operating system - 4

2.Process scheduling

a.) scheduling queues - Queue scheduling mechanism in a data packet transmission system:::::
A queue scheduling mechanism in a data packet transmission system, the data packet transmission system including a transmission device for transmitting data packets, a reception device for receiving the data packets, a set of queue devices respectively associated with a set of priorities each defined by a priority rank for storing each data packet transmitted by the transmission device into the queue device corresponding to its priority rank, and a queue scheduler for reading, at each packet cycle, a packet in one of the queue devices determined by a normal priority preemption algorithm. The queue scheduling mechanism includes a credit device that provides at each packet cycle a value N defining the priority rank to be considered by the queue scheduler whereby a data packet is read by the queue scheduler from the queue device corresponding to the priority N instead of the queue device determined by the normal priority preemption algorithm

b.)schedulers - The scheduler is the component of an operating system that determines which process should be run, and when.

We will specify:

• The service provided-the scheduler specification
• A system that provides this services-the scheduler implementation

c.)context switch - Execution context is determined by the user or login connected to the session, or executing (calling) a module. It establishes the identity against which permissions to execute statements or perform actions are checked. In SQL Server, the execution context can be switched to another user or login by executing the EXECUTE AS statement, or specifying the EXECUTE AS clause in a module. After the context switch, SQL Server checks permissions against the login and user for that account instead of the person calling the EXECUTE AS statement or the module. The database user or SQL Server login is impersonated for the remainder of the session or module execution, or until the context switch is explicitly reverted. For more information about execution context, see Understanding Execution Context.

Explicit Context Switching

The execution context of a session or module can be explicitly changed by specifying a user or login name in an EXECUTE AS statement. The impersonation remains in effect until one of the following events occurs:

• The session is dropped.
  
• Context is switched to another login or user.
  
• Context is reverted to the previous execution context.