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Context
§ Apply general multiprocessor resource sharing
protocols to Ada tasking model
- Scheduling
- Implementation
On the Effect of Protected Entry Servicing Policies on the Response - - PowerPoint PPT Presentation
On the Effect of Protected Entry Servicing Policies on the Response - - PowerPoint PPT Presentation
dit UPM On the Effect of Protected Entry Servicing Policies on the Response Time of Ada Tasks Jorge Garrido Juan Zamorano Alejandro Alonso Juan A. de la Puente Universidad Politcnica de Madrid (UPM), Spain Context Apply general
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Problem statement
§ Multiprocessor protocols define general policies for
accessing shared resources
- Upper-bound number of access requests
- Upper-bound access time per request
§ Servicing policies
- Priority based
- FIFO queues
MSRP MrsP FMLP RNLP SEPP M-BWI
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MSRP & MrsP
§ MSRP – Multiprocessor Stack Resource Prolicy § MrsP – Multiprocessor Resource Sharing Protocol ü PCP/SRP-based protocols ü Fixed priority scheduling ü Access cost to globally shared resources bounded
ü Number of concurrent accesses ü Spin-waiting FIFO order
ü Spin-wait & access
- MSRP : non-preemptable
- MrsP : local ceiling priority + helping mechanism
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MSRP & MrsP
P1 P2 P3 Pn
…
queue access time
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MSRP & MrsP
P1 P2 P3 Pn
…
?
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Self-Service Model
- Simple approach
- Potential parallel
execution on multiprocessors
τ1 τ2
Accesses r Entry access to r Blocked on r closed barrier Evaluates r barrier
- More context switches if
- n same processor
- Difficult to implement
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Proxy Model
τ1 τ2
Accesses r Entry access to r Blocked on r closed barrier Evaluates r barrier
- Less context switches
- Simple implementation
- More complex design
- Overhead for server task
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Which should be the preferred servicing implementation model for multiprocessor systems?
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Ideal
τx τy
Accesses r Entry access to r Blocked on r closed barrier Evaluates r barrier
P1 P2
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Self-service approach (I)
- Directly applying self-service on multiprocessor
- Does not preserve PCP/SRP properties
τ1 τ2 t P1
Blocked on r closed barrier
τx
Opens r closed barrier
P2
Accesses r Entry access to r
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Self-service approach (II)
- Potential solution:
- Revert to previous active priority when queued on entry
But …
τ1 τ2 t P1
Blocked on r closed barrier
τx
Opens r closed barrier
P2
Accesses r Entry access to r
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Self-service approach (III)
… any access in between breaks the eggshell model
τ1 τ2 t P1 τx
Opens r closed barrier Executes new call to r Entry access to r
P1 P2
Blocked on r closed barrier
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Self-service approach (IV)
- Can be solved by servicing open entry barriers first
- But that is a ‘delayed proxy model’
- More complex implementation
τ1 τ2 t P1
Blocked on r closed barrier
τx
Opens r closed barrier Serves entry first, then executes new call to r
P1 P2
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Proxy model
- Proxy model preserves SRP/PCP and eggshell properties
- Remote entry servicing has to be notified
τ1 τ2 t P1
Blocked on r closed barrier
τx
Opens r closed barrier Serves entry on τ1 behalf Accesses r again
P2
Entry serviced
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MSRP & MrsP – Proxy model
§ Entry servicing overhead
§Full Ada tasking: number of task accessing entries §Ravenscar profile: one entry servicing
§ Compatible with MrsP helping mechanism
§Entry servicing considered as part of the resource access §Overhead to be taken into account for potential number of preemptions
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Conclusions
§ Self-service not compatible with MSRP & MrsP
§Might be the preferred option for other multiprocessor systems
§ Proxy model can preserve relevant MSRP & MrsP properties
§Entry servicing overhead can be computed for MSRP & MrsP
§ Proxy model compatible with MrsP helping mechanism
§Entry servicing considered as part of the resource access
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