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GangaAn interface to the LHC computing grid

Matt WilliamsUniversity of Birmingham

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CERN and the LHC

● Largest particle physics experiment in the world

● 27km in circumference ● Over 100m underground ● Thousands of physicists● 100s of petabytes of data

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The Grid

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GANGA

● ~2001 LHCb started GANGA, an in-house tool– Specific to our needs

● By 2010 when the LHC turned on, it was used by many more– ATLAS, NA62, T2K and many more smaller experiements

● Python had always been the obvious choice– Used everywhere in Particle Physics (along with C++)

– Easy to create new plugins for experiments

● Can be scripted or with an IPython-based interactive console● Open source, released as GPL (like most CERN software)

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How is it used

j = Job(name = 'Example job')

j.application = Executable()

j.application.exe = File('test.sh')

j.outputfiles = [LocalFile('out.txt')]

j.backend = Local()

j.submit()

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Retrieving results

In [1]: j.peek()

total 200

-rw-r--r-- 1 phrfbi lhcb 0 Jun 22 2013 __syslog__

-rw-r--r-- 1 phrfbi lhcb 141999 Jun 22 2013 stdout

-rw-r--r-- 1 phrfbi lhcb 53671 Jun 22 2013 stderr

-rw-r--r-- 1 phrfbi lhcb 2463 Jun 22 2013 out.txt

-rw-r--r-- 1 phrfbi lhcb 135 Jun 22 2013 __jobstatus__

In [2]: j.peek('out.txt')

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Using the Grid

Just change backend from Local() to LCG()

Other backends are Interactive, PBS, LSF, SGE, Panda, Jedi, Dirac, Condor, ARC, CREAM...

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Input data and splitting

j = Job(name = 'Input splitter', backend = LCG())

j.application = Executable()

j.application.exe = File('analyse_data')

j.inputfiles = [LocalFile(f.strip()) for f in open('inputs.txt')]

j.splitter = SplitByFiles(filesPerJob = 10)

j.outputfiles = [LocalFile('histogram.root')]

j.submit()

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Mergers

j = Job(name = 'Merger', backend = LCG())

j.application = Executable()

j.application.exe = File('analyse_data')

j.inputfiles = [LocalFile(f.strip()) for f in open('inputs.txt')]

j.splitter = SplitByFiles(filesPerJob = 10)

j.outputfiles = [LocalFile('histogram.root')]

j.merger = RootMerger(files = ['histogram.root'])

j.submit()

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Job catalogue

In [1]: jobs

Out [1]:

fqid | status | name | subjobs | application | backend

----------------------------------------------------------------------

0 | completed | Example job | | Executable | Local

1 | running | Input splitter | 324 | Executable | LCG

2 | running | Merger | 324 | Executable | LCG

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Full API access

In [2]: jobs(2).status

Out [2]: running

In [3]: len([j for j in jobs(2).subjobs if j.status == 'completed'])

Out [3]: 24

In [4]: for subjob in jobs(2).subjobs:

if subjob.status == 'failed':

subjob.resubmit()

Can define custom functions in ~/.ganga.py which will be available at runtime

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Dealing with large files

j = Job(name = 'Large output', backend = Dirac())

j.application = Executable()

j.application.exe = File('analyse_data')

j.inputfiles = [DiracFile('input.root')]

j.outputfiles = [DiracFile('histogram.root')]

j.submit()

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Find more at cern.ch/ganga

Download code from cern.ch/ganga/download/

Thank you