Thread

• Lightweight process responsible for executing a task

Process vs Thread

• Process is isolated. | Threads share memory.
• Process switching uses interface in operating system | Thread switching does not require to call a operating system and cause an interrupt to the kernel.
• PCB, Stack, Address space | PCB, TCB, Stack, common Address space
• A thread is a path of execution within a process. A process can contain multiple threads.
• more time for context switching. | less time for context switching.

PCB

• Process Id
• Process state
• Process priority
• Accounting Information
• Program Counter
• CPU Register
• PCB Pointers
• .....

TCB

• Thread ID
• Thread State
• CPU Information
• Program Counter
• Register contents
• Thread Priority
• Pointer to process that created this thread
• Pointers to other threads that were created by this thread

** https://www.geeksforgeeks.org/thread-control-block-in-operating-system/?ref=rp

Address Space

valid addresses in memory that are available for process.

Process in Java

https://www.geeksforgeeks.org/java-lang-process-class-java/

Links

• https://www.geeksforgeeks.org/thread-models-in-operating-system/?ref=rp
• https://www.geeksforgeeks.org/thread-in-operating-system/?ref=rp
• https://www.geeksforgeeks.org/mutex-lock-for-linux-thread-synchronization/?ref=rp
• https://www.geeksforgeeks.org/kernel-in-operating-system/?ref=rp
• https://www.geeksforgeeks.org/difference-between-user-level-thread-and-kernel-level-thread/?ref=rp
• https://www.geeksforgeeks.org/difference-between-process-and-kernel-thread/?ref=rp
• https://www.geeksforgeeks.org/difference-between-operating-system-and-kernel/?ref=rp

Concurrency vs Parallelism (ToDo)

Parallel

• multiple things at same time
• genuine simultaneous execution

Concurrency

• multiple things together but not same time
• appearance of simultaneous execution (by interleaving of processes in time)

False Sharing

• DZone
• https://medium.com/@rukavitsya/what-is-false-sharing-and-how-jvm-prevents-it-82a4ed27da84
• Jenkov

Critical Section

a piece of code that accesses a shared resource, usually a variable or data structure.

Race Condition

a software error that occurs when multiple threads access a shared resource

Intermediate Program

• Consists of 1 or * race conditions;
• the o/p of program is not deterministic & varies from run to run.

Mutual Exclusion

no two processes can exist in the critical section at any given point of time

InterProcess Communication

• coordinate and synchronize between processes
• ways
  1. Pipes
  2. Signals
  3. Buffers: Message queues etc
  4. Sockets

Shared memory

• Common memory region from PCB

AssemblerInstructions

ldsub

load/store unsigned instruction byte (locked version of atomic exchange)

xchg

atomic exchange

Concurrency models

1. Parallel Workers
   • Disadvantages
     a. Complexity with Shared state - race conditions, deadlock, non blocking not easy implement
     b. Stateless (worker which re-reads state every time is needed, which can be slow)
     c. Non-deterministic job ordering
2. Assembly Line
   • Reactive event driven systems
   • Actors vs Channels
3. Functional Parallelism

Mutex

Only thread access resource at a time.

HappensBefore

Guarntees 1 operation effects are visible to another.

Read/Write Lock

Multiple readers allowed; writes get exclusive access to resource

Future/Promise

Promise sets value; future retrieves it from another

CAS

Atomic operation; update memory only value matches expectations

Atomics

Variables with in divisible operation, no interference from another thread.

Single Writer Principle

• One thread writes multiple thread reades.
• Eliminates race conditions; simplifies concurrency;
• Eliminates lock contention for writes.