DESCRIPTION

Students:  Please note instructions in paper copy of quiz.  Save your
files often, make sure OMSI fills your entire screen at all times, etc.

A question may not fully fit into your OMSI question box, which is not
scrollable.  You can try adjusting the relative size of the question and
answer boxes, but remember that clicking CopyQtoA will copy the entire
question box to the answer box.

QUESTION

Consider the pthreads example, pp.8ff.  Each thread is handling only one
value of 'nextbase' at a time. Say we define a global variable 'nbinc',
set in 'main()'.  This variable will replace the 2 in line 49, so as to
increase the granularity. We will then need to slightly modify lines
52-59.  Show how to do this.  They currently are

if (base <= lim)  {
   if (prime[base])  {
      crossout(base);
      work++;  
   }
}
else return work; 


QUESTION

Consider the pthreads example, pp.8ff.  Suppose I run it under gdb, with
n and nthreads equal to 100 and 2, and set a breakpoint at line 49.  I
then use gdb's thread command to switch to some other thread.  I will
then be informed as to what line that thread is currently executing.
Give the line number(s) that are likely to be indicated.  Note that
there will be three threads in all, including the one for main().

QUESTION 

Consider the following R code:

x <- c(5,12,13)
y <- x
y[2] <- 88
x <- y

State how many words of memory will be written, and state the name of
the relevant R policy.

QUESTION -ext .R -run 'Rscript ./omsi_answer4.R'

Write a "Snow" function with call form

maxpar(cls,x)

that will parallelize R's 'max()' function.  The latter does e.g.

> max(5,2)
[1] 5
> max(c(5,12,13,8))
[1] 13

The arguments in 'maxpar()' will be the "Snow" cluster and the input vector.

Following is partial code, with some lines omitted, as well as free-standing 
test code.  (You may alter those lines if you wish, but you must comply with 
the specified call form, and must leave the test code intact.) Hit CopyQtoA.

maxpar <- function(cls,x) {
   require(parallel)
   clusterExport(cls,'x',envir=environment())
   maxidxgrp <- function(igrp) max(x[igrp])
} 

library(parallel)
cls <- makeCluster(2)
z <- c(5,2,1,22,6,8,9)
print(maxpar(cls,z))