Popular multiprocessor thread-scheduling strategies.

Gang Scheduling: A set of related threads is scheduled to run on a set of processors at the same time, on a 1-to-1 basis. Closely related threads / processes may be scheduled this way to reduce synchronization blocking, and minimize process switching. Group scheduling predated this strategy.
Dedicated processor assignment: Provides implicit scheduling defined by assignment of threads to processors. For the duration of program execution, each program is allocated a set of processors equal in number to the number of threads in the program. Processors are chosen from the available pool.
Dynamic scheduling: The number of thread in a program can be altered during the course of execution.

A must know concept

Scheduling is the process of allocating the CPU a process.Only one process can be executed at a time.They are of three types short-, long- and medium-term scheduling
Long term scheduler determines which programs are admitted to the system for processing. It controls the degree of multiprogramming. Once admitted, a job becomes a process.
Medium term scheduling is part of the swapping function. This relates to processes that are in a blocked or suspended state. They are swapped out of real-memory until they are ready to execute. The swapping-in decision is based on memory-management criteria.
Short term scheduler, also know as a dispatcher executes most frequently, and makes the finest-grained decision of which process should execute next. This scheduler is invoked whenever an event occurs. It may lead to interruption of one process by preemption.

Belady's Anomaly

One important and nice topic in Os that most people like is that of Belady's Anomaly.Also called FIFO anomaly. Usually, on increasing the number of frames allocated to a process' virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Belady's Anomaly. This is true for certain page reference patterns.It is a must for everyone learning OS to know about it.So learn it as soon as possible friends.

Dead lock:The easiest concept in operating system

It is a concept which tells about an error condition.Here the system is deadlocked and no process executes.i Will List the Coffman's conditions that lead to a deadlock.
*Mutual Exclusion: Only one process may use a critical resource at a time.
*Hold & Wait: A process may be allocated some resources while waiting for others.
*No Pre-emption: No resource can be forcible removed from a process holding it.
*Circular Wait: A closed chain of processes exist such that each process holds at least one resource needed by another process in the chain.
Deadlocks can be avoided and also prevented.

Some concepts:reentrancy

I am going to explain some key concepts in operating systems.Let us start with reentrancy.
It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record. Generally, the activation record is kept on the stack.
Note: A reentrant procedure can be interrupted and called by an interrupting program, and still execute correctly on returning to the procedure.

Concepts in operating systems

Friends I like operating systems very much.I have studied every page and corner in the subject.It is a very interesting subject and it must be thoroughly read by the various students studying operating systems.It was a compulsory subject to me during my second year in Engineering.I even had a lab in it.We used a book which had a Dinosaur picture in the front(i couldn't find out what similarity that the author found between a dino and operating system).It had a red cover.The author was Abraham Silberschatz.He explained all the concepts of Operating systems very clearly and in an enjoyable manner.So friends try to use that book as much as possible.

Operating system

One of my favorite subject in my engineering studies was operating system. Do not know why im more crazy about that subject. Let me discuss about the concept that i learnt.. And will soon gather more info about this and will share in my later posts..