Hi Dr. Bangerth,

Thanks a lot for the clarification! They are really helpful!


On Thursday, July 30, 2020 at 11:47:21 AM UTC-5, Wolfgang Bangerth wrote:
> On 7/30/20 10:11 AM, Jimmy Ho wrote: 
> > 
> > As a follow-up question, upon calling compress(), will the local copy of 
> the 
> > system matrix on a specific processor get updated to contain information 
> from 
> > all other processors? In other words, if I print out the system matrix 
> from a 
> > particular processor after calling compress(), is that the same global 
> system 
> > matrix that the linear solver is solving? 
> Each processor only stores a subset of the rows of the matrix. During 
> assembly, each processor computes a number of matrix entries, but it can 
> not 
> compute all entries for the rows of the matrix it owns -- some need 
> contributions from other processes. The call to compress() makes sure the 
> contributions from these other processes are sent to the one that owns 
> these 
> rows of the matrix. 
> In any case, after compress(), the rows each processor owns are correct -- 
> but 
> each processor doesn't know anything about the rows of the matrix it 
> doesn't own. 
> Best 
>   W. 
> -- 
> ------------------------------------------------------------------------ 
> Wolfgang Bangerth          email:                 bang...@colostate.edu 
> <javascript:> 
>                             www: http://www.math.colostate.edu/~bangerth/ 

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