[ewg] Need help for Infiniband optimisation for our cluster (MTU...)

giggzounet giggzounet at gmail.com
Tue Dec 7 07:32:05 PST 2010


Thx for your answer!

Particularly the explication between connected and datagram mode (I see
that with the IMB1 benchmarks of mpi)!

The hardware we are using in details:
- on the master: Mellanox MHGH18-XTC ConnectX with VPI adapter, single
port 20Gb/s, PCIe2.0 x8 2.5GT/s
- on the nodes: Integrated Mellanox DDR Infiniband 20Gbs ConnectX with
QSFP Connector.

How can I know the limit of the MTU size ?

On the Infiniband we are just using mpi with different CFD programs. But
always with mpi (intel mpi or openmpi). Sould I use QoS ?

Thx for your help!

Le 07/12/2010 16:09, Richard Croucher a écrit :
> Connected mode will provide more throughput but datagram mode will provide
> lower latency.  
> You don't say what HCA's you are using.  Some of the optimizations for
> Connected mode are only available for the newer ConnectX QDR HCA's.
> Your HCA will probably limit the MTU size.  Leave this as large as possible.
> If you are only running a single application on the InfiniBand you need not
> bother with QoS.   If you are running multiple, then you do need to set
> this.  This is quite complex since you need to define V'L's, their
> arbitration policies and assign SL's to them.  This is described in the
> OpenSM docs.  This is relevant even if you are using the embedded SM in the
> switch.
> AS a newbie, take a look in the ../OFED/docs  
> There is probably all you need there. Mellanox also have some useful docs on
> their website.
> -----Original Message-----
> From: ewg-bounces at lists.openfabrics.org
> [mailto:ewg-bounces at lists.openfabrics.org] On Behalf Of giggzounet
> Sent: 07 December 2010 14:01
> To: ewg at lists.openfabrics.org
> Subject: [ewg] Need help for Infiniband optimisation for our cluster
> (MTU...)
> Hi,
> I'm new on this list. We have in our laboratory a little cluster:
> - master 8 cores
> - 8 nodes with 12 cores
> - DDR infiniband switch Mellanox MTS3600R
> On these machines we have an oscar cluster with CentOS 5.5. We have
> installed the ofed packages 1.5.1. The default config for the infiniband
> is used. So infiniband is running in connected mode.
> Our cluster is used to solve CFD (Computational Fluid Dynamics)
> problems. And I'm trying to optimize the infiniband network and so I
> have several questions:
> - Is it the right mailing list to ask ? (if not...where should I post ?)
> - Is there a how-to on infiniband optimisation ?
> - CFD computations need a lot of bandwidth. There are a lot of data
> exchange through MPI (we are using intel mpi). Has the infiniband mode
> (connected or datagram) influence in this case ? What is the "best" MTU
> for those computation ?
> Best regards,
> Guillaume
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