748
D. Froidevaux
16.7 Computing and Software
The ATLAS computing and software infrastructure is clearly of paramount importance. The functionality and flexibility of both will determine, to a very large extent,
the rate and quality of the physics output of the experiment. As expected, there are
numerous challenges to be addressed also in these two areas.
On the computing side, the LHC experiments represent a new frontier in highenergy physics. What is genuinely new at the LHC is that the required level of
computing resources can only be provided by a number of computing centres
working in unison with the CERN on-site computing facilities. Off-site facilities
will thus be vital to ATLAS operation to an extent that is completely different
from previous large experiments. Usage of these off-site facilities necessitates the
substantial use of Grid computing concepts and technologies [33]. The latter allow
for the sharing of the responsibility for processing and storing the data, but also for
providing the same level of data access, and making available the same amount of
computing resources to all members of the collaboration.
A second challenge for computing is the development and operation of a data
storage and management infrastructure which is able to meet the demands of a
yearly data volume of O(10) Petabytes and is used by both organised data processing
and individual analysis activities, which are geographically dispersed around the
world.
The architecture which is now in place is geographically distributed and relies
on four levels or tiers, as illustrated in Fig. 16.18. Primary event processing occurs
at CERN in the so-called Tier-0 facility. Raw data are archived at CERN and sent
(along with the reconstructed data) to the Tier-1 centres around the world. These
centres share among themselves the archiving of a second copy of the raw data,
while they also provide the reprocessing capacity and access to the various versions
of the reconstructed data, and allow scheduled analysis of the latter by physics
analysis groups. A more numerous set of Tier-2 centres, which are smaller but
still have substantial CPU and disk storage resources, provide capacity for analysis,
calibration activities and Monte Carlo simulation. Datasets, which are produced at
the Tier-1 centres by physics groups, are copied to the Tier-2 facilities for further
analysis. Tier-2 centres rely upon the Tier-1 centres for access to large datasets and
secure storage of the new data they produce. A final level in the hierarchy is provided
by individual group clusters used for analysis: these are the Tier-3 centres.
The ATLAS collaboration also relies on the CERN Analysis Facility (CAF)
for algorithmic development work and a number of short-latency data-intensive
calibration and alignment tasks. This facility is also expected to provide additional
analysis capacity with, as an example, re-processing of the express-stream data and
short turn-around analysis jobs.
D. Froidevaux
16.7 Computing and Software
The ATLAS computing and software infrastructure is clearly of paramount importance. The functionality and flexibility of both will determine, to a very large extent,
the rate and quality of the physics output of the experiment. As expected, there are
numerous challenges to be addressed also in these two areas.
On the computing side, the LHC experiments represent a new frontier in highenergy physics. What is genuinely new at the LHC is that the required level of
computing resources can only be provided by a number of computing centres
working in unison with the CERN on-site computing facilities. Off-site facilities
will thus be vital to ATLAS operation to an extent that is completely different
from previous large experiments. Usage of these off-site facilities necessitates the
substantial use of Grid computing concepts and technologies [33]. The latter allow
for the sharing of the responsibility for processing and storing the data, but also for
providing the same level of data access, and making available the same amount of
computing resources to all members of the collaboration.
A second challenge for computing is the development and operation of a data
storage and management infrastructure which is able to meet the demands of a
yearly data volume of O(10) Petabytes and is used by both organised data processing
and individual analysis activities, which are geographically dispersed around the
world.
The architecture which is now in place is geographically distributed and relies
on four levels or tiers, as illustrated in Fig. 16.18. Primary event processing occurs
at CERN in the so-called Tier-0 facility. Raw data are archived at CERN and sent
(along with the reconstructed data) to the Tier-1 centres around the world. These
centres share among themselves the archiving of a second copy of the raw data,
while they also provide the reprocessing capacity and access to the various versions
of the reconstructed data, and allow scheduled analysis of the latter by physics
analysis groups. A more numerous set of Tier-2 centres, which are smaller but
still have substantial CPU and disk storage resources, provide capacity for analysis,
calibration activities and Monte Carlo simulation. Datasets, which are produced at
the Tier-1 centres by physics groups, are copied to the Tier-2 facilities for further
analysis. Tier-2 centres rely upon the Tier-1 centres for access to large datasets and
secure storage of the new data they produce. A final level in the hierarchy is provided
by individual group clusters used for analysis: these are the Tier-3 centres.
The ATLAS collaboration also relies on the CERN Analysis Facility (CAF)
for algorithmic development work and a number of short-latency data-intensive
calibration and alignment tasks. This facility is also expected to provide additional
analysis capacity with, as an example, re-processing of the express-stream data and
short turn-around analysis jobs.