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.

XRD_PARALLELEVTLOOP=4

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

XRD_PARALLELEVTLOOP=1

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

XRD_WORKERTHREADS=1 XRD_PARALLELEVTLOOP=1

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:

1. Setting XRD_PARALLELEVTLOOP=1 makes the xrdcp process use 7 thread, of which 3 are explicable by the default value of XRD_WORKERTHREADS, 1 is the event loop, but I still can't reason about the other 3 threads.
2. The two variables seem to be independently adding threads to the 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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