Background execution

    

NESL allows background execution of expressions. This is most useful when evaluating expressions that might take a long time to complete. It is also useful on supercomputers that allow more machine resources (memory, processors, runtime) to be used for batch jobs than for interactive jobs.

name &= exp [,mem := n] [,max_time := n] [,config := config]; 
This evaluate the expression exp in the background and assigns the result to the background variable name. The NESL prompt returns soon as the job has been submitted. For example:

<Nesl> a &= sum([0:100000]);

Compiling..Writing..Submitting..
[1] 12782
background a : int
<Nesl>

The result is retrieved using the get command described below. The &= command has three optional arguments:

• mem: This specifies the amount of memory the job will need. It uses the same units as used by the mem_size field of a configuration, and defaults to the value specified in the current configuration.

• max_time: This specifies the maximum amount of time (in seconds) that the job will run. It is normally used when submitting jobs on supercomputers, since it can be used as a safety cap in case the job goes into an infinite loop.

• config: This is used to specify a configuration other than the current configuration on which to run the job.

get name;  
This is used to retrieve a background variable. If the job is not completed, the message ``Variable waiting for result'' will be printed. If the job is completed, all output generated during execution will be printed, and if there was no error the result will be assigned to the variable name. The status of background jobs can be examined with the show status command described below.