[Beowulf] gigabit ethernet: horrible performance for 0 byte messages

scyld at jasons.us scyld at jasons.us
Wed Feb 11 13:01:21 EST 2004

On Wed, 11 Feb 2004, Bernhard Wegner wrote:

> Hello,
> I have a really small "cluster" of 4 PC's which are connected by a normal
> Ethernet 100 Mbit switch. Because the motherboards have Gigabit-LAN on board
> I thought I might be able to improve performance by connecting the machines
> via a Gigabit switch (which are really cheap nowadays).
> Everything seemed to work fine. The switch indicates 1000Mbit connections to
> the PC's and transfer rate for scp-ing large files is significantly higher
> now, but my software unsing mpich RUNS about a factor of 4-5 SLOWER NOW than
> with the 100 Mbit switch.

Have you tried setting the speed and duplex of the gig NICs to 1000/full
on both the system side and switch side?  I've found that autonegotiate
rarely does especially with 3com gear.  I'm guessing, based on its size,
your switch isn't managed so you may have to stick to locking it on the
systems and watching the behavior to see if the switch gets the
negotiation right.  (if traffic is bursty you have a speed mismatch and if
you get loads of errors it's more likely to be duplex problem)

FWIW I have the same mobo at home but haven't hooked it to gigabit yet so
I'm quite curious to see how this works out.


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