Transcript Document 7282182

2001 Summer Student Lectures
Computing at CERN
Lecture 1 — Looking Around
CERN
Tony Cass — [email protected]
Acknowledgements

The choice of material presented here is entirely my own. However, I
could not have prepared these lectures without the help of
– Charles Granieri, Hans Grote, Mats Lindroos, Franck Di Maio, Olivier Martin,
Pere Mato, Bernd Panzer-Steindel, Les Robertson, Stephan Russenschuck, Frank
Schmidt, Archana Sharma and Chip Watson

who spent time discussing their work with me, generously provided
material they had prepared, or both.
For their general advice, help, and reviews of the slides and lecture
notes, I would also like to thank
– Marco Cattaneo, Mark Dőnszelmann, Dirk Düllmann, Steve Hancock, Vincenzo
Innocente, Alessandro Miotto, Les Robertson, Tim Smith and David Stickland.
CERN
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Some Definitions
General

Computing Power
CERN

– CERN Unit
– MIPS
– SPECint92, SPECint95

Networks
– Unix: WGS & PLUS
» CUTE, SUE, DIANE
– NICE

» CORE
» Normal (10baseT, 10Mb/s)
» Fast (100baseT, 100Mb/s)
» Gigabit (1000Mb/s)


CERN
– PCSF
Other

Data Storage, Data Access & Filesystems
– AFS, NFS, RFIO, HPSS, Objectivity[/DB]
bits and Bytes
– 1MB/s = 8Mb/s
Batch Systems
– Unix: SHIFT, CSF
– Ethernet
– FDDI
– HiPPI
Interactive Systems

CPUs
– Alpha, MIPS, PA-Risc, PowerPC, Sparc
– Pentium, Pentium II, Merced
Factors
– K=1024, K=1000
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How to start?
 Computing
is everywhere at CERN!
– experiment computing facilities, administrative computing, central
computing, many private clusters.
 How
–
–
–
–
should this lecture course be organised?
From a rigorous academic standpoint?
From a historical standpoint
...
From a physics based viewpoint
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Number of Users each Week
Weekly use of Interactive Platforms
1987-2001
12000
Windows 95
10000
Windows NT
WGS and PLUS
CERNVM
8000
VXCERN
6000
4000
2000
0
CERN
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Week
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Computer Usage at IN2P3
CERN
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Computing at CERN
 Computing
“purely for (experimental) physics” will be the
focus of the second two lectures of this series. Leaving this
area aside, other activities at CERN can be considered as
falling into one of three areas:
– administration,
– technical and engineering activities, and
– theoretical physics.
 We
will take a brief look at some of the ways in which
computing is used in these areas in the rest of this first lecture.
CERN
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Administrative Computing
 As
any organisation, CERN has all the usual
Administrative Data Processing activities such as
– salaries, human resource tracking, planning ...

