Monitors: Mesa & Hoare
Hoare
- "wait": causes the calling program to be delayed
- "signal": causes exactly one of the waiting programs to be resumed immediately.
| Hoare | Mesa | Java | |
|---|---|---|---|
| Monitor Lock | ALL | entry | Synchronized[1] |
| Condition | Vars | Y | No/yes (one implicit CV per object) |
| Wait | Yes | Y | Yes |
| Signal | Yes | Y -> nofify & Broadcast | Yes notify(), notifyAll() |
| Granularity/粒度 | module | Monitor class & Monitor record | Monitor/instance, sync. Blocks |
| Aborts | No | Unwind handler, reestablish invariant | KILL, STOP[3] |
| Nested calls | No | Yes4 | Yes5 |
- [1] Every object is a monitor. More than half of the code implementing Object class is about synchronization.
- [2] 1. Synchronized block; 2. Class / Instance associated synchronization
- [3] KILL and STOP have already been deprecated in Java. Instead, a thread t0 set a variable to show the intention to kill another thread t1. t1 then does the clean-up and do the stop.
How are the semantics of wait and notify different
- Java is similar to Mesa.
- In Mesa, wake up only when …
- In Hoare immediately exe ( go back to the ready q) -- guaranteed to get into the monitor.
- In Mesa, cons: need to check again. Pros: fairness,
Hoare
- Signal immediately transfer control
Mesa
- while (not invariant) wait (c)
Questions
Why did Mesa make this change
- Performance: extra context switches
- Remove scheduling from inside of monitor
Where do the extra context switches come from?
- (S -- Signal Process, W -- Wait Process)
- Hoare: S switch to W, W goes, have to swtich back to S
- Mesa:
Why Java does not use condition variables
- simplicity of a single lock per object or class
Why does Mesa have monitor records as well as monitor modules?
- fine-grained concurrency (并发性)
- Note that Java chose fine-grained as the default
Mesa Handling aborts
- Mesa has explicit support for shutting down a process such that it can establish the monitor invariant before dying.
What are the implications for Java where threads can be killed?
- JVM has to release locks
A network socket is an endpoint of a connection across a computer network. Today, most communication between computers is based on the Internet Protocol; therefore most network sockets are Internet sockets.
Priority
priority inversion is a problematic scenario in scheduling in which a high priority task is indirectlypreempted by a medium priority task effectively "inverting" the relative priorities of the two tasks.
- 将low priority task临时提高priority，快点运行完（运行完之后，再降回），这样high priority就不用一直等着那个resource了。一直等着就是说每次轮到CPU了，却只能等／被唤出。