03 02 Processes Part 2 — Transcript

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1. 0:08[Music]
2. 0:21all right so the next thing would be the
3. 0:23process scheduling
4. 0:25or the cpu scheduling okay
5. 0:27so process scheduler selects among
6. 0:30available processes
7. 0:32for the next execution of the cpu or on
8. 0:34the cpu core so the goal here is to
9. 0:37maximize the cpu use
10. 0:39in other terms we have to keep the cpu
11. 0:41as busy as possible so we always want to
12. 0:44get a hundred percent of cpu utilization
13. 0:47here and
14. 0:48quickly switch processes onto the cpu
15. 0:51core
16. 0:52so the two main objectives of process
17. 0:54scheduling systems first are
18. 0:57to keep the cpu busy at all times and
19. 1:00second
20. 1:01deliver acceptable response times for
21. 1:04all the program particularly for
22. 1:06interactive ones
23. 1:08right so i'll repeat
24. 1:10there are two objectives or main
25. 1:12objectives of process scheduling
26. 1:15so the first one is we have to keep the
27. 1:18cpu as busy as possible okay keep the
28. 1:21cpu busy at all times
29. 1:23and deliver acceptable response
30. 1:26for all the programs particularly for
31. 1:28interactive ones
32. 1:30all right so the process scheduler must
33. 1:32meet these objectives by implementing
34. 1:34suitable policies for swapping processes
35. 1:36in and out of the cpu
36. 1:39okay
37. 1:40so
38. 1:41note that these objectives can be
39. 1:44conflicting in particular
40. 1:46every time the system steps into a swap
41. 1:48process
42. 1:49it takes up time on the cpu to do so
43. 1:52which thereby lost from doing any useful
44. 1:56productive work
45. 1:58okay
46. 1:59so aside from that the process scheduler
47. 2:02maintains scheduling cues of a process
48. 2:05so you've got the ready queues and the
49. 2:07weighting cues okay so what's the
50. 2:09difference between these two
51. 2:10when you see a ridicule these are set of
52. 2:13all processes residing in the main
53. 2:15memory
54. 2:16ready and waiting to be executed
55. 2:19okay
56. 2:20and when you say weight queue
57. 2:22these are set of processes waiting for
58. 2:24an event
59. 2:25to occur for example an i o
60. 2:29right so processes migrate from among
61. 2:32the various queues depending upon the
62. 2:35state of the process
63. 2:39okay
64. 2:41so i have here the ready and waiting
65. 2:43queues okay so sometimes when the
66. 2:46process is preempted okay so it will be
67. 2:49going back to the ram
68. 2:51okay so ready and waiting for
69. 2:54the next execution time
70. 2:56so all processes are basically stored in
71. 2:59the job queue
72. 3:00okay or process queue
73. 3:02okay so
74. 3:04next would be
75. 3:06process is in the ready state
76. 3:08they are placed in the ready queue
77. 3:11right so processors waiting for a device
78. 3:15to become available to deliver data are
79. 3:17placed in the device queues
80. 3:20so we have a lot of views here
81. 3:22these are generally a separate device
82. 3:25for queue for each of the device there
83. 3:28so other queues are also be created
84. 3:31and used as needed
85. 3:35all right
86. 3:37okay
87. 3:38so next would be the representation of a
88. 3:41process scheduling
89. 3:44okay
90. 3:44so for the schedulers we have what you
91. 3:47call
92. 3:48a long-term scheduler
93. 3:50this is a typical batch of system
94. 3:53or a very heavily loaded system it runs
95. 3:56infrequently
96. 3:57such as when one process and selecting
97. 4:00one more to be loaded
98. 4:02in from a disk in its place
99. 4:04and can afford to take the time to
100. 4:07implement intelligent
101. 4:09and advanced cpu scheduling algorithm
102. 4:13okay so that's long-term scheduler
103. 4:15we also have a short-term scheduler
104. 4:17another term for that is cpu scheduler
105. 4:21okay so the cpu scheduler runs very
106. 4:24frequently
107. 4:25on the order of 100 milliseconds
108. 4:28and must be very quickly swap one
