multiprocessor x CPU 0 CPU 1 bus Shared

프로세스 간 데이터 공유 multi-processor x++ CPU #0 CPU #1 bus Shared Memory x=11 x++

프로세스 간 데이터 공유 multi-processor CPU #0 CPU #1 bus 1. 2. 3. Read x into register Operation with ALU register Write back to storage box Shared Memory x=11 Also access x

Interleaved execution Race Condition (시간에 따라 다른 결과) multi-processor X++ CPU #0 CPU #1 X++ bus Read x into register Increment ALU register Write back to storage box t 4 t 1 t 3 Shared Memory x=11 t 2 Read x into register t 3 Increment ALU register t 4 t 5 Write back to storage box

Interleaved execution Race Condition (시간에 따라 다른 결과) multi-processor X++ CPU #0 CPU #1 X++ bus Read x into register (11) Increment ALU register Write back to storage box t 4 t 1 t 2 t 3 t 5 Shared Memory x=11 t 2 Read x into register t 3 Increment ALU register t 4 t 5 Write back to storage box

Interleaved execution Race Condition (시간에 따라 다른 결과) multi-processor X++ CPU #0 CPU #1 X++ bus Read x into register (11) Increment ALU register Write back to storage box t 4 t 1 t 2 t 3 t 5 Shared Memory x=11 t 2 Read x into register (11) t 3 Increment ALU register t 4 t 5 Write back to storage box

Interleaved execution Race Condition (시간에 따라 다른 결과) multi-processor X++ CPU #0 CPU #1 X++ bus Read x into register (11) Increment ALU register (12) Write back to storage box t 4 t 1 t 2 t 3 t 5 Shared Memory x=11 t 2 Read x into register (11) t 3 Increment ALU register t 4 t 5 Write back to storage box

Interleaved execution Race Condition (시간에 따라 다른 결과) multi-processor X++ CPU #0 CPU #1 X++ bus Read x into register (11) Increment ALU register (12) Write back to storage box t 4 t 1 t 2 t 3 t 5 Shared Memory x=11 t 2 Read x into register (11) t 3 Increment ALU register (12) t 4 t 5 Write back to storage box

Interleaved execution Race Condition (시간에 따라 다른 결과) multi-processor X++ CPU #0 CPU #1 X++ bus Read x into register (11) t 1 t 2 t 3 Increment ALU register (12) Write back to storage box t 4 (12) t 5 Shared Memory x=11 t 2 Read x into register (11) t 3 Increment ALU register (12) t 4 t 5 Write back to storage box

Interleaved execution Race Condition (시간에 따라 다른 결과) multi-processor X++ CPU #0 CPU #1 X++ bus Read x into register (11) t 1 t 2 t 3 Increment ALU register (12) Write back to storage box t 4 (12) t 5 Shared Memory x=11 t 2 Read x into register (11) t 3 Increment ALU register (12) t 4 t 5 Write back to storage box (12) Incorrect result

Mutual Exclusion multi-processor CPU #0 CPU #1 bus Read x into register (11) Operation with ALU register (12) Write back to storage box (12) Shared Memory x critical section 공유변수를 액세스 하는 code 부분

Mutual Exclusion multi-processor CPU #0 CPU #1 bus Read x into register (11) Operation with ALU register (12) Write back to storage box (12) Shared Memory 한 순간에는 한 프로세스만 critical section 내에서 작업토록 상호배제 (mutual exclusion) 원칙 x critical section 공유변수를 액세스 하는 code 부분 Read x into register (12) Operation with ALU register (13) Write back to storage box (13) correct result

프로세스 동기화 (synchronization) multi-processor CPU #0 CPU #1 bus Read x into register (11) t 1 Increment ALU register (12) t 2 Write back to storage box (12) t 3 한 프로세스가 critical section을 나올때 Shared Memory x

프로세스 동기화 (synchronization) multi-processor CPU #0 CPU #1 bus Read x into register (11) t 1 Increment ALU register (12) t 2 Write back to storage box (12) t 3 한 프로세스가 critical section을 나올때 Shared Memory x 기다렸다가 critical section으로 들어감 t 4 Read x into register (12) t 5 Operation with ALU register (13) t 6 Write back to storage box (13)

프로세스 동기화 (synchronization) multi-processor CPU #0 CPU #1 bus Read x into register (11) t 1 Increment ALU register (12) t 2 Write back to storage box (12) t 3 Shared Memory x 기다렸다가 critical section으로 들어감 t 4 Read x into register (12) t 5 Operation with ALU register (13) t 6 Write back to storage box (13) 한 프로세스가 critical section을 나올때

