Parallelise some CPU -> CPU copies

[jipolanco]

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Benchmark Results (Julia v1)

Time benchmarks

	master	695096f...	master / 695096f...
BiotSavart/add_local_integrals!	13.3 ± 0.18 ms	13.3 ± 0.88 ms	0.998 ± 0.067
BiotSavart/add_point_charges!	12.7 ± 0.45 ms	12.7 ± 0.36 ms	1 ± 0.045
BiotSavart/velocity	0.572 ± 0.0046 s	0.578 ± 0.0047 s	0.989 ± 0.011
BiotSavart/velocity + streamfunction	0.691 ± 0.0078 s	0.692 ± 0.0078 s	0.999 ± 0.016
CellLists/CPU/nsubdiv = 1/foreach_pair (SIMD/sorted)	0.0789 ± 0.0015 s	0.077 ± 0.0021 s	1.03 ± 0.034
CellLists/CPU/nsubdiv = 1/foreach_pair (SIMD/unsorted)	0.0968 ± 0.01 s	0.0889 ± 0.0011 s	1.09 ± 0.11
CellLists/CPU/nsubdiv = 1/foreach_pair (sorted)	0.0566 ± 0.00061 s	0.055 ± 0.00064 s	1.03 ± 0.016
CellLists/CPU/nsubdiv = 1/foreach_pair (unsorted)	0.0672 ± 0.00081 s	0.0648 ± 0.005 s	1.04 ± 0.08
CellLists/CPU/nsubdiv = 1/foreach_source	0.0675 ± 0.00068 s	0.0639 ± 0.00082 s	1.06 ± 0.017
CellLists/CPU/nsubdiv = 1/iterator_interface	0.106 ± 0.025 s	0.0766 ± 0.00091 s	1.39 ± 0.33
CellLists/CPU/nsubdiv = 1/set_elements!	1.98 ± 0.066 ms	1.97 ± 0.073 ms	1 ± 0.05
CellLists/CPU/nsubdiv = 2/foreach_pair (SIMD/sorted)	0.107 ± 0.0018 s	0.108 ± 0.0011 s	0.996 ± 0.019
CellLists/CPU/nsubdiv = 2/foreach_pair (SIMD/unsorted)	0.255 ± 0.0025 s	0.263 ± 0.011 s	0.969 ± 0.042
CellLists/CPU/nsubdiv = 2/foreach_pair (sorted)	0.096 ± 0.001 s	0.0989 ± 0.0052 s	0.97 ± 0.052
CellLists/CPU/nsubdiv = 2/foreach_pair (unsorted)	0.253 ± 0.0097 s	0.252 ± 0.0039 s	1.01 ± 0.041
CellLists/CPU/nsubdiv = 2/foreach_source	0.25 ± 0.0028 s	0.248 ± 0.0021 s	1.01 ± 0.014
CellLists/CPU/nsubdiv = 2/iterator_interface	0.328 ± 0.0017 s	0.328 ± 0.0016 s	0.999 ± 0.0072
CellLists/CPU/nsubdiv = 2/set_elements!	5.59 ± 0.22 ms	6.04 ± 1.3 ms	0.926 ± 0.21
CellLists/OpenCLBackend/nsubdiv = 1/foreach_pair (sorted)	0.0633 ± 0.00058 s	0.064 ± 0.00055 s	0.989 ± 0.012
CellLists/OpenCLBackend/nsubdiv = 1/foreach_pair (unsorted)	0.0651 ± 0.00044 s	0.067 ± 0.00055 s	0.973 ± 0.01
CellLists/OpenCLBackend/nsubdiv = 1/set_elements!	2.32 ± 0.055 ms	2.33 ± 0.059 ms	0.995 ± 0.035
CellLists/OpenCLBackend/nsubdiv = 2/foreach_pair (sorted)	0.185 ± 0.00061 s	0.183 ± 0.0034 s	1.01 ± 0.019
CellLists/OpenCLBackend/nsubdiv = 2/foreach_pair (unsorted)	0.208 ± 0.00091 s	0.207 ± 0.0023 s	1.01 ± 0.012
CellLists/OpenCLBackend/nsubdiv = 2/set_elements!	7.16 ± 0.18 ms	7.22 ± 0.21 ms	0.992 ± 0.038
Diagnostics/energy_flux	1.13 ± 0.0053 s	1.18 ± 0.0089 s	0.955 ± 0.0085
Diagnostics/energy_injection_rate	8.65 ± 0.14 ms	8.67 ± 0.17 ms	0.999 ± 0.025
Diagnostics/energy_spectrum	2.25 ± 0.093 ms	2.25 ± 0.1 ms	1 ± 0.062
Diagnostics/energy_transfer_matrix	1.47 ± 0.22 s	1.55 ± 0.17 s	0.952 ± 0.18
Diagnostics/helicity	5.51 ± 0.071 ms	5.5 ± 0.053 ms	1 ± 0.016
Diagnostics/kinetic_energy	0.874 ± 0.015 ms	0.873 ± 0.016 ms	1 ± 0.025
Reconnections/ReconnectBasedOnDistance	3.14 ± 0.064 s	3.13 ± 0.11 s	1 ± 0.042
Reconnections/ReconnectFast	0.259 ± 0.02 s	0.265 ± 0.02 s	0.979 ± 0.1
Refinement/RefineBasedOnSegmentLength	7.17 ± 2.3 ms	6.86 ± 2 ms	1.05 ± 0.45
Timestepping/forcing	22.1 ± 3.1 ms	22.1 ± 3 ms	0.999 ± 0.2
Timestepping/step!	2.66 ± 0.064 s	2.72 ± 0.072 s	0.978 ± 0.035
time_to_load	1.6 ± 0.0064 s	1.6 ± 0.0071 s	0.998 ± 0.006

