Distributable Threads An overview

1
Distributable Threads An overview

• Venkita Subramonian
• Nov 20 2003
• CS523 Fall 2003

2
Outline

• CORBA and RTCORBA overview
• Dynamic scheduling in RTCORBA
• Distributable threads model
• Preliminary implementation

3
Overview of CORBA

• Common Object Request Broker Architecture (CORBA)
• A family of specifications
• OMG is the standards body
• Over 800 companies
• CORBA defines interfaces, not implementations
• It simplifies development of distributed
  applications by automating/encapsulating
• Object location
• Connection memory mgmt.
• Parameter (de)marshaling
• Event request demultiplexing
• Error handling fault tolerance
• Object/server activation
• Concurrency
• Security

• CORBA shields applications from heterogeneous
  platform dependencies
• e.g., languages, operating systems, networking
  protocols, hardware

4
RTCORBA 1.0

• Limitations of CORBA
• Lack of QoS specifications
• Lack of QoS enforcement
• Lack of real-time programming features
• Lack of performance optimizations

• RT CORBA adds QoS control to regular CORBA to
  improve application predictability, e.g.,
• Bounding priority inversions
• Managing resources end-to-end
• Policies mechanisms for resource
  configuration/control in RT-CORBA include
• Processor Resources
• Thread pools
• Priority models
• Portable priorities
• Communication Resources
• Protocol policies
• Explicit binding
• Memory Resources
• Request buffering

5
RTCORBA 2.0

• RTCORBA 1.0 addresses static scheduling of
  resources
• RTCORBA 2.0 intended for dynamically scheduled
  distributed real-time systems
• Originally referred to as Dynamic Real-Time CORBA
• Addresses the need to spread a real-time
  application across multiple physically dispersed
  computing nodes
• Addresses end-to-end scheduling
• Introduces a Distributable thread abstraction for
  an end-to-end computation

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What is a Distributable Thread?

• Based on the "distributed (trans-node) thread"
  abstraction first introduced in the Alpha
  distributed real-time OS kernel
• An Architectural Overview Of The Alpha Real-Time
  Distributed Kernel (1993) - Raymond K. Clark, E.
  Douglas Jensen, Franklin D. Reynolds
• Also influential in the Distributed Real-Time
  Specification for Java, proposed in Sun Java
  Specification Request JSR-000050
• A programming model abstraction for end-to-end
  control flow
• A thread that can execute operations in objects
  without regard for physical node boundaries
• A program may consist of multiple concurrent
  distributable threads

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How is this relevant to CS523?

• Extending concept of thread scheduling end-to-end
• The distributable thread (not a local thread) is
  the schedulable entity
• Pluggable dynamic schedulers
• Scheduler interfaces standardized by RTCORBA 2.0
• Similar to the hierarchical scheduling framework
  in our project
• Scheduling Parameter interfaces (some of them
  atleast) defined by RTCORBA 2.0
• Similar to concept of rms_attr_t, edf_attr_t (in
  our project)

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Distributed system model
DT carries scheduling parameters with it
Host 1
Host 2
RTCORBA 2.0 Scheduler
RTCORBA 2.0 Scheduler
DT
Physical binding of a single DT with two OS
threads
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DTs within a single node
Scheduler invocation points

• Programming model very similar to transaction
  programming model in databases
• begin_transaction()
• Do atomic stuff
• end_transaction()

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DTs spanning multiple endsystems
BSS begin_scheduling_segment ESS
end_scheduling_segment
Application call Interceptor call
Scheduling parameters serialized at ORB
interceptor points for migration
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Distributable thread implementation

• Vast area of research
• Several issues to be addressed
• Mutual exclusion and TSS semantics with same GUID
  mapped to two different TIDs
• Scheduling parameter propagation
• Heterogeneous schedulers
• Focus of this discussion
• Scheduler implementation

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Condition Variable based implementation
Middleware Scheduler/Dispatcher
bss
uss
broadcast
Most eligible
Condition variable
wait
bss
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CV approach - variation

• Instead of a single CV, associate a CV with each
  thread
• Each thread waits on its own CV
• Only most eligible thread is signalled instead of
  a broadcast to the single CV
• Less number of context switches
• More resource (CV) consumption

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Priority based approach

• Assign thread priority based on scheduling
  parameters
• Scheduling algorithm determines eligibility
  criterion
• OS dispatcher takes care of actual dispatching of
  threads
• Middleware Scheduler just maps from scheduling
  parameters to OS priority
• Only coarse mapping from scheduling parameter to
  OS priority
• T1 Deadline 100 ticks from now Assign
  priority 5
• T2 Deadline 95 ticks from now Assign priority
  6
• T3 Deadline 90 ticks from now What priority
  do we assign?

sched-params
Middleware Scheduler
OS prio
OS Dispatcher
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Solution Use priority levels

• Four priority levels
• Scheduler thread runs at executive level
• Active and Inactive levels to control thread
  execution based on eligibility
• Blocked priority level to park threads that
  have blocked
• This ensures that the thread gets a chance to be
  rescheduled when it unblocks

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Issues with Level based approach

• Threads have to explicitly inform the scheduler
  that they are going to block
• One solution is to write wrappers for each
  blocking function
• Very cumbersome
• Error-prone
• Scheduler activation concept might be useful
• Portability of middleware code an issue
• Idea of a Block Catcher thread (something like
  an idle thread)
• The Block catcher thread just signals the
  scheduler thread to wakeup

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CV vs Priority implementations
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Conclusion

• Lot of concepts from CS523 found applicable to
  this implementation
• Implementation done as part of Kokyu framework
  and made available as part of ACE/TAO source.
• A fertile area for new research
• Future work
• Application in Real-world scenarios (DARPA PCES
  project Dr. Chris Gill)
• Group Scheduling abstraction (KU Dr. Doug
  Niehaus)
• Useful Links
• http//www.omg.org/technology/documents/formal/RT_
  dynamic.htm
• http//www.real-time.org/realtimecobra.htm
• http//www.cs.wustl.edu/schmidt/
• http//www.cs.wustl.edu/cdgill/