Semaphore

Semaphores • Semaphore S – integer 변수 – 지정된 세 operation만 사용가능: P(S), V(S), Init(S) – 이 operation들은 indivisible (atomic) 함 P(S): while (S 0) do no-op ; S--; V(S): S++; S 0? T F Critical Section noop

Semaphores • Semaphore S – integer 변수 – 지정된 세 operation만 사용가능: P(S), V(S), Init(S) – 이 operation들은 indivisible (atomic) 함 음이면 no-op 하며 공회전 (양이 될 때까지) P(S): while (S 0) do no-op; S--; S 0? V(S): S++; T F Critical Section noop

Semaphores • Semaphore S – integer 변수 – 지정된 세 operation만 사용가능: P(S), V(S), Init(S) – 이 operation들은 indivisible (atomic) 함 음이면 no-op 하며 공회전 (양이 될 때까지) P(S): while (S 0) do no-op ; 양이면 -- 하고 진입 S--; V(S): S++; S 0? T F Critical Section noop

Semaphores • Semaphore S – integer 변수 – 지정된 세 operation만 사용가능: P(S), V(S), Init(S) – 이 operation들은 indivisible (atomic) 함 (초기값=1) 음이면 no-op 하며 공회전 (양이 될 때까지) P(S): while (S 0) do no-op ; S--; V(S): S++; 양이면 -- 하고 진입 read/--/store atomic read/++/store atomic S 0? T F Critical Section noop

동시에 Semaphore 액세스? multi-processor P(S) CPU #0 CPU #1 P(S) bus arbitrator bus Shared Memory S=1 둘이 동시에 P(S) 수행 둘이 동시에 bus 사용권 요청 (load Register S) bus arbitrator가 한 CPU 에게만 bus cycle 허가 (예: CPU A) CPU A는 bus 사용 (load S / dec / store S) atomic! mutual exclusion CPU A가 bus 다 쓰면 bus 사용권 해제 CPU B가 bus 사용 (load S …)

Critical Section of n Processes CPU control line semaphore S; /* 초기값은 1 */ bus arbitrator data line Memory • Process: { /* */ critical section 일반 code } while (1);

Critical Section of n Processes CPU control line semaphore S; /* 초기값은 1 */ CPU bus arbitrator data line Memory • Process: { P(S); critical section V(S); 일반 code } while (1); /* 양: dec & 진입*/ /* Zero: wait 후 진입 */ /* Inc S */

Critical Section of n Processes CPU control line semaphore S; /* 초기값은 1 */ bus arbitrator data line Memory • Process: { P(S); critical section V(S); 일반 code } while (1); critical section 직전 – 혼자만 들어가도록 critical section 직후 – 타 프로세스가 진입토록

Critical Section of n Processes CPU control line semaphore S; /* 초기값은 1 */ bus arbitrator data line Memory • Process: { P(S); critical section V(S); 일반 code } while (1); critical section 직전 – 혼자만 들어가도록 critical section 직후 – 타 프로세스가 진입토록 Binary Semaphore – 1/0

Binary Busy-Wait Semaphore Binary Semaphore Busy-Wait loop S 초기값 1 P(S): V(S): while (S 0) do no-op; S--; S++; S 0? T noop F Critical Section 질문: 기다리는 동안 왜 CPU, Memory 소모? 해답: 기다리게 되면 즉시 CPU를 포기 (block itself) 나중에 다른 프로세스가 V(S) 하면 wakeup

Integer (counting) Block-Wakeup Semaphore S 초기값 1 P(S): S--; if (S < 0) do block; V(S): S++; Block-Wakeup S -- S < 0? Zero 음 block wakeup other Critical Section process; 질문: N 프로세스가 동시에 P(S)를 하면? 한 프로세스만 성공 나머지 (N-1) 프로세스는 모두 S-- 하고 block 됨 이때 |S|는 block 된 프로세스의 개수 “Integer Semaphore”

busy-wait 대 block-wakeup semaphore • block-wakeup 시간과 비교해볼 때 – Critical section이 짧으면 Busy-wait – Critical section이 길면 Block-wakeup

Asynchronous/ Synchronous Concurrent Processes Asynchronous Concurrent 아무때나 X 를 access CPU #0 CPU #1 bus Shared Memory 아무때나 X 를 access

Asynchronous/ Synchronous Concurrent Processes Synchronous Concurrent access X 마치면 타 프로세스에게 X 넘김 CPU #0 CPU #1 bus Shared Memory X가 넘어오면 access 마치면 타 프로세스에게 X를 넘김