POSIX API 3 / Scheduling 1 1 last time pipes (fjnish) process - PowerPoint PPT Presentation

POSIX API 3 / Scheduling 1 1

last time pipes (fjnish) process states in xv6 the scheduler thread, switching to and from scheduling queues I/O and CPU bursts 2

note on VM I made mistake in producing the VM… though I started from a clean VM but… some excess fjles on VM from a last semester submission testing please delete them (fat* and life directories) questions/concerns? email/talk to me privately 3

recall: scheduling queues ready queue CPU I/O I/O queues I/O system call timer/etc. interrupt wait/… system call wait queues 4

CPU and I/O bursts wait for I/O … web browser: wait for remote web server drawing program: wait for mouse presses/etc. shell: wait for keypresses examples: and waiting for I/O program alternates between computing … start write … compute on read data wait for I/O start read compute on read data wait for I/O (from fjle/keyboard/…) start read compute 5

CPU bursts and interactivity (one c. 1966 shared system) shows compute time from command entered until next command prompt from G. E. Bryan, “JOSS: 20,000 hours at a console—a statistical approach” in Proc. AFIPS 1967 FJCC 6

CPU bursts and interactivity (one c. 1990 desktop) shows CPU time from RUNNING until not RUNNABLE anymore from Curran and Stumm, “A Comparison of basic CPU Scheduling Algoirithms for Multiprocessor Unix” 7

CPU bursts observation: applications alternate between I/O and CPU especially interactive applications but also, e.g., reading and writing from disk typically short “CPU bursts” (milliseconds) followed by short “IO bursts” (milliseconds) 8

scheduling CPU bursts our typical view: ready queue, bunch of CPU bursts to run to start: just look at running what’s currently in ready queue best same problem as ‘run bunch of programs to completion’? later: account for I/O after CPU burst 9

an historical note historically applications were less likely to keep all data in memory historically computers shared between more users meant more applications alternating I/O and CPU context many scheduling policies were developed in 10

scheduling metrics response time (Anderson-Dahlin) AKA turnaround time (Arpaci-Dusseau) (want low ) (what Arpaci-Dusseau calls response time is slightly difgerent — more later) what user sees: from keypress to character on screen (submission until job fjnsihed) throughput (want high ) total work per second problem: overhead (e.g. from context switching) fairness many defjnitions 11 all confmict with best average throughput/turnaround time

turnaround and wait time wait for input ready running turnaround time (Anderson-Dahlin “response time”) + wait time (= turnaround time - running time) Arpaci-Dusseau’s “response time” common measure: mean turnaround time or total turnaround time same as optimizing total/mean waiting time 12

turnaround and wait time wait for input ready running turnaround time (Anderson-Dahlin “response time”) + wait time (= turnaround time - running time) Arpaci-Dusseau’s “response time” common measure: mean turnaround time or total turnaround time same as optimizing total/mean waiting time 12

turnaround and wait time wait for input ready running turnaround time (Anderson-Dahlin “response time”) + wait time (= turnaround time - running time) Arpaci-Dusseau’s “response time” common measure: mean turnaround time or total turnaround time same as optimizing total/mean waiting time 12

turnaround and wait time wait for input ready running turnaround time (Anderson-Dahlin “response time”) + wait time (= turnaround time - running time) Arpaci-Dusseau’s “response time” common measure: mean turnaround time or total turnaround time same as optimizing total/mean waiting time 12

turnaround time and I/O scheduling CPU bursts? important for fully utilizing I/O devices scheduling batch program on cluster? once program done with CPU, it’s probably done 13 turnaround time ≈ time to start next I/O closed loop: faster turnaround time → program requests CPU sooner turnaround time ≈ how long does user wait

throughput run A … time lost not starting I/O early as possible time lost due to cold caches also other considerations: non-context switch CPU utilization = 14 (2 units) run A (3 units) run B context switch(each .5 units) (3 units) throughput: useful work done per unit time 3 + 3 + 2 3 + . 5 + 3 + . 5 + 2 = 88%

fairness run A easy to answer — but formal defjnition? two timelines above; which is fairer? assumption: one program per user run B run A run B run B timeline 1 run A run B run A timeline 2 run B run A 15

fairness run A easy to answer — but formal defjnition? two timelines above; which is fairer? assumption: one program per user run B run A run B run B timeline 1 run A run B run A timeline 2 run B run A 15

two trivial scheduling algorithms fjrst-come fjrst served (FCFS) round robin (RR) 16

scheduling example assumptions multiple programs become ready at almost the same time alternately: became ready while previous program was running …but in some order that we’ll use e.g. our ready queue looks like a linked list 17

two trivial scheduling algorithms fjrst-come fjrst served (FCFS) round robin (RR) 18

fjrst-come, fjrst-served simplest(?) scheduling algorithm no preemption — run program until it can’t suitable in cases where no context switch e.g. not enough memory for two active programs 19

