Multithreading in C/C++

(can be used in C++ too but instead of using 'gcc' for compiling, use 'g++')

Using multiple thread to complete a certain task to improve code efficiency.

This program is compiled and executed on CentOS system

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Question

Write a program to find points (x,y) in an array such that between no two points there exists another value 'z' in the array which falls between 'x' and 'y'.

ie: the point z should not be between x and y.

---> x < z > y -- if x < y

---> x > z < y -- if x > y

Print the value of the points (x,y) which have no values between them and the distance between them.

Example:

Input :

     int arr[7] = {0,3,5,7,10,1,2};

Output:

      (0,1) or (1,0) = 1

      (3,5) or (5,3) = 2
      
      (3,2) or (2,3) = 1
      
      (5,7) or (7,5) = 2
      
      (7,10) or (10,7) = 3
      
      (1,2) or (2,1) = 1

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1. Using a Single Thread

FileName: singlethread_execution.c

To compile the singlethread_execution.c file

Compile: GCC compiler: gcc singlethread_execution.c -o singlethread_execution

--> An executable file pthread_multithreading will be created.

To execute the singlethread_execution file

Execute: ./singlethread_execution

2. Using POSIX Thread Library (pthread)

FileName: pthread_multithreading.c

To compile the pthread_multithreading.c file

Compile: GCC compiler: gcc pthread_multithreading.c -o pthread_multithreading -lpthread

Note: It is mandatory to compile the file using -lpthread as we are using the POSIX thread library, otherwise the compilation will give an error.

--> An executable file pthread_multithreading will be created.

To execute the pthread_multithreading file

Execute: ./pthread_multithreading

For the syntax of pthread_create() or pthread_join() refer Linux Programmers Manual

The Order of the Output may not be in the same sequence but it will definitely give all the possibilities because the order would depend on the scheduling of the thread.

3. Using OpenMP

OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared memory multiprocessing programming in C, C++, and Fortran, on most platforms, instruction set architectures (ISA) and operating systems, including Solaris, AIX, HP-UX, Linux, macOS, and Windows. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.

You might have noticed the changes that are required in the code to spawn threads using POSIX Thread Library. To overcome that a better option would be to use the OpenMP API.

Filename: openmp_multithreading.c

In your code, do not forget to #include<omp.h> library

To Compile and Execute use the same commands as of Single Threads.

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Whats the difference?

There is a big difference when it comes to Performance which is counted in terms of Latency and Throughput.

Latency (Execution time): Time taken to finish execution of a certain task or CPU intensive calculations the time taken to complete the task will be higher.

Throughput : Number of tasks executed in a fixed time.

To check execution time of the process use 'time' before the Execution command

eg: time ./pthread_multithreading

What is the need?

In the programs above it may not take much time with the given size of array. But try to increase the size of the array to 3000 or 5000 and then you will notice a significant difference in execution time.

This can also be tried on Multiplication of nxn matrices or any code that take lots of CPU time for calculations. It will definitely improve the performance.

To check number of CPU's and CPU threads and other CPU info

Use the command

lscpu

Number of Cores in your system (Shown by CPUS) = No of Sockets x No Cores per socket x No of Threads per Core

To check utilization of CPU threads

Use the command htop in terminal.

If htop is not installed on your Linux system then follow the instructions below

Installation of 'htop'

For RHEL, CentOS & Fedora Systems:

sudo yum install htop

For Debian Systems:

sudo apt-get install htop

Once htop is installed on your system you can simply run it with the command:

htop

You will see something like this

Since on my machine(CPU) has 8 cores we can see numbers between 1 to 8 which represent the CPU core and along with it is the Utilization of CPU (in %).

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This is how I have handled multithreading. I'm open to suggestions. Shoot in your questions or suggestions at samuelpio01@gmail.com