[petsc-dev] petsc4py/numpy: is there a performance difference between these two pieces of code?

[Jed Brown]

On Sun, Nov 27, 2011 at 01:44, Aron Ahmadia <aron.ahmadia at kaust.edu.sa>wrote:

> qbc is a multi-dimensional, Fortran ordered numpy array
>

Why is it a separate array instead of the one inside of the local vector?

Well, let's just think about what actually happens.


> # METHOD 1
>
> # with fset(self,q) defined as follows:
>
        def fset(self,q):
>             self.gqVec.setArray(q.reshape([-1], order = 'F'))
>

This is memcpy


>
>         self.q = qbc[:,mbc:-mbc,mbc:-mbc]
>

This will do another copy to turn the view into contiguous storage that can
be passed on.


Note: I think PyClaw's use of fset for setting elements of an array is
confusing since the call site looks like it should have different semantics
than it does.


>
> # METHOD 2
>
>         self.lqVec.placeArray(qbc.reshape([-1], order = 'F'))
>

I think/hope this does no copy on the Numpy side.


>         self.q_da.localToGlobal(self.lqVec,self.gqVec)
>

This updates the ghost values without an intermediate copy.


>         self.lqVec.resetArray()
>
> # METHOD 3
>
>         self.lqVec.placeArray(qbc)
>

This is the same as before, right?


>         self.q_da.localToGlobal(self.lqVec,self.gqVec)
>         self.lqVec.resetArray()
>
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