arrival order: A , B , C fjrst-come, fjrst-served (FCFS) A 0 ( A ), 24 ( B ), 28 ( C ) turnaround times: (mean=27.7) 24 ( A ), 28 ( B ), 31 ( C ) arrival order: B , C , A B C 0 30 10 20 30 waiting times: (mean=3.7) 7 ( A ), 0 ( B ), 4 ( C ) turnaround times: (mean=14) 31 ( A ), 4 ( B ), 7 ( C ) waiting times: (mean=17.3) 10 20 3 process CPU time needed A 24 B 4 C A (AKA “fjrst in, fjrst out” (FIFO)) CPU-bound B, C I/O bound or interactive A B C 0 20

arrival order: A , B , C fjrst-come, fjrst-served (FCFS) A turnaround times: (mean=27.7) 24 ( A ), 28 ( B ), 31 ( C ) arrival order: B , C , A B C 0 waiting times: (mean=17.3) 10 20 30 waiting times: (mean=3.7) 7 ( A ), 0 ( B ), 4 ( C ) turnaround times: (mean=14) 31 ( A ), 4 ( B ), 7 ( C ) 0 ( A ), 24 ( B ), 28 ( C ) 20 30 C process CPU time needed A 24 B 4 3 (AKA “fjrst in, fjrst out” (FIFO)) A B C 0 10 20 A ∼ CPU-bound B, C ∼ I/O bound or interactive

arrival order: B , C , A fjrst-come, fjrst-served (FCFS) A 0 ( A ), 24 ( B ), 28 ( C ) turnaround times: (mean=27.7) 24 ( A ), 28 ( B ), 31 ( C ) B C 0 30 10 20 30 waiting times: (mean=3.7) 7 ( A ), 0 ( B ), 4 ( C ) turnaround times: (mean=14) 31 ( A ), 4 ( B ), 7 ( C ) waiting times: (mean=17.3) 20 (AKA “fjrst in, fjrst out” (FIFO)) C process CPU time needed A 24 B 4 3 10 A B C 0 20 A ∼ CPU-bound B, C ∼ I/O bound or interactive arrival order: A , B , C

arrival order: B , C , A fjrst-come, fjrst-served (FCFS) A 0 ( A ), 24 ( B ), 28 ( C ) turnaround times: (mean=27.7) 24 ( A ), 28 ( B ), 31 ( C ) B C 0 30 10 20 30 waiting times: (mean=3.7) 7 ( A ), 0 ( B ), 4 ( C ) turnaround times: (mean=14) 31 ( A ), 4 ( B ), 7 ( C ) waiting times: (mean=17.3) 20 (AKA “fjrst in, fjrst out” (FIFO)) C process CPU time needed A 24 B 4 3 10 A B C 0 20 A ∼ CPU-bound B, C ∼ I/O bound or interactive arrival order: A , B , C

fjrst-come, fjrst-served (FCFS) 0 0 ( A ), 24 ( B ), 28 ( C ) turnaround times: (mean=27.7) 24 ( A ), 28 ( B ), 31 ( C ) B C A 10 30 20 30 waiting times: (mean=3.7) 7 ( A ), 0 ( B ), 4 ( C ) turnaround times: (mean=14) 31 ( A ), 4 ( B ), 7 ( C ) waiting times: (mean=17.3) 20 (AKA “fjrst in, fjrst out” (FIFO)) 3 process CPU time needed A 24 B 4 C 20 10 A B C 0 A ∼ CPU-bound B, C ∼ I/O bound or interactive arrival order: A , B , C arrival order: B , C , A

fjrst-come, fjrst-served (FCFS) 0 0 ( A ), 24 ( B ), 28 ( C ) turnaround times: (mean=27.7) 24 ( A ), 28 ( B ), 31 ( C ) B C A 10 30 20 30 waiting times: (mean=3.7) 7 ( A ), 0 ( B ), 4 ( C ) turnaround times: (mean=14) 31 ( A ), 4 ( B ), 7 ( C ) waiting times: (mean=17.3) 20 (AKA “fjrst in, fjrst out” (FIFO)) 3 process CPU time needed A 24 B 4 C 20 10 A B C 0 A ∼ CPU-bound B, C ∼ I/O bound or interactive arrival order: A , B , C arrival order: B , C , A

FCFS orders B “convoy efgect” 31 ( A ), 3 ( B ), 7 ( C ) turnaround times: (mean=14) 7 ( A ), 0 ( B ), 4 ( C ) waiting times: (mean=3.7) 30 20 10 0 A C arrival order: B , C , A arrival order: A , B , C 24 ( A ), 28 ( B ), 31 ( C ) turnaround times: (mean=27.7) 0 ( A ), 24 ( B ), 28 ( C ) waiting times: (mean=17.3) 30 20 10 0 C B A 21

two trivial scheduling algorithms fjrst-come fjrst served (FCFS) round robin (RR) 22

round-robin simplest(?) preemptive scheduling algorithm run program until either it can’t run anymore, or it runs for too long (exceeds “time quantum”) requires good way of interrupting programs like xv6’s timer interrupt requires good way of stopping programs whenever like xv6’s context switches 23