Interesting aspects of this work at CERN are
– the extent to which many tasks are automated
– the heterogeneous nature of the platforms used when
performing administration related tasks.
 The
Web is, as in many other cases at CERN,
becoming the standard interface.
CERN
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Technical and Engineering Computing
 Engineers
and physicists working at CERN must
– design,
– build, and
– operate
both
– accelerators and
– detectors
for experimental physicists to be able to collect the data that
they need.
 As in many other areas of engineering design, computer aided
techniques are essential for the construction of today’s
advanced accelerators and detectors.
CERN
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Accelerator design issues
 Oliver
Brüning’s lectures will tell you more about accelerators.
For the moment, all we need to know is that
– particles travelling in bunches around an accelerator are bent by
dipole magnets and must be kept in orbit.
» Of course, they must be accelerated as well(!), but we don’t consider that here.
 Important
studies for LHC are
– magnet design
» how can we build the (superconducting) dipole magnets that are needed?
– transverse studies
» will any particles leave orbit? (and hit the magnets!)
– longitudinal studies
CERN
» how can we build the right particle bunches for LHC?
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LHC Magnet Design
2D field picture for LHC dipole coil
CERN
Pictures generated with ROXIE.
3D representation of dipole coil end with
magnetic field vectors
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Genetic Algorithms for Magnet Design
Original coil design.
Genetic Algorithm convergence plot.
The algorithm is designed to come up
with a number of alternative solutions
which can then be further investigated.
New coil design found using
a genetic algorithm.
This was further developed
using deterministic methods
and replaced the original
design.
CERN
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Transverse Studies
These images show how particles in a circulating bunch
move about in a 4 dimensional phase space: X position &
angle, Y position and angle.
Particles with chaotic trajectories
in this phase space have orbits
that are unbounded and so will hit
the walls of the accelerator
eventually.
Transverse studies of particle
motion attempt to understand
how these instabilities arise—and
how they can be reduced by
changes to the magnets.
Particles that move like
this in phase space stay in
the accelerator.
Those that move like this
don’t!
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Longitudinal Studies
 Not
all particles in a bunch have the same energy. Studies of
energy distribution show aspects of bunch shape.
– The energy of a particle affects its arrival time at the accelerating
cavity… which then in turn affects the energy.
 Need
to measure both energy and arrival
time, but can’t measure energy directly.
Measuring arrival times is easy
– but difficult to interpret successive slices.
 Tomography
techniques lead to a complete
picture
CERN
– like putting together X-ray slices through
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a person.
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Bunch splitting at the PS
CERN
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Accelerator Controls
Magnet current trace showing some of the many
beam types the PS can handle for different users.
PS Operator Control Windows
CERN
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Detector Design Issues
Detector designs also benefit from computer simulations.
CERN
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Detector Design Issues II
NA45 TPC with field cage
Electric field near the field cage
CERN
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Computing for Theory
Feynmann diagrams for some LHC processes…
… and some at LEP
Theoretical physicists could not calculate probabilities
for processes represented by Feynmann diagrams like
these without using symbolic algebra packages—e.g.
Maple or Mathematica.
These calculations are essential for two reasons:
1 As collision energies increase, and as the precision of experimental measurements increases
with increasing data volume, more Standard Model processes contribute to the data that is
collected.
2 Theorists need to calculate how the effects of theories beyond the standard model, e.g.
SUSY, could affect the data that is collected today.
CERN
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CERN and the World Wide Web

The World Wide Web started as a project to make information
more accessible, in particular, to help improve information
dissemination within an experiment.
– These aspects of the Web are widely used at CERN today. All
experiments have their own web pages and there are now web pages
dedicated to explaining about Particle Physics to the general public.
– In a wider sense, the web is being used to distribute graphical
information on system, accelerator and detector status. The release of
Java has given a big push to these uses.

Web browsers are also used to provide a common interface, e.g.
CERN
» currently to the administrative applications, and
» possibly in future as a batch job submission interface for PCs.
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19981999: What has changed?
 Hardware
– PC hardware has replaced RISC workstations for general purpose
computing.
 Software
– Future operating system developments clearly concentrated on Linux
and Windows
» Linux success & use of PCs is a positive feedback loop!
– Java is coming up fast on the inside lane.
» but C++ investment is large and C++/Java interoperability poor.
 Systems
Management
– Understand costs—one PC is cheap, but managing 200 is not!
CERN
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19992000: What has changed?
 Hardware
– PC hardware has replaced RISC workstations.
 Software
– Future operating system developments are clearly concentrated on
Linux. Windows 2000 will be deployed at CERN but is now a distant
3rd choice for physics
» Linux success & use of PCs is a positive feedback loop!
– Java is still coming up fast on the inside lane.
» C++ investment is still large and C++/Java interoperability is still poor.
 Systems
CERN
Management
– Understand costs—one PC is cheap, but managing 2000 is not!
– And do we have enough space, power and cooling for the LHC
equipment?
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20002001: What has changed? I
 Windows
2000 has arrived and Wireless Ethernet is arriving.
– Portable PCs replacing desktops.
– Integration of home directory, web files, working offline makes things
easier—just like AFS and IMAP revolutionised my life 8 years ago.
I
now have ADSL at home rather than ISDN.
– I am now outside the CERN firewall when connected from home but
this doesn’t matter so much with all my files cached on my portable.
» I just need to bolt on a wireless home network so I can work in the garden!
– The number of people connecting from outside the firewall will grow
CERN
» CERN will probably have to support Virtual Private Networks for privileged
access
» And users will have to worry about securing their home network against
hackers…
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Looking Around—Summary
 Computing
extends to all areas of work at CERN.
 In terms of CERN’s “job”, producing particle physics
results, computing is essential for
– the design, construction and operation of accelerators and
detectors, and
– theoretical studies, as well as
– the data reconstruction and analysis phases.
 The
major computing facilities at CERN, though, are
provided for particle physics work and these will be the
subject of the next two lectures.
CERN
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