There has been some interest in investigating the behaviour of XRD_PARALLELEVTLOOP
variable for ROOT workflows. I would like to report some simple tests here to ask for a clarification. All tests are doing a simple xrdcp
with variations of the two variables mentioned in the title.
In theory this should use 4 threads, but there are 10 instead
$ XRD_PARALLELEVTLOOP=4 xrdcp root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root .
[784MB/2.09GB][ 36%][==================> ][11.04MB/s]
$ ps aux | grep xrdcp
vpadulan 2875 14.5 0.4 698364 77920 pts/0 Sl+ 12:15 0:03 xrdcp root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root .
$ ps hH p 2875 | wc -l
10
This should use 1 thread, I see 7
$ XRD_PARALLELEVTLOOP=1 xrdcp root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root .
[184MB/2.09GB][ 8%][====> ][10.82MB/s]
$ ps aux | grep xrdcp
vpadulan 3000 20.0 0.2 608092 46488 pts/0 Sl+ 12:18 0:00 xrdcp root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root .
$ ps hH p 3000 | wc -l
7
I have found another environment variable in the xrootd docs https://xrootd.slac.stanford.edu/doc/xrdcl-docs/xrdcldocs.pdf described as "Number of threads processing user callbacks." with default value 3 . Setting both variables to 1 leads to 5 threads
$ XRD_WORKERTHREADS=1 XRD_PARALLELEVTLOOP=1 xrdcp root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root .
[192MB/2.09GB][ 8%][====> ][10.67MB/s]
$ ps aux | grep xrdcp
vpadulan 3036 17.3 0.2 460628 48240 pts/0 Sl+ 12:21 0:00 xrdcp root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root .
$ ps hH p 3036 | wc -l
5
So for now:
xrdcp
process when they are increased.To conclude, I would like to understand where those extra 3 threads could be coming from or get any better insight from you. Thanks a lot !
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