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Concurrent Control with “Readers” and “Writers” - PowerPoint PPT Presentation
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Concurrent Control with “Readers” and “Writers”. Sagnik Bhattacharya Siddharth Dalal. Sequential Programs. Terminating Single thread or process Number of possible execution paths finite Constructions like if-then-else, while are strictly defined
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Presentation Transcript
Sequential Programs
- Terminating
- Single thread or process
- Number of possible execution paths finite
- Constructions like if-then-else, while are strictly defined
- Mathematical tools (though complicated) exist.
Concurrent Programs
- Possibly non-terminating
- Infinite number of possible execution paths
- Synchronization
- Scheduling
- Prevention of starvation
- Lack of formal tools to deal with concurrency
The P and V operations
- Sometimes referred to as wait and signal
- P(S) :
- while S < 0 do no-op;
- S := S - 1;
- V(S) :
- S := S + 1;
Waiting readers have higher priority.Problem 1
GLOBALS
mutex, w : semaphore := 1; // Mutual exclusion semaphores
readcount : integer := 0;
Problem 1
READER
P(mutex);
readcount++;
if (readcount=1) then P(w);
V(mutex);
CRITICAL REGION
P(mutex);
readcount--;
if (readcount=0) then V(w);
V(mutex);
WRITER
P(w);
CRITICAL REGION
V(w);
Waiting writers have higher priorityProblem 2
GLOBALS
mutex1 : semaphore := 1; // all semaphores here are mutual exclusionmutex2 : semaphore := 1;mutex3 : semaphore := 1;w : semaphore := 1;r : semaphore := 1;readcount, writecount : integer := 0;
Problem 2
READER
P(mutex3);
P(r);
P(mutex1);
readcount++;
if (readcount is 1) then P(w);
V(mutex1);
V(r);
V(mutex3);
CRITICAL REGION
P(mutex1);
readcount--;
if (readcount is 0) then V(w);
V(mutex1);
Problem 2
WRITER
P(mutex2);
writecount++;
if (writecount is 1) then P(r);
V(mutex2);
P(w);
CRITICAL REGION
V(w);
P(mutex2);
writecount--;
if (writecount is 0) then V(r);
V(mutex2);
Does this guarantee FIFO order?
- For Readers?
- NO!!
- For Writers?
- NO!!
To achieve FIFO order…
- Make P and V operations more powerful
- Use array of semaphores
To achieve prioritized order…
- Inter-process communication
- Complicated
- Shared memory
- Readers and Writers problem
Philosophical question
- Should we build safety critical concurrent systems?
