Definition

What is Program?

A program is a set of instructions and associated data that resides on the disk and is loaded by the operating system to perform some task. An executable file or a python script file are examples of programs. In order to run a program, the operating system's kernel is first asked to create a new process, which is an environment in which a program executes.

What is Process?

A process is a program in execution. A process is an execution environment that consists of instructions, user-data, and system-data segments, as well as lots of other resources such as CPU, memory, address-space, disk and network I/O acquired at runtime. A program can have several copies of it running at the same time but a process necessarily belongs to only one program.

What is Thread?

Thread is the smallest unit of execution in a process. A thread simply executes instructions serially. A process can have multiple threads running as part of it. Usually, there would be some state associated with the process that is shared among all the threads and in turn each thread would have some state private to itself. The globally shared state amongst the threads of a process is visible and accessible to all the threads, and special attention needs to be paid when any thread tries to read or write to this global shared state. There are several constructs offered by various programming languages to guard and discipline the access to this global state.

<aside> 💡 Note a program or a process are often used interchangeably but most of the times the intent is to refer to a process.

There's also the concept of "multiprocessing" systems, where multiple processes get scheduled on more than one CPU. Usually, this requires hardware support where a single system comes with multiple cores or the execution takes place in a cluster of machines. Processes don't share any resources amongst themselves whereas threads of a process can share the resources allocated to that particular process, including memory address space. However, languages do provide facilities to enable inter-process communication.

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Benefits of Threads

1. Higher throughput, though in some pathetic scenarios it is possible to have the overhead of context switching among threads steal away any throughput gains and result in worse performance than a single-threaded scenario. However such cases are unlikely and an exception, rather than the norm.
2. Responsive applications that give the illusion of multi-tasking.
3. Efficient utilization of resources. Note that thread creation is light-weight in comparison to spawning a brand new process. Web servers that use threads instead of creating new processes when fielding web requests, consume far fewer resources.

Problem With Threads

1. Usually very hard to find bugs, some that may only rear head in production environments
2. Higher cost of code maintenance since the code inherently becomes harder to reason about
3. Increased utilization of system resources. Creation of each thread consumes additional memory, CPU cycles for book-keeping and waste of time in context switches.
4. Programs may experience slowdown as coordination amongst threads comes at a price. Acquiring and releasing locks adds to program execution time. Threads fighting over acquiring locks cause lock contention.