109. 4:30process out of the cpu
110. 4:32and swap in another one
111. 4:35so some systems are also employ a medium
112. 4:38term scheduler
113. 4:39so when the system loads gets high
114. 4:42this scheduler will swap one or more
115. 4:44processes out of the ready queue
116. 4:47for a few seconds so in order to allow
117. 4:49smaller faster jobs to finish
118. 4:52so and clear the system
119. 4:54so next would be an efficient scheduling
120. 4:57system
121. 4:58will be select or will select a good
122. 5:01process mix of the cpu bound process and
123. 5:05an i o bound process
124. 5:07okay
125. 5:08now take a look at the cpu scheduling
126. 5:10here so
127. 5:11whenever you run an application okay so
128. 5:14basically
129. 5:15that application will be placed on the
130. 5:18reticule so rediq that means the process
131. 5:21is now on the ram
132. 5:23ready for the execution okay it already
133. 5:26has the resources needed maybe for
134. 5:28execution
135. 5:30so on the red state it will be forwarded
136. 5:32to the cpu for execution
137. 5:34this processes are executed by the cpu
138. 5:37one at the time
139. 5:39all right so one at a time so it's a
140. 5:41matter of milliseconds
141. 5:43all right now if the process burst time
142. 5:46okay so let's say burst time here
143. 5:48okay burst time is the amount of time
144. 5:50needed by the process to complete the
145. 5:52execution so for instance
146. 5:55that process has already served
147. 5:58the first time so that means
148. 6:00the process state would be terminated
149. 6:03okay
150. 6:05now on the cpu if the process is
151. 6:07currently executed it can be preempted
152. 6:11so we say preempted
153. 6:12during the execution it can be pulled
154. 6:15out from the cpu
155. 6:17and if it is not yet done it has to fall
156. 6:19in line back again to the reticule
157. 6:23now what would be that instances
158. 6:26where in
159. 6:27the process can be preempted from the
160. 6:29cpu execution so these are the following
161. 6:32so there might be an i o request coming
162. 6:34from the keyboard
163. 6:36all right
164. 6:37so
165. 6:38that would lead to an i o weight queue
166. 6:41you've got an i o there so the process
167. 6:43will be
168. 6:44on going back to the reticule
169. 6:47or maybe you are using a round robin cpu
170. 6:50scheduling and the time slice
171. 6:52has expired so when you say when you say
172. 6:55time slice
173. 6:57this is similar to the term quantum
174. 7:00okay and quantum is the amount of time
175. 7:02given to a process
176. 7:04to stay in the cpu for execution
177. 7:07all right so we call it time slice next
178. 7:11another instance that the process can be
179. 7:13preempted from the executes on on the
180. 7:15cpu would be
181. 7:16there is a child process that needs to
182. 7:18be executed
183. 7:20and when the child termination
184. 7:22wait you
185. 7:23for that process so the child will be
186. 7:26terminated and again
187. 7:28that process will be going back to the
188. 7:30ridicule for the next execution time
189. 7:34okay
190. 7:35and the last one would be
191. 7:37the wait for an interrupt here
192. 7:39so there might be an interrupt during
193. 7:41the execution of the process that's why
194. 7:44it was preempted and therefore
195. 7:46placed
196. 7:47into the reticule
197. 7:49all right
198. 7:50so that's how process works inside the
199. 7:53cpu
200. 7:54between the memory or the ram and the
201. 7:57cpu
202. 8:01all right
203. 8:02next let's talk about the cpu switch
204. 8:04from one process to another
205. 8:07okay so take note that the cpu can
206. 8:10execute one job at a time for example we
207. 8:13have two processes here
208. 8:15namely p0 and p1 it could be any
209. 8:18applications on your computer let's say
210. 8:20this is microsoft word here for p0 and
211. 8:23p1 is for powerpoint
212. 8:25okay
213. 8:26so what happened here and what happened
214. 8:28here is that
