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

[giggzounet]

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...)
> 
> Hi,
> 
> 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
>>
>> _______________________________________________
>> ewg mailing list
>> ewg at lists.openfabrics.org
>> http://lists.openfabrics.org/cgi-bin/mailman/listinfo/ewg
> 
> 
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