Mike McCarty wrote:
Rob wrote:
Hello,
I have a cluster, master + 8 nodes, connected via
a Netgear Gigabit Ethernet Switch (model JGS516).
The cables are of CAT5 type.
Elsewhere I read that for a gigabit network, one
should use CAT6 type cables, to avoid bad
transmissions. Is that so?
Well, there is some debate about this. First, note that
nominally CAT5 is rated for gigabit speeds. Second, note
that gigabit cable does not really carry gigabit rate
transistions, but "only" 250MHz. However, ISTM that
even that is a bit much for simple twisted pair, even if
it is impedance controlled.
I already have done basic speed checks, that indicate
a throughput of about 0.5 Gbps. Hence the network
appears to operate on half of its spec. speed.
That appears quite good to me. 50% occupancy is not
bad.
Question 1:
Should I worry about this reduced speed?
No, I don't think so. What have you got filling the pipe?
Also, what protocols are you using? If you have to wait
for an ACK from time to time, then you are going to get
less than full BW. Also, protocols have other overhead,
like headers on messages and CRCs and whatnot.
The last time I did anything with this, I learned that the maximum
bandwidth for actual data is ~75% of the rated bandwidth. This
basically says 750Mb of data should take ~1sec. And this depends on all
of the above stated points. If you really want to learn about network
bandwidth testing, look at testing network operations for Beowulf clusters.
Install NetPipe and test your network. At least that is what I used on
our cluster when we installed it.
--
Robin Laing
