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Subject: Re: [OMPI devel] RFC: change default for tuned alltoallv to pairwise
From: Nathan Hjelm (hjelmn_at_[hidden])
Date: 2012-03-22 16:01:47

On Thu, 22 Mar 2012, Shamis, Pavel wrote:

>>> What: Change coll tuned default to pairwise exchange
>>> Why: The linear algorithm does not scale to any reasonable number of PEs
>>> When: Timeout in 2 days (Fri)
>>> Is there any reason the default should not be changed?
>> Nathan,
>> I can see why people think the linear algorithm is bad. However I think it depends on the application communication pattern in the alltoallv. Do you have any numbers to back your claim?
> George,
> In addition the above list of dependencies it also depends on network technology. The linear algorithm does not play well with IB.

Nor does it play well with Gemini. I will get some hard numbers early next week on both our XE6 and one of our infiniband clusters.