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Subject: Re: [OMPI devel] MTT parameters vor really big nodes?
From: Yevgeny Kliteynik (kliteyn_at_[hidden])
Date: 2012-11-03 18:33:31


Hi Paul,

On 10/31/2012 10:22 PM, Paul Kapinos wrote:
> Hello Yevgeny, hello all,
>
> Yevgeny, first of all thanks for explaining what the MTT parameters do and why there are two of them! I mean this post:
> http://www.open-mpi.org/community/lists/devel/2012/08/11417.php
>
> Well, the official recommendation is "twice the RAM amount".
>
> And here we are: we have 2 nodes with 2 TB (that with a 'tera') RAM and a couple of nodes with 1TB, each with 4x Mellanox IB adapters. Thus we should have raised the MTT parameters in order to make up to 4 TB memory registrable.

You don't really *have* to be able to register twice the available RAM.
It's just heuristics. It depends on the application that you're running
and fragmentation that it creates in the MTT.

However:
 
> I've tried to raise the MTT parameters in multiple combinations, but the maximum amount of registrable memory I was able to get was one TB (23 / 5). All tries to get more (24/5, 23/6 for 2 TB) lead to not responding InfiniBand HCAs.
>
> Is there any another limits in the kernel have to be adjusted in order to be able to register that a bunch of memory?

Unfortunately, current driver has a limitation in this area so 1TB
(23/5 values) is probably the top what the driver can do.
IIRC, log_num_mtt can reach 26, so perhaps you can try 26/2 (same 1TB),
and then, if it works, try 26/3 (fingers crossed), which will bring you
to 2 TB, but I'm not sure it will work.

This has already been fixed, and the fix was accepted to the upstream
Linux kernel, so it will be included in the next OFED/MLNX_OFED versions.

-- YK

> Best,
>
> Paul Kapinos
>
>
>
>
>
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