/[escript]/trunk/doc/user/execute.tex
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revision 2736 by caltinay, Mon Nov 2 04:15:27 2009 UTC revision 2780 by jfenwick, Thu Nov 26 03:54:24 2009 UTC
# Line 127  which is read by all processors needs to Line 127  which is read by all processors needs to
127  In general a script based on the \escript module does not require modifications when running under \MPI. However, one needs to be careful if other modules are used.  In general a script based on the \escript module does not require modifications when running under \MPI. However, one needs to be careful if other modules are used.
128    
129  When \MPI is used on more than one process ($\var{nn} \cdot \var{np} >1$) the user needs to keep in mind that several copies of his script are executed at the same time  When \MPI is used on more than one process ($\var{nn} \cdot \var{np} >1$) the user needs to keep in mind that several copies of his script are executed at the same time
130  \footnote{In case of OpenMP only one copy is running but \escript temporarily spawns threads.} while data exchange is performed through the \escript module. At any time,  \footnote{In case of OpenMP only one copy is running but \escript temporarily spawns threads.} while data exchange is performed through the \escript module.
131  \escript assumes that an argument of the type \var{int}, \var{float}, \var{str}  
132  and \numpy has an identical value across all processors. All  This has three main implications:
133  values of these types returned by \escript have the same value on all processors.  \begin{enumerate}
134  If values produced by other modules are used as arguments the user has to make sure that the argument values are identical on all processors. For instance, the usage of a random number generator to create argument values bears the risk that the value may depend on the processor.   \item most arguments (\var{Data} excluded) should the same values on all processors. eg \var{int}, \var{float}, \var{str}
135    and \numpy parameters.
136    \item the same operations will be called on processors.
137    \item different processors may store different amounts of information.
138    \end{enumerate}
139    
140    With a few exceptions\footnote{getTupleForDataPoint}, values of types \var{int}, \var{float}, \var{str}
141    and \numpy returned by \escript will have the same value on all processors.
142    If values produced by other modules are used as arguments, the user has to make sure that the argument values are identical
143     on all processors. For instance, the usage of a random number generator to create argument values bears the risk that
144    the value may depend on the processor.
145    
146    Some operations in \escript require communication with all processors executing the job.
147    It is not always obvious which operations these are.
148    For example, \var{Lsup} returns the largest value on all processors.
149    \var{getValue} on \var{Locator} may refer to a value stored on another processor.
150    For this reason it is better if scripts do not have conditional operations (which manipulate data) based on which processor the script is on.
151    Crashing or hanging scripts can be an indication that this has happened.
152    
153    It is not always possible to divide data evenly amongst processors.
154    In fact some processors might not have any data at all.
155    Try to avoid writing scripts which iterate over data points,
156    instead try to describe the operation you wish to perform as a whole.
157    
158  Special attention is required when using files on more than one processor as  Special attention is required when using files on more than one processor as
159  several processors access the file at the same time. Opening a file for  several processors access the file at the same time. Opening a file for
# Line 150  f = FileWriter('test.txt') Line 172  f = FileWriter('test.txt')
172  f.write('test message')  f.write('test message')
173  f.close()  f.close()
174  \end{python}  \end{python}
175  It is highly recommendable to use this class rather than the built-in \function{open}  We strongly recommend using this class rather than the built-in \function{open}
176  function as it will guarantee a script which will run in single processor mode as well as under \MPI.  function as it will guarantee a script which will run in single processor mode as well as under \MPI.
177    
178  If there is the situation that one of the processors is throwing an exception,  If there is the situation that one of the processors is throwing an exception,
179  for instance as opening a file for writing fails, the other processors  for instance as opening a file for writing fails, the other processors
180  are not automatically made aware of this as \MPI  are not automatically made aware of this since \MPI
181  is not handling exceptions. However, \MPI will terminate the other processes but  dioes not handle exceptions.
182    However, \MPI will terminate the other processes but
183  may not inform the user of the reason in an obvious way. The user needs to inspect the  may not inform the user of the reason in an obvious way. The user needs to inspect the
184  error output files to identify the exception.  error output files to identify the exception.

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