[Beowulf] gigabit ethernet: horrible performance for 0 byte messages
Douglas Eadline, Cluster World Magazine
deadline at linux-mag.com
Wed Feb 11 19:31:43 EST 2004
On Wed, 11 Feb 2004, Bernhard Wegner wrote:
> 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.
> I wasn't able to actually track down the problem, but it seems that there is
> a problem with small messages. When I run the performance test provided with
> mpich, it reports (bshort2/bshort4) extremely long times (e.g. 1500 us) for 0
> byte message length, while for larger messages everything looks fine (linear
> dependancy of transfer time on message length, everything below 300 us). I
> have also tried mpich2 which shows exactly the same behavior.
> Does anyone have any idea?
First, I assume you were running the 100BT through the same
onboard NICs and got reasonable performance. So some possible
- the switch is a dog or it is broken
- your cables may be old or bad (but worked fine for 100BT)
- negotiation problem
Some things to try:
Use a cross over cable (cat5e) and see if you get the same problem.
You might try using a lower level benchmark (of the micro variety)
like netperf and netpipe.
The Beowulf Performance Suite:
has these tests. Also, the December and January issues of ClusterWorld
show how to test a network connection using netpipe. At some point this
content will be showing up on the web-page.
Also, the MPI Link-checker from Microway (www.microway.com)
> Here are the details of my system:
> - Suse Linux 9.0 (kernel 2.4.21)
> - mpich-126.96.36.199
> - motherboard ASUS P4P800
> - LAN (10/100/1000) on board (3COM 3C940 chipset)
> - LevelOne 10/100/1000 8-port Fast Ethernet Switch (chipset: TC9208M
> 8x88E1111-BAB, AT89C2051-24PI)
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