[Users] Benchmarking results for McLachlan rewrite

[Ian Hinder]

On 25 Jul 2015, at 12:26, Frank Loeffler <knarf at cct.lsu.edu> wrote:

> Am 25. Juli 2015 11:35:27 MESZ, schrieb Ian Hinder <ian.hinder at aei.mpg.de>:
>> 
>> On 25 Jul 2015, at 10:35, Frank Loeffler <knarf at cct.lsu.edu> wrote:
>> 
>>> If I remember correctly, the default is zero dissipation, so by
>> default it already is effectively off now. 
>> 
>> I'm referring to parameter files which set the McLachlan dissipation
>> parameter.  Those will fail if we disable dissipation (at Kranc time).
> 
> I didn't think you mean at Kranc time. I would think we shouldn't do that. Rerunning Kranc shouldn't be necessary to enable something like dissipation.

Actually I do.  On the rewrite branch, McLachlan has a switch to enable dissipation at Kranc time.  Once enabled, it is unconditionally computed at run time, which takes a certain amount of time.  This cannot currently be disabled at run time.  Erik was asking (I think) whether this Kranc-time dissipation should be disabled, since in the case that users want to use dissipation from the Dissipation thorn, they incur an unavoidable performance penalty (dissipation terms will be computed twice, though one of them will be multiplied by zero).

Dissipation can be provided:

1. By the Dissipation thorn, which provides several useful (needed) features, such as being able to control the dissipation strength on a given refinement level, the dissipation order, as well as having some options for excision masks (which I have never used, but others might); or 
2. By McLachlan, which is more limited, but which Erik expects to be faster

The pre-rewrite version of McLachlan had very slow dissipation, but it was not executed if the dissipation strength was set to 0 at runtime, so this problem did not arise.  

-- 
Ian Hinder
http://members.aei.mpg.de/ianhin

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