CASINO manual - Theory of Condensed Matter

• Track down bugs either by (i) inserting write statements in the code and recompiling or (ii)
using a debugger (such as gdb) in conjunction with binaries that have been compiled with the
debugging flags set, which you can use by passing the --debugger=debugger-binary option to
runqmc.
• Any utilities you write should also be tested by compiling and running with full debugging flags.
Use, e.g., man f90 to find out about the available debugging flags for your compiler.
• If you get stumped and have to send a bug report to us in Cambridge, then please make sure that
everything we need to reproduce the problem is attached or otherwise available. Our record so
far is an exchange of 24 emails—beginning with one stating ‘casino doesn’t work!’—before we
discovered precisely what it was the user was complaining about (that is, what the problem was,
rather than the solution). We shall leave it to you to guess which distinguished London-based
professor we are referring to.
A.4 Performance
casino can print a detailed timing analysis at the end of the output file when it has finished a
calculation. This information can be useful to the programmer in deciding what optimizations to do
to make the program run faster. It is generally disabled by default since, annoyingly, the process
of calculating the timings slows down the code significantly. To activate the detailed analysis, set
timing info=T in the input file.
The total time is printed as ‘CPU’ time, and the ratio between ‘System’ and ‘User’ time is displayed
as well. These timings are broken down according to specific tasks, although by no means all activities
are timed—only those thought likely to contribute significantly. For very small systems such as atoms
a significant amount of CPU time will remain unaccounted for since the traditional heavyweight
tasks are very fast. For medium and large systems, often the calculation of the orbitals and their
derivatives will dominate, closely followed by Jastrow factors and Ewald interactions. In systems with
pseudopotentials, the nonlocal integration is very expensive indeed (since it calls the orbital evaluation
routine a lot) and this expense will increase the larger the nonlocal cutoff radius (lower nlcutofftol
in the input file to see the effect of this).
The ‘User’ time is the time spent executing the instructions that make up the user code, including,
e.g., calls to subroutine libraries linked to the code.
The ‘System’ time is the time spent in kernel mode, processing I/O requests, for example, or other
situations which require the intervention of the operating system.
Together these comprise the total CPU time. Note that the User time is usually significantly greater
than the System time. Indeed, a high ratio of system to user CPU time may indicate a problem.
Exceptions such as page faults, misaligned memory references, and floating-point exceptions consume
a large amount of system time. Note that time spent doing things like waiting for input from the
terminal, seeking on a disk, or rewinding a tape does not appear in the CPU time; these situations
do not require the CPU and thus it can (and generally does) work on something else while waiting.
The elapsed (‘wallclock’ or ‘real’) time is simply the total real time that passed during the execution
of the program. This will typically be greater than the total CPU time due primarily to sharing the
CPU with other programs. In addition, programs that perform a large number of I/O operations
requiring more memory bandwidth than is available on the machine (i.e., the CPU spends significant
time waiting for data to arrive from memory, or is paged or swapped) will show a relatively low ratio
of CPU time used to elapsed time.
A.5 Bug reports
We cannot of course guarantee that casino is free of bugs, and if you find one, please tell us. If
you don’t know what the problem is yourself, than please include as much detailed information you
can about the nature of the error in order to ensure a quick fix. In particular, please send us all the
relevant input files to enable us to reproduce your problem.
A.6 Requests for new features
We are of course always happy to discuss such requests.
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