[Beowulf] Small packets causing context switch thrashing?

Tracy R Reed treed at ultraviolet.org
Wed Dec 1 17:30:44 EST 2004


Ok, this is not exactly beowulf or supercomputer related but it is
definitely a form of high performance computing and I am hoping
the beowulf community has applicable experience.

I am building a box to convert VOIP traffic from H323 to SIP. The system
is an AMD64. Both of these protocols use RTP to transmit the voice data
which means many many small packets. We are currently looking at 8000
packets per second due to 96 simultaneous voice channels and the box is
already at 50% cpu. I really think this box should be able to handle a lot
more than this. I have seen people talk about proxying 2000 RTP streams on
a P4. We get around 15,000 context switches and 8000 interrupts per second
and the box is heavily loaded and the load average starts going up. Is
9000 packets per second a lot? I would not have thought so but it is
hammering our box. I have applied several of the applicable tuning
suggestions (tcp stuff is not applicable since RTP is all UDP) from: 

http://216.239.57.104/search?q=cache:0VItqrkQdO0J:datatag.web.cern.ch/datatag/howto/tcp.html+linux+maximum+network+buffer+size&hl=en

but the improvement has been minimal. We have some generic 100Mb ethernet
chipset in the box. I have seen a number of high performance computing
guys talk about interrupt coalescence in mailing list archives found via
google while researching this problem. Can the Pro 100 card do this or do
I need the 1000? Does it seem likely that if I run down to the store and
pay my $25 for an Intel Pro 1000 card and load up the driver with the
InterruptThrottleRate set (and what is a good value for this?) that I will
get dramatically improved performance? I would do it right now just to see
but the box is in a colo a considerable drive away so I want to have a
good idea that it will work before we make the drive. Ideally I would like
to get 10x (76800pps) the performance out of the box but could settle for
5x (38400pps).

Thanks for any tips you can provide!

-- 
Tracy Reed    http://copilotcom.com 
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