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Previous Year Question (PYQs)
4
Consider the following program fragment that deals with a table T with 17 rows and 1024 columns, computing an average for each column and printing it to screen, where i is row index and j is column index:
for j = [0.....1023] {
temp = 0;
for i = [0.....16]:
temp = temp + T[i][j];
print(temp/17.0);
}
T[i][j] and temp are 32 bit floating point values and memory is word addressable. The temporary variable temp is kept in a processor register so access to temp does not involve a memory reference. The main memory is paged and holds 16 pages of size 1024 words, the page replacement policy is least recently used.
Consider that T is stored in column major format, how many page faults will be encountered?
Solution
Total elements in table T:
$17 \times 1024 = 17408$
Page size:
$1024$ words
Number of pages required:
$\frac{17408}{1024}=17$
In column major format, the program accesses elements column by column, so memory access is sequential.
Each page will cause one page fault.
Therefore,
Total page faults = $17$
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