Outline for Today. Readers/Writers Problem

Transcription

1 Objective: Outline for Today Reader-writer problem Message Passing Administrative details: Check that you know where your demo will be For me, Microsoft Lab on 2 nd floor LSRC. 1 Readers/Writers Problem Synchronizing access to a file or data record in a database such that any number of threads requesting read-only access are allowed but only one thread requesting write access is allowed, excluding all readers. 2

2 Template for Readers/Writers Reader() {while (true) { Writer() {while (true) { startread(); startwrite(); read write endread(); endwrite(); } } } } 5 A Semaphore Solution int readcount = 0; semaphore mutex = 1; semaphore writeblock = 1; 7

3 Reader(){ while (TRUE) { other stuff; P(mutex); readcount = readcount +1; if(readcount == 1) P(writeBlock); V(mutex); access resource; P(mutex); readcount = readcount -1; if(readcount == 0) V(writeBlock); V(mutex); Writer(){ while(true){ other stuff; P(writeBlock); access resource; V(writeBlock); }} }} 8 Attempt at Writer Priority int readcount = 0, writecount = 0; semaphore mutex1 = 1, mutex2 = 1; semaphore readblock = 1; semaphore writeblock = 1; 9

4 Reader2 Reader(){ while (TRUE) { other stuff; P(readBlock); P(mutex1); readcount = readcount +1; if(readcount == 1) P(writeBlock); V(mutex1); V(readBlock); access resource; P(mutex1); readcount = readcount -1; if(readcount == 0) V(writeBlock); V(mutex1); }} Reader1 Writer(){ while(true){ other stuff; P(mutex2); writecount = writecount +1; if (writecount == 1) P(readBlock); V(mutex2); P(writeBlock); access resource; V(writeBlock); P(mutex2); writecount - writecount - 1; if (writecount == 0) V(readBlock); V(mutex2); }} Writer1 10 Reader2 Reader(){ while (TRUE) { other stuff; P(readBlock); P(mutex1); readcount = readcount +1; if(readcount == 1) P(writeBlock); V(mutex1); V(readBlock); access resource; P(mutex1); readcount = readcount -1; if(readcount == 0) V(writeBlock); V(mutex1); }} Reader1 Writer(){ while(true){ other stuff; P(mutex2); writecount = writecount +1; if (writecount == 1) P(readBlock); V(mutex2); P(writeBlock); access resource; V(writeBlock); P(mutex2); writecount - writecount - 1; if (writecount == 0) V(readBlock); V(mutex2); }} Writer1 11

5 Reader(){ while (TRUE) { other stuff; P(writePending); P(readBlock); P(mutex1); readcount = readcount +1; if(readcount == 1) P(writeBlock); V(mutex1); V(readBlock); V(writePending); access resource; P(mutex1); readcount = readcount -1; if(readcount == 0) V(writeBlock); V(mutex1); }} Writer(){ while(true){ other stuff; P(mutex2); writecount = writecount +1; if (writecount == 1) P(readBlock); V(mutex2); P(writeBlock); access resource; V(writeBlock); P(mutex2); writecount - writecount - 1; if (writecount == 0) V(readBlock); V(mutex2); }} 12 Reader2 Reader(){ while (TRUE) { other stuff; P(writePending); P(readBlock); P(mutex1); readcount = readcount +1; if(readcount == 1) P(writeBlock); V(mutex1); V(readBlock); V(writePending); access resource; P(mutex1); readcount = readcount -1; if(readcount == 0) V(writeBlock); V(mutex1); }} Reader1 Assumptions about semaphore semantics? Writer(){ while(true){ other stuff; P(mutex2); writecount = writecount +1; if (writecount == 1) P(readBlock); V(mutex2); P(writeBlock); access resource; V(writeBlock); P(mutex2); writecount - writecount - 1; if (writecount == 0) V(readBlock); V(mutex2); }} Writer1 13

6 Guidelines for Choosing Lock Granularity 1. Keep critical sections short. Push noncritical statements outside of critical sections to reduce contention. 2. Limit lock overhead. Keep to a minimum the number of times mutexes are acquired and released. Note tradeoff between contention and lock overhead. 3. Use as few mutexes as possible, but no fewer. Choose lock scope carefully: if the operations on two different data structures can be separated, it may be more efficient to synchronize those structures with separate locks. Add new locks only as needed to reduce contention. Correctness first, performance second! 16 Interprocess Communication - Messages Assume no explicit sharing of data elements in the address spaces of processes wishing to cooperate/communicate. Essence of message-passing is copying (although implementations may avoid actual copies whenever possible). Problem-solving with messages - has a feel of more active involvement by participants. 17

