1 Advanced Operating Systems - Fall 2009 Lecture 7 – February 2, 2009 Dan C. Marinescu Office: HEC 439 B. Office hours: M, Wd 3 – 4:30.

2 Last, Current, Next Lecture Last time: Process synchronization Today Discussion of a problem regarding threads and I/O. More about condition variables and monitors Discussion of a problem about condition variables and monitors. Next time: Atomic transactions

3 Dictionary Monitor: programming language construct for controlled access to critical sections. Monitor: programming language construct for controlled access to critical sections. Transaction (general): an agreement, communication, or movement carried out between separate entities involving the exchange of information, goods, services, or money. Transaction (general): an agreement, communication, or movement carried out between separate entities involving the exchange of information, goods, services, or money. Database transaction: a unit of work performed by a database management system in a coherent and reliable way, independent of other transactions. Atomic transaction: a group of database operations that either all occur, or none occurs. Atomicity prevents updates to the database occurring only partially. Storage: volatile, non-volatile, stable  Can the information be recovered in case of failure?

4 Dictionary (cont’d) Condition variables: indicate an event and have no value. One cannot store a value into nor retrieve a value from a condition variable. If a thread must wait for an event to occur, that thread waits on the corresponding condition variable. Two operations can be performed on a condition variable: Wait(v) and Signal(v). Condition variables: indicate an event and have no value. One cannot store a value into nor retrieve a value from a condition variable. If a thread must wait for an event to occur, that thread waits on the corresponding condition variable. Two operations can be performed on a condition variable: Wait(v) and Signal(v). Memory-mapped file Memory-mapped file: a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource could be a file physically present on-disk, or a device, shared memory object, or other resource that the OS can reference through a file descriptor. Memory-mapped I/O uses the same address bus to address both memory and I/O devices, and the CPU instructions used to access the memory are also used for accessing devices. Regions of CPU's addressable space are reserved for I/O.

5 Dictionary (cont’d) Condition variables: indicate an event and have no value. One cannot store a value into nor retrieve a value from a condition variable. If a thread must wait for an event to occur, that thread waits on the corresponding condition variable. Two operations can be performed on a condition variable: Wait(v) and Signal(v). Condition variables: indicate an event and have no value. One cannot store a value into nor retrieve a value from a condition variable. If a thread must wait for an event to occur, that thread waits on the corresponding condition variable. Two operations can be performed on a condition variable: Wait(v) and Signal(v). Memory-mapped file Memory-mapped file: a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource could be a file physically present on-disk, or a device, shared memory object, or other resource that the OS can reference through a file descriptor. Memory-mapped I/O uses the same address bus to address both memory and I/O devices, and the CPU instructions used to access the memory are also used for accessing devices. Regions of CPU's addressable space are reserved for I/O.

6 Problem We wish to separate the logical from physical reading/writing to a file and implement a file server process which receives from a user process requests to read to files and sends back the data write files and writes the data to the file Two possible implementations: The server maintains a cache of recently used files in memory reading and writing from/cache whenever possible. How should we use multithreading in this case: (a) one thread per process or (b) multiple threads per process. The server maintains a large buffer space and allows a user process to read and write asynchronously to file buffers.

7 Design questions Why separate logical from physical I/O operations? What do we gain and what is the price to pay? Can we use the concept for all types of I/O (e.g., disk and networking?) What are the main features of each one of the two alternatives? Can a user process use multiple files? How many? Could it support multiple user processes? How many? What are the limitations of each approach. Sketch the design for each of the two alternatives. Which one will be easier to implement and why? Which one is more effective at run time and why? Which one of the two options for the first alternative would you consider and why? Are both options feasible? Compare these two alternative with a memory-mapped file. Discuss memory mapped I/O. Is it related to the methods above?

8 Monitors Programming languages constructs. The compiler handles calls to monitors differently than other calls. Only one process may be active within the monitor at a time monitor monitor-name { // shared variable declarations procedure P1 (…) { …. } … procedure Pn (…) {……} Initialization code ( ….) { … } … }

9 Schematic view of a Monitor

10 Blocking a process when it cannot proceed Condition variables condition x, y; Two operations on a condition variable: x.wait () – a process that invokes the operation is suspended. x.signal () – resumes one of processes (if any) that invoked x.wait ()

11 Monitor with Condition Variables

12 A Producer-Consumer Monitor monitor ProducerConsumer; condition full, empty; integer count; procedure enter; if count = N then wait(full); add_Item; count++; if count = 1 then signal(empty); end; procedure remove; if count = 0 then wait(empty); remove_Item; count--; if count = N-1 then signal(full); end; count=0; end monitor;

13 A Producer-Consumer Monitor (cont’d) procedure producer; begin while true do begin produce_Item; ProducerConsumer_enter; end procedure producer; begin while true do begin ProducerConsumer_remove; consume_Item; end

14 Solution to Dining Philosophers monitor DP { enum { THINKING; HUNGRY, EATING) state [5] ; condition self [5]; void pickup (int i) { state[i] = HUNGRY; test(i); if (state[i] != EATING) self [i].wait; } void putdown (int i) { state[i] = THINKING; // test left and right neighbors test((i + 4) modulo 5); test((i + 1) modulo 5); }

15 Dining Philosophers (cont’d) void test (int i) { if ( (state[(i + 4) modulo 5] != EATING) && (state[i] == HUNGRY) && (state[(i + 1) modulo 5] != EATING) ) { state[i] = EATING ; self[i].signal () ; } initialization_code() { for (int i = 0; i < 5; i++) state[i] = THINKING; }

16 Dining Philosophers (cont’d) Each philosopher I invokes the operations pickup() and putdown() in the following sequence: dp.pickup (i) EAT dp.putdown (i)

17 Monitor Implementation Using Semaphores Variables semaphore mutex; // (initially = 1) semaphore next; // (initially = 0) int next-count = 0; Each procedure F will be replaced by wait(mutex); … body of F; … if (next-count > 0) signal(next) else signal(mutex); Mutual exclusion within a monitor is ensured.

18 Monitor Implementation For each condition variable x, we have: semaphore x-sem; // (initially = 0) int x-count = 0; The operation x.wait can be implemented as: x-count++; if (next-count > 0) signal(next); else signal(mutex); wait(x-sem); x-count--;

19 Monitor Implementation The operation x.signal can be implemented as: if (x-count > 0) { next-count++; signal(x-sem); wait(next); next-count--; }

20 Problem for discussion Monitors use condition variables and two operations: WAIT(v) and SIGNAL(v). Why not use a single primitive with a Boolean predicate p: WAITUNTIL(p). For example: WAITUNTIL (v1 n) How would you implement WAITUNTIL?