Parallel Algorithm - Analysis

Analysis of an algorithm helps us determine whether the algorithm is useful or not. Generally, an algorithm is analyzed based on its execution time (Time Complexity) and the amount of space (Space Complexity) it requires.

Since we have sophisticated memory devices available at reasonable cost, storage space is no longer an issue. Hence, space complexity is not given so much of importance.

Parallel algorithms are designed to improve the computation speed of a computer. For analyzing a Parallel Algorithm, we normally consider the following parameters −

  • Time complexity (Execution Time),
  • Total number of processors used, and
  • Total cost.

Time Complexity

The main reason behind developing parallel algorithms was to reduce the computation time of an algorithm. Thus, evaluating the execution time of an algorithm is extremely important in analyzing its efficiency.

Execution time is measured on the basis of the time taken by the algorithm to solve a problem. The total execution time is calculated from the moment when the algorithm starts executing to the moment it stops. If all the processors do not start or end execution at the same time, then the total execution time of the algorithm is the moment when the first processor started its execution to the moment when the last processor stops its execution.

Time complexity of an algorithm can be classified into three categories−

  • Worst-case complexity − When the amount of time required by an algorithm for a given input is maximum.

  • Average-case complexity − When the amount of time required by an algorithm for a given input is average.

  • Best-case complexity − When the amount of time required by an algorithm for a given input is minimum.

Asymptotic Analysis

The complexity or efficiency of an algorithm is the number of steps executed by the algorithm to get the desired output. Asymptotic analysis is done to calculate the complexity of an algorithm in its theoretical analysis. In asymptotic analysis, a large length of input is used to calculate the complexity function of the algorithm.

Note − Asymptotic is a condition where a line tends to meet a curve, but they do not intersect. Here the line and the curve is asymptotic to each other.

Asymptotic notation is the easiest way to describe the fastest and slowest possible execution time for an algorithm using high bounds and low bounds on speed. For this, we use the following notations −

  • Big O notation
  • Omega notation
  • Theta notation

Big O notation

In mathematics, Big O notation is used to represent the asymptotic characteristics of functions. It represents the behavior of a function for large inputs in a simple and accurate method. It is a method of representing the upper bound of an algorithm’s execution time. It represents the longest amount of time that the algorithm could take to complete its execution. The function −

f(n) = O(g(n))

iff there exists positive constants c and n0 such that f(n) ≤ c * g(n) for all n where n ≥ n0.

Omega notation

Omega notation is a method of representing the lower bound of an algorithm’s execution time. The function −

f(n) = Ω (g(n))

iff there exists positive constants c and n0 such that f(n) ≥ c * g(n) for all n where n ≥ n0.

Theta Notation

Theta notation is a method of representing both the lower bound and the upper bound of an algorithm’s execution time. The function −

f(n) = θ(g(n))

iff there exists positive constants c1, c2, and n0 such that c1 * g(n) ≤ f(n) ≤ c2 * g(n) for all n where n ≥ n0.

Speedup of an Algorithm

The performance of a parallel algorithm is determined by calculating its speedup. Speedup is defined as the ratio of the worst-case execution time of the fastest known sequential algorithm for a particular problem to the worst-case execution time of the parallel algorithm.

speedup =
Worst case execution time of the fastest known sequential for a particular problem / Worst case execution time of the parallel algorithm

Number of Processors Used

The number of processors used is an important factor in analyzing the efficiency of a parallel algorithm. The cost to buy, maintain, and run the computers are calculated. Larger the number of processors used by an algorithm to solve a problem, more costly becomes the obtained result.

Total Cost

Total cost of a parallel algorithm is the product of time complexity and the number of processors used in that particular algorithm.

Total Cost = Time complexity × Number of processors used

Therefore, the efficiency of a parallel algorithm is −

Efficiency =
Worst case execution time of sequential algorithm / Worst case execution time of the parallel algorithm
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