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# Homework 1: Odd-Even Sort

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Homework 1: Odd-Even Sort

Goal
This assignment helps you get familiar with MPI by implementing odd-even sort. We encourage
you to optimize your program by exploring different parallelizing strategies.
Problem Description
In this assignment, you are required to implement odd-even sort algorithm using MPI Library
under the restriction that MPI process can only send data messages to its neighbor processes.
Odd-even sort is a comparison sort which consists of two main phases: even-phase and odd-phase.
In even-phase, all even/odd indexed pairs of adjacent elements are compared. If a pair is in the
wrong order, the elements are switched. Similarly, the same process repeats for odd/even indexed
pairs in odd-phase. The odd-even sort algorithm works by alternating these two phases until the list
is completely sorted.
In order for you to understand this algorithm better, the execution flow of odd-even sort is
illustrated step by step as below: (We are sorting the list into ascending order in this case)
1. [Even-phase] even/odd indexed adjacent elements are grouped into pairs.
2. [Even-phase] elements in a pair are switched if they are in the wrong order.
3. [Odd-phase] odd/even indexed adjacent elements are grouped into pairs.
4. [Odd-phase] elements in a pair are switched if they are in the wrong order.
5. Run even-phase and odd-phase alternatively until no swap-work happens in both even-phase
and odd phase.
Input / Output Format
1. Your programs are required to read an input file, and generate output in another file.
2. Your program accepts 3 input parameters. They are:
1. (integer) the size of the array n (1 <= n <= 536870911)
2. (string) the input file name
3. (string) the output file name
We will run your program with an equalviant of:
srun -nNPROC ./hw1 n inputfile outputfile
Where NPROC is the number of processes
3. The input file contains n 32-bit floats in binary format. The first 4 bytes represents the first
floating point number, the fifth to eighth byte represents the second one, and so on. Please refer
to the sample input files
4. The output file should follow the same format of the input file. Please refer to the sample output
files.
Note:
The float here refers to IEEE754 binary32, as known as single-precision floating-point.
You can use the float type in C/C++.
The input is guaranteed to contain none of the following:
-INF
+INF
NAN
Any other valid float values are possible to show up in the input.
Optimization Hints & Rules
During the odd/even iterations, you can only send & receive array data to & from neighboring
MPI processes. There is no restriction on how much data you can send in 1 iteration.
There is no restriction on how the data should be sorted within an MPI process. You can use
library functions for this.
There is no restriction on how the data can be passed between MPI processes before & after the
odd/even iterations (e.g. when doing I/O).
There is no restriction on how messages other than array data (for example, the terminate
condition) can be passed betwenn MPI processes.
You can try to overlap computation time and communication time as much as possible.
If you are not sure whether your implementation follows the rules, please discuss with TA for
approval.
1. Correctness (70%)
There are 40 test cases at /home/ipl19/x/hw1/cases. You get 1.75 points for each case
passed.
2. Performance (20%)
You need to pass all the correctness test cases in order to get performance points.
Points are given according to the relative performance of your program among all the
students.
3. Demo (10%)
Each student is given 5 minutes to explain the implementation followed by some questions
from TA.
Points are given according to your understanding and explanation of your code, and your
answers of the TA questions.
Submission
Put your source code under at ~/homework/hw1 in apollo31:
~/homework/hw1/Makefile
~/homework/hw1/hw1.cc or ~/homework/hw1/hw1.c
Then use hw1-judge to check & submit your code, before the deadline. You can submit as many
times as you want.
View submissions at http://ipl.cs.nthu.edu.tw/s/hw1.
If you found any problems with the judge script or the scoreboard, contact TA immediately.
Note:
Example Makefile at /home/ipl19/x/hw1/Makefile
Example code for MPI-IO at /home/ipl19/x/hw1/mpiio.cc
Testcases at /home/ipl19/x/hw1/cases/
You can use the hw1-floats command to view the values in the input/output files.
Warning:
You are allowed to discuss and exchange ideas with others, but you are required to write the
code on your own. You’ll got 0 points if we found you cheating.

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