Lecture 13 ProducerConsumer and Semaphores 1 Review Mutual

Lecture 13: Producer-Consumer and Semaphores 1

Review: Mutual Exclusion Solutions • Software solution – Disabling interrupts – Strict alternation – Peterson’s solution • Hardware solution – TSL/XCHG • Key issue – Busy waiting 2

Review: Mitigate Busy Waiting /* set lock to non-zero, proceed if it was 0 earlier */ enter_region: TSL Reg, Lock /* set Lock to 1, copy old value of Lock into Reg*/ If (Reg != 0) then /* not the first to set to zero */ { thread_yield() ; /* let somebody else run */ Jump enter_region /* try again */ } return leave_region: Lock = 0 return 3

In this lecture • sleep() and wakeup() • Producer-consumer problem • Semaphores 4

Avoiding Busy Waiting • If a process cannot enter critical region, the process calls sleep() to give up CPU. • A process wakes up another process using wakeup() 5

Review: Mutual Exclusion using TSL /* set lock to non-zero, proceed if it was 0 earlier */ enter_region: TSL Reg, Lock /* set Lock to 1, copy old value of Lock into Reg*/ If (Reg != 0) then /* not the first to set to zero */ Jump enter_region /* try again */ return leave_region: Lock = 0 return 6

TSL without busy waiting /* set lock to non-zero, proceed if it was 0 earlier */ enter_region: TSL Reg, Lock /* set Lock to 1, copy old value of Lock into Reg*/ If (Reg != 0) then /* not the first to set to zero */ { sleep() Jump enter_region /* try again */ return leave_region: Lock = 0 wakeup(process_id) return 7

Producer – Consumer Problem Producer Consumer • Mutual exclusion • Buffer Full • Buffer Empty 8

How is this solution? #define N 100 int count = 0 PRODUCER While (TRUE) { if (count < N){ item = produce_item(); insert_item(item); count = count + 1; } } No mutual exclusion CONSUMER While (TRUE) { if (count > 0){ item = remove_item(); count = count - 1; consume_item(item); } } 9

Using sleep() and wakeup() #define N 100 int count = 0 PRODUCER CONSUMER While (TRUE) { item = produce_item(); if (count == N) sleep(); insert_item(item); count = count + 1; if (count == 1) wakeup(CONSUMER); } While (TRUE) { if (count == 0) sleep(); item = remove_item(); count = count - 1; if (count == N-1) wakeup(PRODUCER); consume_item(item); } 10

Producer – Consumer Problem • Need a way to block till some condition is satisfied – Semaphores – Condition variables 11

Semaphore: Interface • S: Integer variable • down(&S): when (S > 0) S = S – 1; • up(&S): S = S + 1; • Atomic actions • down(&S) block when S =0 • up(S) never blocks 12

Semaphore: Implementation • down(&S): – If (S=0) then • Suspend thread, put into a waiting queue • Schedule another thread to run – Else decrement S and return • up(&S): – Increment S – If any threads in waiting queue, then • release one of them (make it ‘ready’) • Both the above are done atomically – by disabling interrupts – by TSL/XCHG 13

Producer Consumer using Semaphores #define N 100 int mutex =1; int empty = N; int full = 0 PRODUCER CONSUMER While (TRUE) { item = produce_item(); down(&empty); down(&mutex); insert_item(item); up(&mutex); up(&full); } While (TRUE) { down(&full); down(&mutex); item = remove_item(); up(&mutex); up(&empty); consume_item(item); } 14

Mutual Exclusion Solutions • Software solution – Disabling interrupts – Strict alternation – Peterson’s solution • Hardware solution – TSL/XCHG 15

Mutual Exclusion via Semaphore • Initialize Semaphore mutex = 1 • lock(&mutex)= down(&mutex) • unlock(&mutex)= up(&mutex) 16

Mutual Exclusion via Semaphore lock(&mutex); critical_region(); unlock(&mutex); 17

Example • Web Server can handle only 10 threads at a time – Multiple points where threads are being created – How to ensure no more than 10 active threads? • Semaphore with initial value = 10 – down() before Thread creation – up() once Thread finishes 18