xroot: improve performances

[sbinet]

this is possibly related to #399.

consider:

$> time root-dump root://ccxrootdgotest.in2p3.fr:9001/tmp/rootio/testdata/SMHiggsToZZTo4L.root > /dev/null

real	1m8.557s
user	0m14.863s
sys	0m3.562s

running over the same file but with http[s]:// (which, currently, downloads the whole file and then serves it locally):

$> time root-dump https://cern.ch/binet/big-file.root > /dev/null

real	0m5.454s
user	0m6.156s
sys	0m0.228s

a factor 10x is left on the floor.
(ok, not the same machine, on different networks, etc... but still)

[Moelf]

I was just about to post a performance question (apparently not the same file, but the branch is exactly the same type):

julia> using UnROOT
[ Info: Precompiling UnROOT [3cd96dde-e98d-4713-81e9-a4a1b0235ce9]

julia> const txrd = LazyTree("root://eospublic.cern.ch//eos/root-eos/cms_opendata_2012_nanoaod/Run2012B_DoubleMuParked.root", "Events");

julia> @time sum(txrd.nMuon[1:10^5])
  1.723536 seconds (3.45 M allocations: 173.713 MiB, 2.67% gc time, 29.63% compilation time)
0x0000000000039bbb

julia> @time sum(txrd.nMuon[1:10^5])
  1.174588 seconds (1.27 k allocations: 1.049 MiB)
0x0000000000039bbb

julia> const thttp = LazyTree("https://jiling.web.cern.ch/jiling/public/Run2012B_DoubleMuParked.root", "Events");

julia> @time sum(thttp.nMuon[1:10^5])
  0.847792 seconds (146.22 k allocations: 9.412 MiB, 8.69% compilation time)
0x0000000000039647

julia> @time sum(thttp.nMuon[1:10^5])
  0.287621 seconds (752 allocations: 1.516 MiB)
0x0000000000039647

julia> @time sum(thttp.nMuon[1:10^5])
  0.279659 seconds (719 allocations: 1.516 MiB)
0x0000000000039647

both of these have the same underlying logic beyond I/O source/sink. The HTTP one is not using Multipart GET so it's a fair comparison. Both of these are "CERN" but evidently not the same network, before I've noticed eospublic have pretty good infrastructure for the root:// so it's telling that it's this much slow.

One thing is very different is root:// doesn't seem to benefit from async call, I wonder if it's because I'm not using Go library correctly: should ReadAt() be capable of being called in parallel?

[sbinet]

#923 brought the gap of performances to:

  $> time root-dump root://ccxrootdgotest.in2p3.fr:9001/tmp/rootio/testdata/SMHiggsToZZTo4L.root > /dev/null

  real	0m7.279s
  user	0m8.221s
  sys	0m1.256s

  $> time root-dump https://cern.ch/binet/big-file.root > /dev/null

  real	0m5.454s
  user	0m6.156s
  sys	0m0.228s

(and that's with the https plugin downloading the whole file locally and serving it as a local file)

so there are still some performance bits to recoup.
but the 10x drop has been fixed.