Fix volume_integral_kernel! for DGMulti, VolumeIntegralWeakForm

[DanielDoehring]

See https://github.com/trixi-framework/Trixi.jl/pull/2809/files/7f0ceb876379295836b99d2c009993aac9036d5c#r2820350827

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.10%. Comparing base (aef23e1) to head (925365a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/solvers/dgmulti/flux_differencing.jl	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2812      +/-   ##
==========================================
- Coverage   97.10%   97.10%   -0.00%     
==========================================
  Files         589      589              
  Lines       45539    45543       +4     
==========================================
+ Hits        44218    44221       +3     
- Misses       1321     1322       +1     

Flag	Coverage Δ
unittests	97.10% <87.50%> (-<0.01%)	⬇️

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[ranocha]

I think there was another place in the original PR with the same pattern. Moreover, this introduces a change from one big matri multiplication to many smaller ones, which can have an impact on performance. Did you benchmark it?

[DanielDoehring]

Moreover, this introduces a change from one big matri multiplication to many smaller ones, which can have an impact on performance. Did you benchmark it?

Alternatively, we could dispatch calc_volume_integral! for VolumeIntegralWeakForm and do the multiplication there. This bears the danger, though, that people might think they can use calc_volume_kernel! as in the DGSEM case, which would not be true.

[jlchan]

Could you move the multiplication by rd.Vq into calc_volume_integral instead, outside the element loop?

This should preserve the original performance but factor out common code for both affine and non-affine meshes.

[jlchan]

Alternatively, we could dispatch calc_volume_integral! for VolumeIntegralWeakForm and do the multiplication there. This bears the danger, though, that people might think they can use calc_volume_kernel! as in the DGSEM case, which would not be true.

Since it's not exported I'm less worried about that. It can be unified by ensuring that the cache has some consistency across different DGMulti volume integrals, which I am hoping to do anyways.

[DanielDoehring]

Since it's not exported I'm less worried about that. It can be unified by ensuring that the cache has some consistency across different DGMulti volume integrals, which I am hoping to do anyways.

I will add a comment, should be fine then 👍

[jlchan]

Some minor comments

[jlchan]

Thanks - sorry I missed issue previously.

[ranocha]

Thanks!