[petsc-users] matsetvaluesblocked4_

[Mark Adams]

On Wed, May 27, 2020 at 7:37 PM Matthew Knepley <knepley at gmail.com> wrote:

> On Wed, May 27, 2020 at 7:34 PM Jed Brown <jed at jedbrown.org> wrote:
>
>> Mark Adams <mfadams at lbl.gov> writes:
>>
>> > Nvidias's NSight with 2D Q3 and bs=10. (attached).
>>
>> Thanks; this is basically the same as a CPU -- the cost is searching the
>> sorted rows for the next entry.  I've long thought we should optimize
>> the implementations to fast-path when the next column index in the
>> sparse matrix equals the next index in the provided block.  It'd just
>> take a good CPU test to demonstrate that payoff.
>>
>
> So you first check whether the next index is the one in the set passed in,
> and otherwise
> fall back on the search? Good idea.
>


The existing code seems to have *a line (low=i+1) *that seems to be
trying to exploit consecutive indices but it is not quite right, I don't
think,

This is the existing code fragment (this code has been cloned many times
and there are several instances of this kernel).

I've *added code* that I think might make this do the right thing.

     if (col <= lastcol) low = 0;
      else high = nrow;
      lastcol = col;
      while (high-low > 5) {
        t = (low+high)/2;

*if (rp[low] == col) high = low+1;else *       if (rp[t] > col) high = t;
        else low = t;
      }
      for (i=low; i<high; i++) {
        if (rp[i] > col) break; *// delete this check if you don't add new
columns*
        if (rp[i] == col) {
 ap[i] += value;

*  low = i + 1;*          goto noinsert;
        }
      }

I'll  experiment with this.

Thanks,
Mark


>
>    Matt
>
> --
> 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
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>
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