[petsc-users] Question about memory usage in Multigrid preconditioner

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:

>> On Sep 15, 2016, at 1:10 PM, Dave May <dave.mayhem23 at gmail.com> wrote:
>> 
>> 
>> 
>> On Thursday, 15 September 2016, Barry Smith <bsmith at mcs.anl.gov> wrote:
>> 
>>    Should we have some simple selection of default algorithms based on problem size/number of processes? For example if using more than 1000 processes then use scalable version etc?  How would we decide on the parameter values?
>> 
>> I don't like the idea of having "smart" selection by default as it's terribly annoying for the user when they try and understand the performance characteristics of a given method when they do a strong/weak scaling test. If such a smart selection strategy was adopted, the details of it should be made abundantly clear to the user.
>> 
>> These algs are dependent on many some factors, thus making the smart selection for all use cases hard / impossible.
>> 
>> I would be happy with unifying the three inplemtationa with three different options AND having these implantation options documented in the man page. Maybe even the man page should advise users which to use in particular circumstances (I think there is something similar on the VecScatter page).
>> 
>> I have these as suggestions for unifying the options names using bools 
>> 
>> -matptap_explicit_transpose
>> -matptap_symbolic_transpose_dense
>> -matptap_symbolic_transpose
>> 
>> Or maybe enums is more clear
>> -matptap_impl {explicit_pt,symbolic_pt_dense,symbolic_pt}
>> 
>> which are equivalent to these options
>> 1) the current default
>> 2) -matrap 0
>> 3) -matrap 0 -matptap_scalable
>> 
>> Maybe there could be a fourth option
>> -matptap_dynamic_selection
>> which chooses the most appropriate alg given machine info, problem size, partition size,.... At least if the user explicitly chooses the dynamic_selection mode, they wouldn't be surprised if there were any bumps appearing in any scaling study they conducted.
>
>    I like the idea of enum types with the final enum type being "dynamically select one for me".

I also like enums and "-matptap_impl auto" (which could be the default).
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