7 Issues System calls for sending and receiving messages with the OS(s) acting as courier. Variations on exact semantics of primitives and in the definition of what comprises a message. Naming - direct (to/from pids), indirect (to distinct objects - e.g., mailboxes, ports, sockets) How do unrelated processes find each other? Buffering - capacity and blocking semantics. Guarantees - in-order delivery? no lost messages? 18 Send and Receive 19

8 5 DP Direct Send/Receive Message Passing Between Philosophers Philosopher 4 Fork please? Philosopher 0 (thinking) Philosopher 3 (eating) Philosopher 1 Philosopher 2 20 Philosopher 4 Philosopher 0 (thinking) Umm. Oh yeah. Philosopher 3 (eating) Philosopher 1 Philosopher 2 21

9 Philosopher 4 Fork please? Philosopher 0 (thinking) Philosopher 3 (eating) Philosopher 1 Philosopher 2 22 Philosopher 4 I ll ignore that request until I m done Philosopher 3 (eating) Fork please? Philosopher 0 (thinking) Philosopher 1 Philosopher 2 23

10 Philosopher 4 Philosopher 0 (thinking) Fork please? Philosopher 3 (eating) Philosopher 2 Philosopher 1 Fork please? 24 Client / Server server-> 25

11 Example: Time Service 26 Example: Time Service via Messages 27

12 Client / Server with Threads 28 Hiding Message-Passing: RPC 29

13 Remote Procedure Call - RPC Looks like a nice familiar procedure call P 0 P 1 result = foo(param); Receive 30 Remote Procedure Call - RPC Looks like a nice familiar procedure call P 0 P 1 result = foo(param); blocked here please do foo for P 0 with param Receive 31

14 Remote Procedure Call - RPC Looks like a nice familiar procedure call P 0 P 1 result = foo(param); blocked here please do foo for P 0 with param Receive r = foo(param); // actual call 32 Remote Procedure Call - RPC Looks like a nice familiar procedure call P 0 P 1 result = foo(param); blocked here returning r to P 0 Receive r = foo(param); // actual call Reply 33

15 Remote Procedure Call - RPC Looks like a nice familiar procedure call P 0 P 1 result = foo(param); returning r to P 0 Receive r = foo(param); // actual call Reply 34 Remote Procedure Call - RPC Looks like a nice familiar procedure call P 0 P 1 result = foo(param); Receive r = foo(param); // actual call Reply 35

16 5DP via RPC with Fork Manager Looks like a nice familiar procedure call Philosopher 0 Fork Server result = PickupForks (0); Receive r = proc(param); // explicit queuing when necessary Reply 36 Example: Time Service via RPC 37

17 RPC Issues 39 Naming Destinations for Messages: Ports replyport->receive(response); replyport-> 40

18 Advantages of Ports 41 Port Issues 42

19 Examples of Ports in Real Systems 43 Sockets for Client-Server Message Passing Server 1. Create a named socket syscalls: sfd = socket( ) bind (sfd, ptr, ) 2. Listen for clients listen(sfd, ) 4. Connection made and continue listening cfd=accept(sfd, ) 5. Exchange data write(cfd, ) 6. Done: close(cfd); close(sfd); name name Client 3. Create unnamed socket & ask for connection syscalls: cfd=socket( ) err=connect(cfd, ptr, ) 5. Exchange data read(cfd, ) 6. Done: close(cfd); 44

20 Notification of Pending Messages 45 Polling: Select 46

21 Immediate Notification: Upcalls 47 Advantages of Server Isolation Afforded by Message Passing 48

22 Reconsidering the Kernel Interface and OS Structure 49 Servers and Microkernels 50

23 Microkernel with User-Level OS Server Processes fork open 51 End-to-End Argument Application-level correctness requires checking at the endpoints to ensure that the message exchange accomplished its purpose Application semantics involved Notification of successful delivery (UPS tracking) is not as good as a direct response (thank you note) from the other end. Reliability guarantees in the message-passing subsystem provide performance benefits (shortcircuiting corrective measures). Re-transmitting packet may save re-transferring whole file. 52