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Summary Concurrency and Parallelism in Operating Systems
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talks about various methods of using parallelism in operating systems to achieve efficiency
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Concurrency- Doing things that make it seem like they’re going in parallel or even doing things in true
parallelism
Problem: Concurrency inside the OS
- Kernels have multiple processes and threads running parallely
- Kernels maintain different data structures (E.g., process tables) which can be accessed
at the same time. (-> Race Conditions)
Race Conditions
- Multiple threads/processes try to access shared data at the same time
- Result: data can be corrupted or changed
- Need to avoid at all costs
Critical Section
- Piece of code, share resources between threads/processes are changed
- Only one thread should be able to use this at a point in time (Problem)
Solution: Critical Section Problem
- Mutual exclusion (only one at a time)
- Progress: process should be able to enter the critical section at some point in time
- Fairness: wait times are distributed fairly, process cannot be made to wait forever
Mutual Exclusion
- Only one process at a time in the critical section
- Process need to get “permission” before entering the critical section
- Request a lock
- Hold a lock
- Release a lock
, Implementing Enforcing Mutual Exclusion: Software
Peterson’s Solution
2 Shared Variables Used
- Turn: indicates which process is next to enter the
critical section
- Boolean Flag[2]: indicates that the process is ready
to enter the critical section
Side Note 1: Works well on older machines
Side Note 2: Can generalize to multi-process
Conclusion: Satisfies all critical section requirements
(Mutual Exclusion, Progress, Fairness)
Implementing Enforcing Mutual Exclusion: Hardware
- User code can’t access the hardware directly (requires system calls)
Disabling Interrupts - Disable the interrupts
- Carry out the critical section
- Re-enable interrupts
test_and_set() - Using an external function
- Set a boolean variable to true/false
swap_and_compare() - Same stuff as test_and_set() but uses
integer values and compares old/new
values
Implementing Enforcing Mutual Exclusion: OS Based
Solution Mutexes Semaphores
About Contains a boolean variable lock (to Contains an integer variable
indicate availability)
Functions to Manipulate Semaphores
Function to Manipulate a Mutex - wait(): resource acquired and
- Acquire(): call before CS var decremented
- Release(): call after CS - post(): resource released and
var incremented
Needs to be serialized
Needs to be atomic
When m is acquired, it needs to be
released
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