[ewg] Need help for Infiniband optimisation for our cluster (MTU...)
giggzounet at gmail.com
Tue Dec 7 07:58:32 PST 2010
ok Thx for all your explanations.
So the MTU value which I'm seeing on the ib0 interface (65520) is not
connected to the "real" infiniband MTU value ?
Le 07/12/2010 16:52, Mike Heinz a écrit :
> Heh. I forgot Intel sells an mpi, I thought you were saying you had recompiled one of the OFED mpis with icc.
> 1) For your small cluster, there's no reason not to use connected mode. The only reason for providing a datagram mode with MPI is to support very large clusters where there simply aren't enough system resources for every node to connect with every other node.
> I would still suggest experimenting with mvapich-1 (and recompiling it with icc) to see if you get better performance.
> 2) Similarly, for a small cluster, QoS won't give you any benefit. The purpose of QoS is to divide up the fabric's bandwidth so that multiple simultaneous apps can share it in a controlled way. If you're only running one app at a time (which seems likely) you want that app to get all available bandwidth.
> I'm not sure how you check the MTU size when using stock OFED, but my memory for those HCAs is that they can use 2k MTUs. You only use a smaller MTU size when the larger size causes reliability problems.
> -----Original Message-----
> From: ewg-bounces at lists.openfabrics.org [mailto:ewg-bounces-ZwoEplunGu2DiMhRP7xlEA at public.gmane.orgcs.org] On Behalf Of giggzounet
> Sent: Tuesday, December 07, 2010 10:32 AM
> To: ewg at lists.openfabrics.org
> Subject: Re: [ewg] Need help for Infiniband optimisation for our cluster (MTU...)
> 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
>> 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
>> 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,
>> ewg mailing list
>> ewg at lists.openfabrics.org
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