Parallel 2dtree

This repository contained the parallel implementation of a 2d-tree produced for the Assignment 2 of FHPC course 2021-2022 @ DSSC, Units.

Description of the problem

Kd-trees are a data structures presented originally by Friedman, Bentley and Finkel in 1977 to represent a set of k-dimensional data in order to make them efficiently searchable.

In spite of its age, or more likely thanks to it and to the large amount of research and improvements accumulated in time, kd-trees are still a good pragmatical choice to perform knn (k-nearest neighbours) operations in many cases.

In this assignment you are required to write a parallel code that builds a kd-tree for k=2.

You must implement both the MPI and the OpenMP version. In order to simplify the task, the following 2 assumptions hold:

the data set, and hence the related kd-tree, can be assumed immutable, i.e. you can neglect the insertion/deletion operations;
the data points can be assumed to be homogeneously distributed in all the k dimensions.

You find a formal introduction 2 to kd-trees in the paper “The k-d tree data structure and a proof for neighborhood computation in expected logarithmic time”.

To have more information about the task visit : https://github.com/Foundations-of-HPC/Foundations_of_HPC_2021/tree/main/Assignment2

Compile

To compile do:

$ make

It is needed OpenMPI and MPI. This command produce the executable file named kdtree.x.

In order to use double instead float compile with:

make double

Run

To run with M MPI processes and N threads use the command:

export OMP_NUM_THREADS= <N>
mpirun -np <M> ./kdtree <PUT-HERE-SIZE-OF-THE-PROBLEM>

For example to run with 4 MPI processes, 9 threads and $10^8$ points do:

export OMP_NUM_THREADS= 9
mpirun -np 4  ./kdtree 100000000

Output

The program will produce a text file time where you can find the time taken in the different part of the program.

If the size is $<100$ the program will print the tree in ASCII on standard output.

If you want to generate a file with the tree saved as csv files add --print as arguments after the size of the problem:

mpirun -np 4  ./kdtree 100000000 --print

WARNING : This may take a lot of time and the file produced could be VERY large.

Structure of the source code

Files	content
`tree.c tree.h`	This files contain all the libraries, macro, structures and functions used from all the other files
`main.c main.h`	Main of the program and creation of the data-set
`mpi_tree.c mpi_tree.h`	Functions to divide the work from the master to all MPI processes and re-build back the tree to the master.
`omp_tree.c omp_tree.h`	Functions to build the tree using OMP and convert the tree to an array

Structure of the repository

In the branch main there is an implementatation of the tree using only arrays. However, this implementation has better performance.