[petsc-dev] Efficient shifting vectors one position in array of vectors.

[Matthew Knepley]

Stefano is of course correct. Another method which I have found easier is
just to make the index for
the set of vectors circular, that is shift it by some amount mod the size.

  Thanks,

    Matt

On Wed, Aug 30, 2017 at 9:21 AM, Stefano Zampini <stefano.zampini at gmail.com>
wrote:

> sorry, forgot the last line of pointers swapping
> Vec tmp = Z_Vec[pipe_l];
> for(int i = pipe_l;  i>0; i--){
>    Z_Vec[i] = Z_Vec[i-1];
>    }
> Z_Vec[0] = tmp;
>
> 2017-08-30 16:19 GMT+03:00 Wim Vanroose <wim at vanroo.se>:
>
>> I've tried that in an earlier version.  But it does not work. Then my
>> MatMult complains that
>>
>> [0]PETSC ERROR: Object is in wrong state
>>
>> [0]PETSC ERROR: x and y must be different vectors
>>
>> Because Vec[1] is then referring to  Vec[0] where I am going to store the
>> result of the MatMult.
>> Also later on there are some AXPY that are complaining that that x and y
>> are the same vector.
>>
>> Wim
>>
>>
>> On Wed, Aug 30, 2017 at 2:55 PM, Stefano Zampini <
>> stefano.zampini at gmail.com> wrote:
>>
>>> if you don't use KSPCreateVecs, but just a sequence of VecDuplicates,
>>> then you can swap pointers
>>>
>>> Vec tmp = Z_Vec[pipe_l];
>>> for(int i = pipe_l;  i>0; i--){
>>>    Z_Vec[i] = Z_Vec[i-1];
>>>    }
>>>
>>> if you still want to use KSPCreateVecs, you need to keep track of what
>>> was the first vector of the sequence at the time of creation, because its
>>> address in memory points to the malloced memory for all the Vec pointers.
>>>
>>>
>>>
>>> 2017-08-30 15:45 GMT+03:00 Wim Vanroose <wim at vanroo.se>:
>>>
>>>> Dear Petsc Developers,
>>>>
>>>> We are writing a pipelined CG with deep pipelines.    For this
>>>> algorithm I need to
>>>> store   "pipe_l +1"  auxiliary variables  Z_Vec.    I create these
>>>> vectors as follows:
>>>>
>>>>  Vec *Z_VEC;
>>>>  ierr = KSPCreateVecs(ksp,pipe_l+1,&Z_VEC,0,NULL);CHKERRQ(ierr);
>>>>
>>>>
>>>> Then we have a loop of iterations.   However at some point I have to
>>>> drop the last
>>>> the auxiliary vector and shift the remaining vectors one position, and
>>>> replace the first vector with the result of a matvec.
>>>>
>>>> This is in the current prototype implemented as
>>>>
>>>>       for(int i = pipe_l;  i>0; i--){
>>>>         VecCopy( Z_VEC[i-1], Z_VEC[i] );
>>>>       }
>>>>
>>>>       ierr = MatMult(A, Z_VEC[1], Z_VEC[0]);CHKERRQ(ierr);
>>>>
>>>> It looks very inefficient to me to copy the vector, if we only want to
>>>> shift the data.
>>>>
>>>> Do you have examples code where this is done more efficiently?  Or is
>>>> this already a good way to do it?
>>>> Do you have suggestions?
>>>>
>>>>
>>>> Wim
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> Stefano
>>>
>>
>>
>
>
> --
> Stefano
>



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

http://www.caam.rice.edu/~mk51/
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