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

Brent M. Clements bclem at rice.edu
Thu Feb 12 22:22:02 EST 2004


The best switch that we have found both in price and speed are the GigE
Switches from Dell. We use them in a few of our test clusters and smaller
clusters. They are actually pretty good performers and top even some of
the cisco switches.

-Brent

Brent Clements
Linux Technology Specialist
Information Technology
Rice University


On Wed, 11 Feb 2004, Gerry Creager N5JXS wrote:

> Realize that not all switches are created equal when working with small
> (and, overall, 0-byte == small) packets.  A number of otherwise decent
> network switches are less than stellar performers with small packets.
> We've evaluated this in my lab with an Anritsu MD-1230 Ethernet test
> system running under the RFC-2544 testing suite...
>
> There are switches that perform well with small packets, but it's been
> our experience that most switches, especially your lower cost switches
> (Cisco 2900/2950/3500, 4000/4500; Allied Telesyn *; Cabletron *; some
> others I can't recall right now) didn't perform well with smaller
> packets but did fine when the packet size was about 1500 bytes.
>
> Going with cheap switches is usually not a good way to improve performance.
>
> gerry
>
> Douglas Eadline, Cluster World Magazine wrote:
> > 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.
> >>
> >>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
> > things:
> >
> > - 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:
> > http://www.clusterworld.com/article.pl?sid=03/03/17/1838236
> >
> > 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)
> >
> > http://www.clusterworld.com/article.pl?sid=04/02/09/1952250
> >
> > May help.
> >
> >
> > Doug
> >
> >
> >>Here are the details of my system:
> >> - Suse Linux 9.0 (kernel 2.4.21)
> >> - mpich-1.2.5.2
> >> - 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)
> >>
> >>
> >
> >
>
> --
> Gerry Creager -- gerry.creager at tamu.edu
> Network Engineering -- AATLT, Texas A&M University
> Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
> Page: 979.228.0173
> Office: 903A Eller Bldg, TAMU, College Station, TX 77843
>
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