Parallel IO

[henry2004y]

For the static virtual satellite tracing (845 frames), I do not see decent improvements with 2, 4 threads over 1 thread:

Threads	Julia [s]
1	78
2	70
4	68

---

With more tests, I found that the login nodes Turso as well as compute nodes Vorna have large variations in performance, which also affects multi-threading. Here is another set of run results:

200 frames, serial:
  2.052 s (15663 allocations: 2.95 MiB)
200 frames, 2 threads:
  1.490 s (15080 allocations: 2.94 MiB)
845 frames, serial:
  45.341 s (66213 allocations: 12.49 MiB)
845 frames, 2 threads:
  33.814 s (63695 allocations: 12.42 MiB)

I don't know why it doesn't scale linearly.

Vlasiator.jl/src/vlsv/vlsvreader.jl

Lines 652 to 673 in b76474b

	function extractsat(files::AbstractVector{String}, var::String, cid::Int)
	v = open(files[1], "r") do fid
	footer = getfooter(fid)
	nodevar = findall("//VARIABLE", footer)
	T, _, _, _, vsize = getvarinfo(nodevar, var)
	Array{T,2}(undef, vsize, length(files))
	end
	for i in eachindex(files)
	fid = open(files[i], "r")
	footer = getfooter(fid)
	nodevar = findall("//VARIABLE", footer)
	cellid = getcellid(fid, nodevar)
	c_ = findfirst(isequal(cid), cellid)
	_, offset, _, dsize, vsize = getvarinfo(nodevar, var)
	seek(fid, offset + (c_ - 1)*vsize*dsize)
	@views read!(fid, v[:,i])
	end
	v
	end