Transcript Lecture7

Advanced Operating Systems - Fall 2009
Lecture 7 – February 2, 2009
Dan C. Marinescu
 Email: [email protected]
 Office: HEC 439 B.
 Office hours: M, Wd 3 – 4:30.
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Last, Current, Next Lecture
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Last time:
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Today
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Process synchronization
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:
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Atomic transactions
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Dictionary
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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.
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?
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Dictionary (cont’d)
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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: 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.
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Dictionary (cont’d)
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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: 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.
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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
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read to files and sends back the data
write files and writes the data to the file
Two possible implementations:
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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.
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Design questions
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Why separate logical from physical I/O operations?
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What are the main features of each one of the two alternatives?
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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?)
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?
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Monitors
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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 ( ….) { … }
…
}
}
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Schematic view of a Monitor
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Blocking a process when it cannot proceed
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Condition variables
 condition x, y;
Two operations on a condition variable:
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x.wait () – a process that invokes the operation is suspended.
x.signal () – resumes one of processes (if any) that invoked
x.wait ()
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Monitor with Condition Variables
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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;
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A Producer-Consumer Monitor (cont’d)
procedure producer;
begin
while true do
begin
produce_Item;
ProducerConsumer_enter;
end
end
procedure producer;
begin
while true do
begin
ProducerConsumer_remove;
consume_Item;
end
end
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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);
}
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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;
}
}
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Dining Philosophers (cont’d)
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Each philosopher I invokes the operations pickup()
and putdown() in the following sequence:
dp.pickup (i)
EAT
dp.putdown (i)
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Monitor Implementation Using Semaphores
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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.
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Monitor Implementation
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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--;
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Monitor Implementation
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The operation x.signal can be implemented as:
if (x-count > 0) {
next-count++;
signal(x-sem);
wait(next);
next-count--;
}
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Problem for discussion
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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 < 0 or v2 + v3 > n)
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How would you implement WAITUNTIL?
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