Lec 24: Parallel Processors. Announcements

Transcription

1 Lec 24: Parallel Processors Kavita ala CS 3410, Fall 2008 Computer Science Cornell University P 3 out Hack n Seek nnouncements The goal is to have fun with it Recitations today will talk about it Pizza party on Tuesday after Thanksgiving Final project (distributed ray tracer) out last week Demos: Dec 16 or 17 1

2 Opening the ox Technology Trends DRM capacity Increased Reduced cost DRM capacity Moore s Law Speed? Not really 2

3 Moore s Law Law about transistor count Review: Relative Performance Define Performance = 1/Execution Time X is n time faster than Y Performance Performance X Y = Execution time Y Execution time X = n Example: time taken to run a program 10s on, 15s on Execution Time / Execution Time = 15s / 10s = 1.5 So is 1.5 times faster than 3

4 Review: CPU Time CPU Time = CPU Clock Cycles Clock Cycle Time = CPU Clock Cycles Clock Rate Performance improved by Reducing number of clock cycles Increasing clock rate Hardware designer must often trade off clock rate against cycle count Review: CPU Time Example Computer : 2GHz clock, 10s CPU time Designing Computer im for 6s CPU time Can do faster clock, but causes 1.2 clock cycles How fast must Computer clock be? Clock Rate Clock Cycles = CPU Time 1.2 Clock Cycles = 6s Clock Cycles = CPU Time Clock Rate = 10s 2GHz = Clock Rate = 6s = 6s 9 = 4GHz 4

5 Review: Instruction Count and CPI Clock Cycles = Instruction Count Cycles per Instruction CPU Time = Instruction Count CPI Clock Cycle Time Instruction Count CPI = Clock Rate Instruction Count for a program Determined by program, IS and compiler verage cycles per instruction Determined by CPU hardware If different instructions have different CPI verage CPI affected by instruction mix Review: CPI Example Computer : Cycle Time = 250ps, CPI = 2.0 Computer : Cycle Time = 500ps, CPI = 1.2 Same IS Which is faster, and by how much? CPU Time CPU Time CPU Time CPU Time = Instruction Count CPI Cycle Time = I ps = I 500ps = Instruction Count CPI Cycle Time = I ps = I 600ps I 600ps = = 1.2 I 500ps is faster by this much 5

6 Review: Performance Summary Instructions Clock cycles CPU Time = Program Instruction Seconds Clock cycle Performance depends on lgorithm: affects IC, possibly CPI Programming language: affects IC, CPI Compiler: affects IC, CPI Instruction set architecture: affects IC, CPI, T c Moore s Law Law about transistor count 6

7 Why Multicore? Moore s law law about transistors Smaller means faster transistors Power consumption growing with transistors 7

8 Power Trends In CMOS IC technology Power = Capacitive load Voltage 2 Frequency 30 5V 1V 1000 High Power at Idle Look back at X4 power benchmark t 100% load: 295W t 50% load: 246W (83%) t 10% load: 180W (61%) Google data center Mostly operates at 10% 50% load t 100% load less than 1% of the time 8

9 Moore s law Why Multicore? law about transistors Smaller means faster transistors Power consumption growing with transistors The power wall We can t reduce voltage further We can t remove more heat How else can we improve performance? Uniprocessor Performance Constrained by power, instruction-level parallelism, memory latency 9

10 Inside the Processor MD arcelona: 4 processor cores Intel s argument 10

11 frequency prop Voltage 1.2x 11

12 1.2x 0.8x 1.2x 1.6x 12

13 Multicore: Do you believe? 13