vault backup: 2022-10-30 13:55:53

content/notes/19-cpu-scheduling.md

@@ -24,7 +24,53 @@ criteria
 - throughput - number of processes completed per time unit
 - turnaround time - (for a single process) length of time from submitted to completed
 - waiting time - (for a single process) time spent waiting for the CPU
-- response time - (for a signle process) average timefrom the submission of a request to a process util the first response in produced
+	- does include time wating for IO
+- response time - (for a signle process) average time from the submission of a request to a process until the first response is produced
+
+Preemption
+- 4 situations for scheduling
+	1. a process switches from running to waiting
+	2. a process switches from running to ready (due to interrupt)
+	3. a process switches from wauting to ready (due to comletion of IO)
+	4. a process terminates
+- non preemptive - scheduling take splace only under 1 nd 4, which has no choice
+- premptive - scheduling can take place under 2 and 3 as well. that means a running process has to give way to the processes with higher priority
+
+if you allow interruption it is preemptive
 
 # Scheduling algorithms
+## Gantt charts
+The operation of  scheduling algortihm is commonly represented in a Gantt chart
 
+![example chart|400](https://i.imgur.com/haqjMOW.png)
+![gantt evaluation|400](https://i.imgur.com/iqtT90Q.png)
+
+## First come first served (FCFS)
+![diagram|400](https://i.imgur.com/yAd4LRl.png)
+
+simplest method- executes processes in the ready queue on a first come first served basis
+
+- when a proces become ready it is put at the tail of the queue
+- when the currently executig process terminates, or waits for IO, the process at the front of the queue is selected next
+- processes can come at any time - what if two at the same time? choose process with smallest ID to serve first
+
+non-preemptive
+
+pro
+- easy to understand and implement
+con
+- waiting time bad
+- convoy effect: lots of small process can get stuck behind a big on
+- bad response time
+
+fair but not efficient
+
+## Shortest job first (SJF)
+choose the job in the queue with the shortest burst time.
+
+- equal burst time are executed FCFS (by process id)
+
+![SJF example|400](https://i.imgur.com/nzwBwjc.png)
+
+pro
+- prova