Memory benchmarks

	master	695096f...	master / 695096f...
BiotSavart/add_local_integrals!	7.04 k allocs: 4.16 MB	7.04 k allocs: 4.16 MB	1
BiotSavart/add_point_charges!	6.04 k allocs: 0.677 MB	6.04 k allocs: 0.677 MB	1
BiotSavart/velocity	14.1 k allocs: 4.89 MB	14.9 k allocs: 4.94 MB	0.99
BiotSavart/velocity + streamfunction	14.6 k allocs: 4.99 MB	15.5 k allocs: 5.05 MB	0.988
CellLists/CPU/nsubdiv = 1/foreach_pair (SIMD/sorted)	0.066 k allocs: 5.97 kB	0.066 k allocs: 5.97 kB	1
CellLists/CPU/nsubdiv = 1/foreach_pair (SIMD/unsorted)	22 allocs: 2.22 kB	22 allocs: 2.22 kB	1
CellLists/CPU/nsubdiv = 1/foreach_pair (sorted)	0.066 k allocs: 5.97 kB	0.066 k allocs: 5.97 kB	1
CellLists/CPU/nsubdiv = 1/foreach_pair (unsorted)	22 allocs: 2.22 kB	22 allocs: 2.22 kB	1
CellLists/CPU/nsubdiv = 1/foreach_source	22 allocs: 2.16 kB	22 allocs: 2.16 kB	1
CellLists/CPU/nsubdiv = 1/iterator_interface	1.2 M allocs: 0.0834 GB	22 allocs: 2.16 kB	4.06e+04
CellLists/CPU/nsubdiv = 1/set_elements!	0.044 k allocs: 3.69 kB	0.044 k allocs: 3.69 kB	1
CellLists/CPU/nsubdiv = 2/foreach_pair (SIMD/sorted)	0.066 k allocs: 5.97 kB	0.066 k allocs: 5.97 kB	1
CellLists/CPU/nsubdiv = 2/foreach_pair (SIMD/unsorted)	22 allocs: 2.22 kB	22 allocs: 2.22 kB	1
CellLists/CPU/nsubdiv = 2/foreach_pair (sorted)	0.066 k allocs: 5.97 kB	0.066 k allocs: 5.97 kB	1
CellLists/CPU/nsubdiv = 2/foreach_pair (unsorted)	22 allocs: 2.22 kB	22 allocs: 2.22 kB	1
CellLists/CPU/nsubdiv = 2/foreach_source	22 allocs: 2.16 kB	22 allocs: 2.16 kB	1
CellLists/CPU/nsubdiv = 2/iterator_interface	22 allocs: 2.16 kB	22 allocs: 2.16 kB	1
CellLists/CPU/nsubdiv = 2/set_elements!	0.044 k allocs: 3.69 kB	0.044 k allocs: 3.69 kB	1
CellLists/OpenCLBackend/nsubdiv = 1/foreach_pair (sorted)	0.641 k allocs: 0.0398 MB	0.641 k allocs: 0.0398 MB	1
CellLists/OpenCLBackend/nsubdiv = 1/foreach_pair (unsorted)	0.122 k allocs: 9.11 kB	0.122 k allocs: 9.11 kB	1
CellLists/OpenCLBackend/nsubdiv = 1/set_elements!	0.731 k allocs: 0.0433 MB	0.731 k allocs: 0.0433 MB	1
