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

[Wim Vanroose]

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
>
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