[Beowulf] Performance characterising a HPC application

stephen mulcahy smulcahy at aplpi.com
Wed Mar 21 10:34:24 EDT 2007

Mark Hahn wrote:
> I don't follow why that indicts latency - multiple smaller packets don't
> each require a round trip, for instance.  with TCP, I've only
> ever seen jumbo packets resulting in modestly higher bandwidth and often
> noticibly lower CPU overhead.  TCP with 1500B packets will _certainly_
> have multiple packets in flight, so on a lan is not terribly
> latency-sensitive.
> if this traffic is to some hotspot, I'd be more inclined to think that
> small packets are overloading the CPU with interrupt and TCP-stack
> overhead.

True. I guess I'm leaning toward latency because I don't see bandwidth
as being the issue - if the overhead is lots of small packets
overloading the CPU shouldn't I see a lot of system time on some nodes?

>> looking for. Do you eyeball raw tcpdump data or use wireshark to
>> browse it?
> I use perl to browse it ;)

Hehe, fair enough. I am well versed in the art of perl hacking, just not
sure it's the best way to use my analysis time.

> pricing is always squishy for this category of hardware.  you can see
> the myrinet list prices on the myricom website.  for 16 ports:
> 16*(495+75)+6600
> comes to just under $1k/port.  using local (Canadian) HP public-sector
> prices, IB is $1600/port.  I'd be surprised of both couldn't be bettered.

Thanks - interesting to get even this kind of comparison, I'm sure its
in the right ballpark anyways.


Stephen Mulcahy, Applepie Solutions Ltd, Innovation in Business Center,
   GMIT, Dublin Rd, Galway, Ireland.      http://www.aplpi.com
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