CellLists/OpenCLBackend/nsubdiv = 2/foreach_pair (sorted)	0.641 k allocs: 0.0398 MB	0.641 k allocs: 0.0398 MB	1
CellLists/OpenCLBackend/nsubdiv = 2/foreach_pair (unsorted)	0.122 k allocs: 9.11 kB	0.122 k allocs: 9.11 kB	1
CellLists/OpenCLBackend/nsubdiv = 2/set_elements!	0.731 k allocs: 0.0433 MB	0.731 k allocs: 0.0433 MB	1
Diagnostics/energy_flux	0.0556 M allocs: 8.59 MB	0.0568 M allocs: 8.66 MB	0.992
Diagnostics/energy_injection_rate	2.06 k allocs: 0.203 MB	2.06 k allocs: 0.203 MB	1
Diagnostics/energy_spectrum	26 allocs: 3.22 kB	26 allocs: 3.22 kB	1
Diagnostics/energy_transfer_matrix	25 k allocs: 0.0392 GB	26.3 k allocs: 0.0393 GB	0.998
Diagnostics/helicity	1.71 k allocs: 0.0703 MB	1.71 k allocs: 0.0703 MB	1
Diagnostics/kinetic_energy	2.05 k allocs: 0.0805 MB	2.05 k allocs: 0.0805 MB	1
Reconnections/ReconnectBasedOnDistance	4.18 M allocs: 0.312 GB	4.18 M allocs: 0.312 GB	1
Reconnections/ReconnectFast	7.49 M allocs: 0.196 GB	7.49 M allocs: 0.196 GB	1
Refinement/RefineBasedOnSegmentLength	5.59 k allocs: 0.263 MB	5.59 k allocs: 0.263 MB	1
Timestepping/forcing	2.04 k allocs: 0.102 MB	2.29 k allocs: 0.119 MB	0.855
Timestepping/step!	2.24 M allocs: 0.0656 GB	2.24 M allocs: 0.0658 GB	0.996
time_to_load	0.149 k allocs: 11.1 kB	0.149 k allocs: 11.1 kB	1

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.97297% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.90%. Comparing base (e27b3a4) to head (695096f).

Files with missing lines	Patch %	Lines
src/BiotSavart/host_device_transfers.jl	47.36%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   93.01%   92.90%   -0.11%     
==========================================
  Files         126      126              
  Lines        8407     8433      +26     
==========================================
+ Hits         7820     7835      +15     
- Misses        587      598      +11     

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