[petsc-users] petsc4py Vec().setSizes()

[Jed Brown]

On Tue, Feb 5, 2013 at 7:59 AM, Weston Lowrie <wlowrie at uw.edu> wrote:

> That makes sense now.  It was just creating 'mpi_size' (in my case 4)
> separate Vec objects and distributing them evenly among the processes.
>  That is why it appeared to give the correct sizes.
>
> Good to know about the petsc4py "size" argument.  It was unclear to me
> that it could take multiple values, but that makes sense to keep the API
> general and include as much of the functionality of the C++/Fortran APIs.
>
> I have been referencing this:
> http://packages.python.org/petsc4py/apiref/index.html
>
> where it shows:
> setSizes(self, size, bsize=None)
>
> Maybe there is a better reference somewhere, or I missed where it
> describes the size argument?
>

AFAIK, it's not documented. Most of petsc4py is a direct mapping from PETSc
and I think we (Lisandro) have been hoping to find a way to automatically
generate Python docs. For those features that are actually different (like
the size arguments), it would be very useful to write doc strings. (They
would show up in the apiref above.)

I'm sure Lisandro would be happy to accept documentation patches.


>
> Thanks,
> Wes
>
>
>
> On Mon, Feb 4, 2013 at 6:40 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>
>> petsc4py is "too clever" in the sense that it tries to interpret many
>> different kids of "sizes" arguments. You can always pass the pair
>> (localsize, globalsize), but you can also pass only a global size (in which
>> case the vector will be split apart). If you want to set only the local
>> size, you should pass (localsize, None).
>>
>> Your example is invalid, with each process passing different global
>> sizes. petsc-dev will now error if you do this.
>>
>> I changed your example to:
>>
>>
>> X = PETSc.Vec().create(comm=PETSc.COMM_WORLD)
>> X.setSizes((sizes[mpi_rank],PETSc.DECIDE),bsize=1)
>> X.setFromOptions()
>> ilow,ihigh = X.getOwnershipRange()
>>
>> PETSc.Sys.syncPrint("rank: ",mpi_rank,"low/high: ",ilow,ihigh)
>> PETSc.Sys.syncFlush()
>>
>> and now get the output:
>>
>> rank:  0 low/high:  0 35675
>> rank:  1 low/high:  35675 401185
>> rank:  2 low/high:  401185 766927
>> rank:  3 low/high:  766927 802370
>>
>>
>>
>> On Mon, Feb 4, 2013 at 4:01 PM, Weston Lowrie <wlowrie at uw.edu> wrote:
>>
>>> Hi,
>>> I'm confused what the Vec().setSizes() routine is doing in petsc4py.
>>>  Consider this example:
>>>
>>> #!/usr/bin/env python
>>> import sys,os
>>> from petsc4py import PETSc
>>> from numpy import *
>>>
>>> mpi_rank = PETSc.COMM_WORLD.getRank()
>>> mpi_size = PETSc.COMM_WORLD.getSize()
>>>
>>> sizes = zeros(4)
>>> sizes[0] = 35675
>>> sizes[1] = 365510
>>> sizes[2] = 365742
>>> sizes[3] = 35443
>>>
>>> X = PETSc.Vec().create(comm=PETSc.COMM_WORLD)
>>> X.setSizes(mpi_size*sizes[mpi_rank],bsize=1)
>>> X.setFromOptions()
>>> ilow,ihigh = X.getOwnershipRange()
>>>
>>> print "rank: ",mpi_rank,"low/high: ",ilow,ihigh
>>>
>>>
>>>
>>> Why is it that when setting the local sizes explicitly do I need to
>>> multiply by the mpi_size?  My understanding is that when using this routine
>>> it is telling PETSc what the local processor core size should be.  It seems
>>> to divide it by total number of processors cores.
>>>
>>> Thanks,
>>> Wes
>>>
>>
>>
>
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