[petsc-dev] [petsc4py] Vec.getArray()

[Matthew Knepley]

On Fri, Aug 27, 2010 at 6:14 PM, Lisandro Dalcin <dalcinl at gmail.com> wrote:

> I cannot figure out how to implement a copy-free and safe
> VecGetArray()/VecRestoreArray() pattern in Python (not even by using
> the 'with' statement, it leaks the target variable!!!!).
>
> 1) Provide a 100% safe but slow, copy-based way:
>
> a = x.getArray() #gives you a copy. It is implemented with
> VecGetArrayRead(x, &p), memcpy p->a.data, VecRestoreArray(x,&p) on a
> freshly allocated numpy array that is returned to the user.
> a.base is None # True, the array owns its memory buffer
> x.setArray(a) #writes array on the vector. It is implemented with
> VecGetArray(x,&p) and memcpy a.data -> p, VecRestoreArray(x,&p)
>
>
> 2) Provide a unsafe but fast, copy-free way to get a numpy array
> sharing memory with PETSc vectors:
>
> a = numpy.asarray(x) # gives you a numpy array that shares mem with
> the vec, it is implemented with VecGetArray() and special Python/NumPy
> protocols for buffer sharing.
> a.base is x # True, the base attr holds a ref to the Vec instance, the
> array does not own its memory buffer.
> del a # force garbage collection explicitily, then VecRestoreArray()
> will be called when a gets deallocated.
>
> Relying in explicit use of del for garbage collection is not reliable.
> NumPy is designed to support array views, these views hold references
> to the base array. So users have to be very careful about how the
> arrays obtained the fast way are used.
>
>
> Comments ? Suggestions? Complaints?


I am for 2). PETSc users generally want to sacrifice safety for performance.

   Matt


>
> --
> Lisandro Dalcin
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-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
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