[CS61C FA20] Lecture 21.1 - Pipelining I: Pipelining — Transcript

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1. 0:00[Music]
2. 0:10hello welcome back to 61c
3. 0:12we are starting a new module this module
4. 0:16will deal with pipelining
5. 0:17so far we have designed the functional
6. 0:19unit and evaluated its performance
7. 0:21but we really didn't get a sense is that
8. 0:23performance
9. 0:24good or bad and
10. 0:27is that the the cycle time the only
11. 0:30measure of performance
12. 0:32what we'll do in the this unit will
13. 0:33actually start measuring the performance
14. 0:35or different ways how we measure the
15. 0:37performance and then
16. 0:39figuring out the way how to improve it
17. 0:42so let's get going
18. 0:45in our new school of machine structures
19. 0:48we have looked at different
20. 0:50layers of hierarchy and how can they
21. 0:53support
22. 0:53parallelism now we are going to take a
23. 0:56look
24. 0:57at parallelism at the instructional
25. 1:00level
26. 1:01and what do we need to do in the
27. 1:02hardware in the execution units
28. 1:04to support simultaneous
29. 1:08execution of instructions
30. 1:12we are also introducing uh
31. 1:15one of our six great ideas in computer
32. 1:18architecture
33. 1:19which is performance measurement and
34. 1:21improvement
35. 1:23we simply what we mean by that is
36. 1:26we need to know how to measure the
37. 1:28performance in order to be able to
38. 1:30improve it so let's get going
39. 1:36we had one measure of performance which
40. 1:38was the minimum cycle time that we
41. 1:40needed to execute an instruction
42. 1:42in our single cycle risk five cpu
43. 1:46and we determine the time by trying to
44. 1:48measure the time that it takes to
45. 1:50execute
46. 1:51each of the five phases of execution so
47. 1:54we looked at the instruction fetch
48. 1:55and found out the instruction fetch
49. 1:57takes 200
50. 2:00picoseconds that instruction decode
51. 2:03takes about 100 picoseconds
52. 2:05the alu operation that
53. 2:10is our execute phase would take another
54. 2:12200 picoseconds
55. 2:13memory access will take yet another 200
56. 2:16picoseconds and finally right back would
57. 2:18be done we completed in 100 picoseconds
58. 2:22when you sum all of that up the minimum
59. 2:24cycle time was
60. 2:25800 picoseconds now
61. 2:28not every single instruction goes
62. 2:31through all five phases of execution
63. 2:33and we found out that it was just only
64. 2:35one of them load word that
65. 2:37went through all five phases of
66. 2:40execution
67. 2:41um but we can have a processor that
68. 2:44can you know we could possibly think of
69. 2:48a
70. 2:48trying to clock this processor faster
71. 2:50but then it will not be able to do
72. 2:53a load word instruction although it will
73. 2:55be able to do
74. 2:56most of the other instructions so
75. 2:59always our critical path is set by the
76. 3:02longest
77. 3:03instruction here that the longest time
78. 3:06that an instruction takes
79. 3:08in this case that was 800 picoseconds
80. 3:10and
81. 3:12we determined that the maximum clock
82. 3:14frequency is equal to 1 over that
83. 3:16those 800 picoseconds and that was 1.25
84. 3:21gigahertz
85. 3:24and that was one measure of a
86. 3:25performance so our single
87. 3:27cycle cpu can execute instructions at
88. 3:321.25 gigahertz that means that it can
89. 3:35start an instruction
90. 3:36and completed it um within
91. 3:39800 picoseconds and then can start
92. 3:41another one that would take also 800
93. 3:43picoseconds and so on
94. 3:46it would finish 1.25 billion
95. 3:51instructions per second
96. 3:54so the question here is can we do better
97. 3:56than that
98. 3:58and is that the only measure of
99. 4:01performance
100. 4:02so generally when we take a look at the
101. 4:05spec sheet
102. 4:06for any kind of a product there are
103. 4:08usually different kinds of performance
104. 4:10measures
105. 4:12so in this case let's try to see what do
106. 4:14we mean by a person
107. 4:15performance of a processor
108. 4:20in some cases it may mean that it may
109. 4:23have a quicker response time so
110. 4:25if we give it one job it will will it
111. 4:27finish it
112. 4:28sooner so the time to execute one job
113. 4:31may be
114. 4:32the the the
115. 4:35a performance measure or how many jobs
116. 4:39can you do
117. 4:39in a unit time so you know how many
118. 4:42web pages can it serve or how many
119. 4:45spoken words can it recognize or
120. 4:47how many chess games can it do
121. 4:50can it play in an hour
122. 4:54or um how much energy does it take
123. 4:57meaning how many uh how
124. 5:01long will our battery last when running
125. 5:04this processor running tasks on this
126. 5:07processor like
127. 5:08watching movies how many movies can we
128. 5:10watch on a battery charge
129. 5:13what is useful to think of
130. 5:17while we are getting a sense for a
131. 5:18process of performance to think of
132. 5:20something that is that is
133. 5:21much more familiar like transportational
134. 5:23knowledge and in transportation we can
135. 5:25take a look
136. 5:26at the performance metrics of a sports
137. 5:28car
138. 5:29versus performance metrics of a bus and
139. 5:31usually people think
140. 5:33of a sports car as a performance car
141. 5:38but bus is also a performance vehicle
142. 5:41for certain under certain metrics
143. 5:45so let's take a look at uh on the left
144. 5:48hand side uh specs for a sports car
145. 5:52and specs for a bus so the sports car
146. 5:54can carry two passengers the bus can
147. 5:56carry 50 passengers
