All about the subject Operating System

Its been a while my friends??
how are you today??


first gusto ko malaman nyo na kahit may thesis kami ngaun and even hanggang hindi parin kami makapagdecide ng partner ko kung anu gagawin...


pero d2 nalang muna tayo sa subject na Operating System..
Marahil marami kayong nalalaman sa Operating system lalo na pag mahilig kayo sa computer..Bilang student sa universidad at kumukuha ng COMSCI naging subject namin ito.. Alam niyo ba ang  OS ay isa sa pinaka masarap at mahirap pag aralan na subject lalo na pag ikaw ang gagawa ng program mostly ang pinag aaralan kasi dito ay proper of memory management at scheduling system sa pag aaral namin nito natuto kami kung paano mas mapabilis ang bawat gawain sa programming mostly sa mga job sequencing gumagamit kami ng Array para ma test at mapagaralan ang mga to at syempre dahil given ang gagawin next week kada hands on kami na gumagawa ng program bali nag reresearch kami about sa gagawin kasi baka mabigla kami pag mismo dun na namin ginawa ng harapan..

Ang mga types ng scheduling algorithms ay
1 First-come, first-served scheduling (FCFS) algorithm
2 Shortest Job First Scheduling (SJF) algorithm
3 Non-preemptive priority Scheduling algorithm
4 Preemptive priority Scheduling algorithm
5 Round-Robin Scheduling algorithm
6 Multilevel Queue Scheduling algorithm




Ipapaliwanag ko ang ilan sa mga ito:
First-come, first-served scheduling (FCFS) algorithm
First-come First-served Scheduling follow first in first out method. As each process becomes ready, it joins the ready queue. When the current running process ceases to execute, the oldest process in the Ready queue is selected for running. That is first entered process among the available processes in the ready queue.The average waiting time for FCFS is often quite long. It is non-preemptive.


TURN AROUND TIME=WAITING TIME + SERVICE TIME





Advantages

• Better for long processes
• Simple method (i.e., minimum overhead on processor)
• No starvation

Disadvantages

• Convoy effect occurs.Even very small process should wait for its turn to come to utilize the CPU. Short process behind long process reults in lower CPU utilization.
• Throughput is not emphasized.

Shortest Job First Scheduling (SJF) algorithm
This algorithm associates ith each process the length of the next CPU burst.Shortest-job-first scheduling is also called as shortest process next (SPN). The process with the shortest expected processing time is selected for execution, among the available processes in the ready queue. Thus, a short process will jump to the head of the queue over long jobs. If the next CPU bursts of two processes are the same then FCFS scheduling is used to break the tie.SJF scheduling algorithm is probably optimal. It givs the minimum average time for a given set of processes.It cannot be implemented at the level of short term CPU scheduling. There is no way of knowing the shortest CPU burst.
SJF can be premptive or non-preemptive. 
A premptive SJF algorithm will preempt the currently executing process if the next CPU burst of newly arrived process may be shorter than what is left to the currently executing process.
A Non-premptive SJF algorithm will allow the currently running process to finish.Preemptive SJF Scheduling is sometimes called Shortest Remaining Time First algorithm.





Advantages

• It gives superior turnaround time performance to shortest process next because a short job is given immediate preference to a running longer job.
• Throughput is high.

Disadvantages

• Elapsed time (i.e., execution-completed-time) must be recorded, it results an additional overhead on the processor.
• Starvation may be possible for the longer processes.

Round-Robin Scheduling algorithm
This type of scheduling algorithm is basically designed for time sharing system. It is similar to FCFS with preemption added.Round-Robin Scheduling is also called as time-slicing scheduling and it is a preemptive version based on a clock. That is a clock interrupt is generated at periodic intervals usually 10-100ms. When the interrupt occurs, the currently running process is placed in the ready queue and the next ready job is selected on a First-come, First-serve basis. This process is known as time-slicing, because each process is given a slice of time before being preempted.
One of the following happens:

• The process may have a CPU urst of less than the time quantum or
• CPU burst of currently executing process be longer than the time quantum. In this case the a context switch occurs the process is put at the tail of the ready queue.

In round-robin scheduling, the principal design issue is the length of the time quantum or time-slice to be used. If the quantum is very short, then short processes will move quickly.





Advantages

• Round-robin is effective in a general-purpose, times-sharing system or transaction-processing system.
• Fair treatment for all the processes.
• Overhead on processor is low.
• Overhead on processor is low.
• Good response time for short processes.

Disadvantages

• Care must be taken in choosing quantum value.
• Processing overhead is there in handling clock interrupt.
• Throughput is low if time quantum is too small.

Performance of RR Scheduling

• If there are n processes in the ready queue and time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once.
• No process waits for more than (n-1)*q time units until the next time quantum.
• The performance of RR depends on time slice.If it is large then it is the same as FCFS. If q is small then overhead is too high.

---alam nyu bang ang galing ng teacher namin d2 c sir Daez sa saglit naming nakasama cya naging msaya ung subject namin dito bukod dito interesado kasi ako about sa mga OS ang sarap kasi ng pakiramdam na na momodified mu ang isang bagay ^^