215. 8:29for example at the current time or at
216. 8:31time zero okay process zero is being
217. 8:34executed
218. 8:35okay so because of the interrupt or
219. 8:38system call okay so p0 will be stored or
220. 8:42save the state into the pcb
221. 8:44for process zero take note that each
222. 8:47process has their own process control
223. 8:49block
224. 8:51all right
225. 8:52so what will happen is okay
226. 8:55so if there is an interrupt here
227. 8:57there might be the instance or an
228. 8:59instance where in the process is idle
229. 9:01waiting for another chance of execution
230. 9:04so on this area here the process is in
231. 9:09the waiting queue
232. 9:11or in the ready queue so that means they
233. 9:13are ideal
234. 9:15okay
235. 9:15now if one process is idle there might
236. 9:18be another process that is currently
237. 9:20being executed
238. 9:22okay
239. 9:22and when you say cpu switch from one
240. 9:25process to the other okay so take note
241. 9:28that the cpu is processing
242. 9:30one process or one job at a time
243. 9:33so if the process is not able to finish
244. 9:36the execution or if the burst time
245. 9:39has not been served
246. 9:40and it has been
247. 9:42preempted
248. 9:44okay so that's the time the cpu is
249. 9:46executing another process here
250. 9:48okay so take a look at this
251. 9:51at this time p0 is executed and then
252. 9:54you've got idle state here now while it
253. 9:57is in idle state
254. 9:58process one is currently being executed
255. 10:02okay
256. 10:02so a context switch of course when the
257. 10:05cpu switches from one process to another
258. 10:08so that scenario is what you call
259. 10:11context switch
260. 10:14all right
261. 10:15so
262. 10:16what is the context switch so when cpu
263. 10:19switches to another process
264. 10:21the system must save the state of the
265. 10:23old process
266. 10:24and load the saved state
267. 10:27of the new process via context switch
268. 10:30okay so context of a process
269. 10:34is represented in the pcb
270. 10:36okay
271. 10:37so whenever an interrupt arrives the cpu
272. 10:40must do a state save
273. 10:42of the currently running process then
274. 10:45switch into a kernel mode to handle the
275. 10:47interrupt
276. 10:49and then
277. 10:50do a state restore of the interrupt
278. 10:53process so that's how it works so
279. 10:55similarly okay
280. 10:57so a contact switch of course when the
281. 10:59time slice for one process has expired
282. 11:02we're talking about round rolling here
283. 11:04okay and a new process is to be loaded
284. 11:07from the red eq
285. 11:09okay
286. 11:10so this will be instigated by the timer
287. 11:12interrupt which will then cause the
288. 11:15current process state to be saved and
289. 11:18the new process state to be restored
290. 11:21okay so we have a lot of event that is
291. 11:24happening during the context switch
292. 11:26so saving and restoring states involved
293. 11:28saving and restoring all of the
294. 11:30registers and program counters
295. 11:32as well as the process control block
296. 11:34described
297. 11:36earlier
298. 11:37okay
299. 11:38so context switching happens in a very
300. 11:41very frequently and
301. 11:43the overhead of doing the switching is
302. 11:45just lost cpu time
303. 11:47so contacts switches
304. 11:50states saves and restores
305. 11:52need to be fast as possible because
306. 11:54during the context switch
307. 11:56during the movement from one process to
308. 11:58another the cpu is doing nothing
309. 12:02so some hardware has special provisions
310. 12:04for speeding this up such a single
311. 12:06machine instruction for saving and
312. 12:08restoring all registers at once
313. 12:12all right so can you now imagine how the
314. 12:15operating systems manages this process
315. 12:17process switching
316. 12:19okay you've got requests coming from the
317. 12:21processes
318. 12:32[Music]
319. 12:40you