1. G53SRP: Resource Sharing Issues
Real Time Scheduling
Resource sharingReal Time Scheduling
29/11/2018
G53SRP: Resource Sharing Issues
Chris Greenhalgh
School of Computer Science 1
G53SRP
G53SRPG53SRP 1

2. Resource sharingReal Time Scheduling
Contents
Resource sharing
Priority inversion
Priority inheritance
Priority ceiling emulation
RTSJ support
Wait-free interactions
Summary
Book: Wellings ch. 14 (esp & 14.2) 2
G53SRPG53SRP

3. Resource sharing Locking => blocking
lower-priority process locking a resource shared with a higher-priority process delays it
Additional source of interference – delays worst case response time of higher-priority process
By longest time for which lower priority process holds lock for each acquisition in worst case

4. Example of Blocking Process b has higher priority
Process b attempts to lock shared resource Q
Process
Process a releases lock on Q and is preempted b a 2 4 6 8 10 12 14 16 18
Executing
Preempted
Executing with Q locked
Blocked

5. Priority inversion But its worse than that:
P3 (lowest priority) holds lock
P1 (highest priority) is blocked waiting for lock
P2 (medium priority) executes
Pre-empts P3 – higher priority
so P1 is further delayed by P2 (no shared resource) as well as P1

6. Example of Priority Inversion
Process
Process c attempts to lock shared resource Q c
Process c delayed by
b which pre-empts a
Process a releases
lock on Q b a 2 4 6 8 10 12 14 16 18
Executing
Preempted
Executing with Q locked
Blocked

7. Priority inheritance
Dynamically adjusts priority of a process holding a lock:
(a) Simple priority inheritance
=> to maximum priority of all processes currently waiting for lock
(b) Priority ceiling emulation
=> to maximum priority of all processes that ever acquire lock

8. Example of Simple Priority Inheritance
Process
Process c attempts to lock shared resource Q c
Process a inherits
priority of c
Process a releases
lock on Q b a 2 4 6 8 10 12 14 16 18
Executing
Preempted
Executing with Q locked
Blocked

9. Example of Priority Ceiling Emulation
Process
Process a has normal
priority when not locking Q c b
Process a has elevated
priority while locking Q a 2 4 6 8 10 12 14 16 18
Executing
Preempted
Executing with Q locked
Blocked

10. Priority inheritance notes
Simple priority inheritance
Prevents priority inversion – limits blocking delay
No additional information required
Priority ceiling emulation
Easier to implement
Requires input from programmer or program analysis to decide in advance what priority to associate with each lock/resource
May reduce worst case blocking compared to simple priority inheritance, but may increase delays in common case (always elevates priority)

11. Priority inheritance in RTSJ
Simple priority inheritance required by default
Priority ceiling emulation may also be available
In the specification, but optional to implement
NOT implemented in the lab JVM
Specifiable per monitor (i.e. Java object)
With process-wide default
Implemented by MonitorControl and subclasses…

12. Priority Inheritance classes
<<abstract>> javax.realtime. MonitorControl
extends
javax.realtime. PriorityInheritance
javax.realtime. PriorityCeilingEmulation

13. MonitorControl class package javax.realtime;
public abstract class MonitorControl {
// default
public static MonitorControl getMonitorControl();
public static MonitorControl setMonitorControl(
MonitorControl mc);
// one monitor
public static MonitorControl getMonitorControl(
Object monitor);
Object monitor, }

14. PriorityInheritance class
package javax.realtime;
public class PriorityInheritance
extends MonitorControl {
// singleton
public static PriorityInheritance instance(); }

15. PriorityCeilingEmulation class
package javax.realtime;
public class PriorityCeilingEmulation
extends MonitorControl {
// instance per priority
public PriorityCeilingEmulation
instance(int ceiling);
public int getCeiling();
// max
public static int getMaxCeiling(); }
N.B. throws UnsupportedOperationException
if not supported by JVM

16. Examples of use … // actually default anyway…
MonitorControl.setMonitorControl(
PriorityInheritance.instance());
MonitorControl.setMonitorControl(sharedObj,
PriorityCeilingEmulation.instance(20));
synchronized(sharedObj) {
// will run with priority 20… }
N.B. it is an error to lock an object
with a lower ceiling priority than the thread

17. Wait-free interactions
An alternative to locks and blocking…
Interactions from a (e.g. high priority) thread fail rather than block
So no risk of delaying the (e.g. high priority) thread
E.g. javax.realtime.WaitFreeReadQueue & WaitFreeWriteQueue
Will block write/read (respectively) if queue full/empty (respectively)
Will not block read/write (respectively) but return null/false (respectively) to indicate failure

18. Summary (1)
Resource sharing with locks (mutual exclusion) may introduce additional delays due to blocking
Priority inversion can result
Intermediate priority processes pre-empt lower priority processes holding locks needed by higher priority processes
Priority inheritance prevents this => limits blocking time
Simple priority inheritance increases lock-holder’s priority when higher priority processes are waiting for lock
Priority ceiling emulation always increases lock-holder’s priority

19. Summary (2) RTSJ requires priority inheritance by default
Others (e.g. priority ceiling emulation) can be specified as default or per-monitor
These are optional in implementation(s)
See MonitorControl
Wait-free interaction can be used to avoid some forms of conflict & blocking
E.g. WaitFreeRead/WriteQueue