148. 5:58um a sports car can go at 200 miles per
149. 6:01hour and the bus can only go at 50 miles
150. 6:03per hour
151. 6:05and gas mileage for the sports car is
152. 6:07five miles per gallon
153. 6:09um so it can go more miles per gallon
154. 6:12than the bus that can only go
155. 6:13two miles per gallon so let's see how
156. 6:16they perform on a task
157. 6:19that involves transporting a hundred
158. 6:21passengers
159. 6:22over a 50 mile trip and in this case
160. 6:25we're just going to assume that
161. 6:27we are measuring only one-way
162. 6:30travel and
163. 6:33both the car and the bus um
164. 6:36immediately somehow come back to the
165. 6:40starting point you know reload the
166. 6:43passengers
167. 6:43and go back so we we're just measuring
168. 6:46the time that
169. 6:47it is taking to transport the passengers
170. 6:51so a sports car um will travel
171. 6:5550 miles in 15 minutes the bus will take
172. 6:58an hour
173. 6:59but in every trip the
174. 7:02the pal the sports car will only deliver
175. 7:04two passengers and
176. 7:05go somehow transport itself empty and
177. 7:08take another two passengers
178. 7:10so in total to trans transport a hundred
179. 7:12people will take 750 minutes on the
180. 7:15other hand the bus will take only two
181. 7:17trips
182. 7:17so it'll be done in 120 minutes
183. 7:22the energy efficiency of a bus is a lot
184. 7:24better than the energy efficiency of a
185. 7:26car
186. 7:27the car um uh takes
187. 7:30five gallons per passenger to deliver
188. 7:33them
189. 7:34uh 50 miles to the destination if it's
190. 7:3850 miles away
191. 7:39and a bus takes only
192. 7:430.5 gallons per passenger
193. 7:46so if we had so we see a difference here
194. 7:51and
195. 7:51what does it mean something having a
196. 7:53higher performance
197. 7:55yeah a sports car is better if we have
198. 7:58something that is of a very high value
199. 8:00that needs to be
200. 8:00transported 50 miles away
201. 8:04an injured person for example
202. 8:09but bus is much better in delivering
203. 8:12moving a lot of people over a distance
204. 8:15so
205. 8:15it will do it in a much shorter time and
206. 8:18save a lot of energy
207. 8:19let's see how does this relate to the
208. 8:21compute performance
209. 8:23so in the transportation what we had was
210. 8:27trip time in computer world that
211. 8:30metric of performance will be program
212. 8:32execution time
213. 8:34how long does it take to execute a
214. 8:37a program or for example time to update
215. 8:40the
216. 8:40display with a new picture
217. 8:46the second metric that we had was the
218. 8:49time
219. 8:49to for in in transportation world time
220. 8:52to transport 100 passengers
221. 8:55in the compute world it is a metric
222. 8:59that measures how many tasks can
223. 9:03can we do in a given time for example
224. 9:06in this case it would be how many web
225. 9:09pages can be served in a given time um
226. 9:12or how many
227. 9:12server requests can be performed in an
228. 9:14hour
229. 9:16and then third one is the energy
230. 9:18efficiency the way how we measure the
231. 9:19energy efficiency in the cars is
232. 9:21you know gallons per passenger in a
233. 9:24computer
234. 9:25we measure them we also measure energy
235. 9:28efficiency
236. 9:29by the amount of energy used electrical
237. 9:32energy used
238. 9:33so in this case we will measure the
239. 9:36energy per task or
240. 9:37how much energy does playing a movie
241. 9:40take
242. 9:41or how many movies can we watch from a
243. 9:42battery charge
244. 9:44keep in mind that the energy is
245. 9:46important for
246. 9:47both mobile devices where we kind of
247. 9:50have a better sense for this
248. 9:51how much energy is stored in in a
249. 9:54battery
250. 9:56energy is also very important for high
251. 9:58performance servers that live in
252. 10:00warehouse computers
253. 10:01in data centers over there the cost of
254. 10:04running a data center is essentially
255. 10:07the cost of energy the hardware
256. 10:10um you know the energy cost
257. 10:14overshadows the hardware cost after
258. 10:16about a
259. 10:17year or you know year and a half after
260. 10:19deployment so that's a that's a big deal
261. 10:22in every
262. 10:24[Music]
263. 10:25application domain or in every domain of
264. 10:27computing
265. 10:29finally one thing that is important to
266. 10:31notice that power is generally not a
267. 10:33good measure
268. 10:35of efficiency energy is much better
269. 10:38power is a rate of exchange of energy so
270. 10:41they're kind of related
271. 10:43but um the the way how we can think of
272. 10:48it
273. 10:48is we can have um
274. 10:51a really low performance cpu that is
275. 10:54very low power
276. 10:55very very low power but it would take so
277. 10:59long to complete
278. 11:01the task um that
279. 11:04it would basically drain the energy by
280. 11:06the time it's done on the other hand
281. 11:07some better optimized cpu can probably
282. 11:10do
283. 11:11multiple tasks long tasks with
284. 11:15one battery charge we'll
285. 11:18see a bit more about that later but for
286. 11:20now what we want to
287. 11:22remember is we really care about the
288. 11:24energy
289. 11:25energy is what is stored in the battery
290. 11:28and that's what
291. 11:29the utility charges charges us
292. 11:34in units for delivering us the energy
293. 11:37not the power so that's it for the quick
294. 11:42introduction
295. 11:43so we are going to try to take a
296. 11:46deeper look into the processor
297. 11:48performance after a quick break