CSE 3320 Operating Systems Synchronization Jia Rao

Transcription

1 CSE 3320 Operating Systems Synchrnizatin Jia Ra Department f Cmputer Science and Engineering

2 Recap f the Last Class Multiprcessr scheduling Tw implementatins f the ready queue Lad balancing Parallel prgram scheduling } Synchrnizatins n shared data and executin phases } Causality amng threads Inter-prcess r thread cmmunicatins

3 Inter-Prcess Cmmunicatin (IPC) Three fundamental issues: Hw ne prcess can pass infrmatin t anther Hw t make sure tw r mre prcesses d nt get int each ther s way when engaging in critical activities Hw t maintain prper sequencing when dependencies present Hw abut inter-thread cmmunicatin?

4 Race Cnditins Race cnditins: when tw r mre prcesses/threads are reading r writing sme shared data and the final results depend n wh runs precisely when Interrupts, interleaved peratins/executin Dir[in] = X; in ++; printer daemn Dir[in] = Y; in ++;

5 Mutual Exclusin and Critical Regins Mutual exclusin: makes sure if ne prcess is using a shared variable r file, the ther prcesses will be excluded frm ding the same thing Main challenge/issue t OS: t design apprpriate primitive peratins fr achieving mutual exclusin Critical regins: the part f the prgram where the shared memry is accessed Fur cnditins t prvide mutual exclusin N tw prcesses simultaneusly in critical regin N assumptins made abut speeds r numbers f CPUs N prcess running utside its critical regin may blck anther prcess N prcess must wait frever t enter its critical regin

6 Mutual Exclusin Using Critical Regins Mutual exclusin using critical regins

7 Mutual Exclusin with Busy Waiting Disabling interrupts: OS technique, nt users multi-cpu? Lck variables: test-set is a tw-step prcess, nt atmic Busy waiting: cntinuusly testing a variable until sme value appears (spin lck)

8 Busy Waiting: Strict Alternatin 0 1 Prpsed strict alternatin slutin t critical regin prblem (a) Prcess 0. (b) Prcess 1. What if P1 s nncritical_regin() has lts mre wrk than P0 s?

9 Busy Waiting: Petersn s sharing Different frm strict alternatin Petersn's slutin fr achieving mutual exclusin

10 Busy Waiting: TSL TSL (Test and Set Lck) Indivisible (atmic) peratin, hw? Hardware (multi-prcessr) Hw t use TSL t prevent tw prcesses frm simultaneusly entering their critical regins? Entering and leaving a critical regin using the TSL instructin

11 Sleep and Wakeup Issue I with Petersn s & TS: hw t avid CPU-cstly busy waiting? Issue II: pririty inversin prblem Cnsider tw prcesses, H with (strict) high pririty and L with (strict) lw pririty, L is in its critical regin and H becmes ready; des L have chance t leave its critical regin? Sme IPC primitives that blck instead f wasting CPU time when they are nt allwed t enter their critical regins Sleep and wakeup

12 Sleep and Wakeup Prducer-Cnsumer Prblem Q1: What if the wakeup signal sent t a nn-sleep (ready) prcess? Q2: what is a wakeup waiting bit? Is ne enugh?

13 Semaphres and P&V Operatins Semaphres: a variable t indicate the # f pending wakeups Dwn peratin (P; request): lck Checks if a semaphre is > 0, - if s, it decrements the value and just cntinue - Otherwise, the prcess is put t sleep withut cmpleting the dwn fr the mment Up peratin (V; release): unlck Increments the value f the semaphre - if ne r mre prcesses are sleeping n the semaphre, ne f them is chsen by the system (randmly) and allwed t cmplete its dwn (semaphre will still be 0) P & V peratins are atmic, hw t implement? Single CPU: system calls, disabling interrupts temprally Multiple CPUs: TSL help

14 The Prducer-cnsumer Prblem w/ Semaphres Fr mutual exclusin and synchrnizatin Binary semaphres: if each prcess des a dwn befre entering its critical regin and an up just leaving it, mutual exclusin is achieved UC. Clrad Springs

15 Mutexes Mutex: a variable that can be in ne f tw states: unlcked r lcked A simplified versin f the semaphres [0, 1] Give ther chance t run s as t save self; What is mutex_trylck()?

16 Mutexes User-space Multi-threading What is a key difference between mutex_lck and enter_regin in multithreading and multi-prcessing? Fr user-space multi-threading, a thread has t allw ther threads t run and release the lck s as t enter its critical regin, which is impssible with busy waiting enter_regin Tw prcesses entering and leaving a critical regin using the TSL instructin

17 Mnitrs Mnitr: a higher-level synchrnizatin primitive Only ne prcess can be active in a mnitr at any instant, with cmpiler s help; thus, hw abut t put all the critical regins int mnitr prcedures fr mutual exclusin? But, hw prcesses blck when they cannt prceed? Cnditin variables, and tw peratins: wait() and signal()

18 Mnitrs (2) Wakeup and sleep signals can lst, but nt Wait and signal signals, why? Cnditins are nt cunters; wait() befre signal() Outline f prducer-cnsumer prblem with mnitrs nly ne mnitr prcedure active at ne time (a prcess ding signal must exit the mnitr immediately); buffer has N slts

19 Mnitr (3) Prs Make mutual exclusin autmatic Make parallel prgramming less errr-prne Cns Cmpiler supprt

20 Message Passing Cmmunicatin withut sharing memry The prducer-cnsumer prblem with N messages

21 Barriers Use f a barrier (fr prgrams perate in phases, neither enters the next phase until all are finished with the current phase) fr grups f prcesses t d synchrnizatin (a) prcesses appraching a barrier (b) all prcesses but ne blcked at barrier (c) last prcess arrives, all are let thrugh

22 Class IPC Prblems: Dining Philsphers Philsphers eat/think Eating needs 2 frks Pick ne frk at a time Hw t prevent deadlck & starvatin Deadlck: bth are blcked n sme resurce Starvatin: bth are running, but n prgress made The prblem is useful fr mdeling prcesses that are cmpeting fr exclusive access t a limited number f resurces, such as I/O devices

23 Dining Philsphers (2) A nn-slutin t the dining philsphers prblem What happens if all philsphers pick up their left frks simultaneusly? Or, all wait fr the same amunt f time, then check if the right available? What if randm waiting, then check if the right frk available? What perfrmance if dwn and up n mutex befre acquiring/replacing a frk?

24 Dining Philsphers (3): Slutin part1

25 Dining Philsphers (4): Slutin part2

26 The Readers and Writers Prblem UC. Clrad Springs

27 Summary Race cnditins Mutual exclusin and critical regins Tw simple appraches Disabling interrupt and Lck variables Busy waiting Strict alternatin, Petersn s and TSL Sleep and Wakeup Semaphres Mutexes Classical IPC prblems Additinal practice Read Linux dcumentatin: LINUX_SRC/Dcumentatin/spinlcks.txt Find the implementatin f dwn and up in LINUX_SRC/kernel/semaphre.c Spinlck v.s. Mutex: