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Subject: Re: [OMPI users] Strange TCP latency results on Amazon EC2
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2012-01-12 10:10:50

Hi Roberto.

We've had strange reports of performance from EC2 before; it's actually been on my to-do list to go check this out in detail. I made contact with the EC2 folks at Supercomputing late last year. They've hooked me up with some credits on EC2 to go check out what's happening, but the pent-up email deluge from the Christmas vacation and my travel to the MPI Forum this week prevented me from testing yet.

I hope to be able to get time to test Open MPI on EC2 next week and see what's going on.

It's very strange to me that Open MPI is getting *better* than raw TCP performance. I don't have an immediate explanation for that -- if you're using the TCP BTL, then OMPI should be using TCP sockets, just like netpipe and the others.

You *might* want to check hyperthreading and process binding settings in all your tests.

On Jan 12, 2012, at 7:04 AM, Roberto Rey wrote:

> Hi again,
> Today I was trying with another TCP benchmark included in the hpcbench suite, and with a ping-pong test I'm also getting 100us of latency. Then, I tried with netperf and the same result....
> So, in summary, I'm measuring TCP latency with messages size between 1-32 bytes:
> Netperf over TCP -> 100us
> Netpipe over TCP (NPtcp) -> 100us
> HPCbench over TCP -> 100us
> Netpipe over OpenMPI (NPmpi) -> 60us
> HPCBench over OpenMPI -> 60us
> Any clues?
> Thanks a lot!
> 2012/1/10 Roberto Rey <eros.83_at_[hidden]>
> Hi,
> I'm running some tests on EC2 cluster instances with 10 Gigabit Ethernet hardware and I'm getting strange latency results with Netpipe and OpenMPI.
> If I run Netpipe over OpenMPI (NPmpi) I get a network latency around 60 microseconds for small messages (less than 2kbytes). However, when I run Netpipe over TCP (NPtcp) I always get around 100 microseconds. For bigger messages everything seems to be OK.
> I'm using the BTL TCP in OpenMPI, so I can't understand why OpenMPI outperforms raw TCP performance for small messages (40us of difference). I also have run the PingPong test from the Intel Media Benchmarks and the latency results for OpenMPI are very similar (60us) to those obtained with NPmpi
> Can OpenMPI outperform Netpipe over TCP? Why? Is OpenMPI doing any optimization in BTL TCP?
> The results for OpenMPI aren't so good but we must take into account the network virtualization overhead under Xen
> Thanks for your reply
> --
> Roberto Rey Expósito
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> users_at_[hidden]

Jeff Squyres
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