Process Management | All Concept of Process Management

Process
A process is a program in execution. For example, when we write a program in C or C++ and compile it, the compiler creates a binary code. The original code and Binary code, both are programs. When we actually run the binary code, it becomes a process.

Program vs Process

process is program under execution ,process is an active entity

while a program is in form of programming language and program is a passive entity.

moreover,a program is stored in secondary storage but process is stored in memory

What does a process look like in Memory?

Text Section:A Process is also sometimes known as the Text Section. It also includes the current activity represented by the value of Program Counter.
Stack: Stack contains the temporary data such as function parameters, returns address and local variables.
Data Section: Contains the global variable.
Heap Section: Dynamically allocated memory to process during its run time.

Attributes or Characteristics of a Process
A process has following Attributes.

1. Process Id:    A unique identifier assigned by operating system
2. Process State: Can be ready, running, .. etc
3. CPU registers: Like Program Counter (CPU registers must be saved and 
                  restored when a process is swapped out and in of CPU)
5. Accounts information:This includes the amount of CPU used for process execution,time limits, execution ID etc.

6. I/O status information: For example devices allocated to process, 
                           open files, etc
8. CPU scheduling information: For example Priority (Different processes 
                               may have different priorities, for example
                               a short process may be assigned a low priority
                               in the shortest job first scheduling)

All the above attributes of a process are also known as Context of the process.
Every Process has its known Program control Block(PCB) i.e each process will have a unique PCB. All the Above Attributes are the part of the PCB.

Mode switching

A user process undergoes a mode switch when it needs access to system resources. This is implemented through the system call interface or by interrupts such as page faults.

There are two modes:

• User mode
• Kernel mode

Processor time spent in user mode (application and shared libraries) is reflected as user time in the output of commands such as the vmstat, iostat, and sar commands. Processor time spent in kernel mode is reflected as system time in the output of these commands.

• User mode
  
  Programs that execute in the user protection domain are user processes.
• Kernel mode
  
  Programs that execute in the kernel protection domain include interrupt handlers, kernel processes, the base kernel, and kernel extensions (device driver, system calls and file systems).
• Mode switches
  
  The use of a system call by a user-mode process allows a kernel function to be called from user mode. Access to functions that directly or indirectly invoke system calls is typically provided by programming libraries, which provide access to operating system functions.

difference between process switching and context switching in operating system

context switch happens only in kernel mode. If context switching happens between two user mode processes, first cpu has to change to kernel mode, performcontext switch, return back to user mode and so on. So there has to be a mode switch associated with a context switch.

translation lookaside buffer (TLB)
A translation lookaside buffer (TLB) is a memory cache that stores recent translations of virtual memory to physical addresses for faster retrieval.



When a virtual memory address is referenced by a program, the search starts in the CPU. First, instruction caches are checked. If the required memory is not in these very fast caches, the system has to look up the memory’s physical address. At this point, TLB is checked for a quick reference to the location in physical memory.


When an address is searched in the TLB and not found, the physical memory must be searched with a memory page crawl operation. As virtual memory addresses are translated, values referenced are added to TLB. When a value can be retrieved from TLB, speed is enhanced because the memory address is stored in the TLB on processor. Most processors include TLBs to increase the speed of virtual memory operations through the inherent latency-reducing proximity as well as the high-running frequencies of current CPU’s.


TLBs also add the support required for multi-user computers to keep memory separate, by having a user and a supervisor mode as well as using permissions on read and write bits to enable sharing.


TLBs can suffer performance issues from multitasking and code errors. This performance degradation is called a cache thrash. Cache thrash is caused by an ongoing computer activity that fails to progress due to excessive use of resources or conflicts in the caching system.