data volumes 
per node imply larger surfaces to communicate which imply larger messages. 
These transfers can be hidden behind compute cycles.  Of course, one 
has to worry about faster processors shrinking compute cycles. 

There are better more detailed analyses in the archives as I recall.

rbw
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