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

[Stefano Zampini]

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
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