NotesProblems
Quite a few:
- Performance
- Interaction with thread priorities
- Deadlocks
- Starvation
- Synchronization priorities
Performance
Blocking synchronization
- Context switch if the mutex is not available
- Makes CPU available to others
Spin-lock synchronization
- Wastes CPU resources
- But if mutex is available, or will be soon, then we avoid the context switch overhead
Hybrid Synchronization (Solaris)
- Starts out with a spin-lock
- if mutex is available, enter the critical section
- Spin for a limited time, if mutex is not available, use blocking synchronization
Starvation
If the synchronization solution is not well implemented, a thread may starve.
EX: A semaphore implemented in LIFO order (a stack)
Deadlocks
There are 4 conditions that must be true for deadlock to occur, they are:
- Mutual Exclusion: A process or thread must be the only one able to access a resource, forcing others to not be able to use it
- No Preemption: A resource can only be given up when the holding process wants to, it can not be preempted to do so
- Circular Wait: A process waits for a resource held by another process, which is also waiting for the original process to release the resource
- Hold and Wait: A process is holding onto a resource and requesting/waiting for a resource held by another process.
Example
| Thread 1 | Thread 2 |
|---|---|
| P(s1) | P(s2) |
| P(s2) | P(s1) |
| V(s1) | V(s2) |
| V(s2) | V(s1) |
| In this case, the threads both need to access s1 and s2, which leads them into a deadlock. |