G53SRP: Resource Sharing Issues

1. G53SRP: Resource Sharing Issues Chris Greenhalgh School of Computer Science 1

2. Contents • Resource sharing • Priority inversion • Priority inheritance • Priority ceiling emulation • RTSJ support • Wait-free interactions • Summary • Book: Wellings ch. 14 (esp. 14.1 & 14.2) 2

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. Process b has higher priority Process b attempts to lock shared resource Q Process a releases lock on Q and is preempted Example of Blocking Process b a 0 2 4 6 8 10 12 14 16 18 Preempted Executing 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. Process c attempts to lock shared resource Q Process c delayed by b which pre-empts a Process a releases lock on Q Example of Priority Inversion Process c b a 0 2 4 6 8 10 12 14 16 18 Preempted Executing 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. Process c attempts to lock shared resource Q Process a inherits priority of c Process a releases lock on Q Example of Simple Priority Inheritance Process c b a 0 2 4 6 8 10 12 14 16 18 Preempted Executing Executing with Q locked Blocked

9. Process a has elevated priority while locking Q Process a has normal priority when not locking Q Example of Priority Ceiling Emulation Process c b a 0 2 4 6 8 10 12 14 16 18 Preempted Executing 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); public static MonitorControl setMonitorControl( Object monitor, MonitorControl mc); }

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