CS 603 Threads, Processes, and Agents

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CS 603Threads, Processes, and Agents

• March 18, 2002

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Goal Operate at Remote Site

• Client-Server Model
• Client requests service
• Scope of available services known in advance
• Remote Procedure Call
• Call existing procedure at remote site
• Problem Remote Site must already know what we
  want to do!

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Goal Operate at Remote Site

• How is this handled in non-distributed system?
• Write and run a program!
• Are there single-system analogues to starting
  arbitrary operation?
• Processes
• Threads

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Process Model

• Basic idea Virtual MachineModel complete
  ownership of machine
• Memory
• Files
• Processing
• How does this fit with Distributed System?
• It doesnt
• Solution Multiple Processes
• Each process on different virtual machine
• In Distributed System, different real machine

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Multiple Processes How

• Fork Create child as clone of current process
• Wouldnt execution be same as parent?Process ID
  is new, as are a few other things
• Exit Terminate process
• Frees resources, closes files, etc.
• Wait Parent waits for child to terminate
• Block until child exits
• What about multiple children?

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Inter-process Communication

• Not very interesting if processes completely
  independent
• Files Process can write and read same file
• Synchronization difficulties
• Pipes One-way FIFO buffer between processes
• Sounds like stream-oriented communication
• Shared Memory
• Explicitly declared shared area
• Also semaphore primitive
• This is beginning to sound like a distributed
  system!

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Remote Fork

• Why isnt Remote Fork a primitive
• Fork clones process
• Copies memory
• Same open file descriptors
• Expensive at best, infeasible at worst
• Solution Process as local proxy
• Fork creates process
• Child uses RPC/? to start remote execution

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Threads

• Multiple activities in same address space
• Process model creates new address space
• Threads share i.e., all memory is shared
• Shared variables for communication
• Mutex mutual exclusion primitive
• Condition variable wait/signal
• Join wait for particular process
• Even more difficult in distributed system
• Requires distributed shared memory model
• Again use as local proxy to start remote code

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So where are we?

• Models for Remote Processing
• Server Request documented service
• RPC Request execution of existing procedure
• What if operation we want isnt offered remotely?
• Solution Agents / Code Migration

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Code Migration

• Process consists of
• Code
• Resources (files, devices, etc.)
• Execution (data, stack, registers, etc.)
• Fork copies everything
• Is this needed?
• Solution Copy part of the process

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Types of Code Migration

• Weak mobility Copy only code
• Program starts from initial state
• Example Java applets
• Strong mobility Copy code and execution
• Resume execution where it stopped
• But doesnt necessarily have same resources (less
  than fork)
• Example DAgents (later), cluster computing
  (Condor, LSF)

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Types of Code Migration

• Sender Initiated
• Receiver Initiated
• Examples
• Java Applets
• Receiver Initiated
• Cluster computing
• Sender Initiated?
• Central manager initiated?

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Types of Code Migration

• Where executed?
• In target process
• In new process
• Strong Mobility Move vs. Copy
• Migrate process Ceases at originating site
• Clone process Two copies in parallel

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Types of Code Migration
From Andrew Tanenbaum, Distributed Operating
Systems, 1995.
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The Hard Part Resources

• Migrated process still needs resources
• Options to Connect to a Resource (Fugetta et al.,
  1998)
• Binding by identifier (e.g., URL)
• Attach to the same resource
• Binding by value (e.g., standard libraries)
• Bind to equivalent resource
• Bind by type (e.g., local printer)
• Bind to resource with same function

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The Hard Part Resources

• Alternative Move the Resource
• Unattached resources (e.g., data files)
• Relatively easy to move
• Fastened resource (e.g., database)
• Expensive to move
• Fixed resource (e.g., communications endpoint)
• Cant be moved

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Resource Binding
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What about Heterogeneity?

• Assumption Same code runs at both source and
  target
• Weak mobility Can compile/execute same source
• Strong mobility More difficult
• C/Pascal/etc. Mobile only at subroutine call
• Use same mechanisms as RPC
• Translate parameters and stack/globals
• Return goes back to original machine
• Java Hide heterogeneity in virtual machine

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What Next?

• Specific Example DAgents
• Research Project at Dartmouth
• Basic idea Mobile agents
• Based on Tcl
• Project 3 will use DAgents
• Still working on appropriately scoped task
• Feel free to start reading
• http//agent.cs.dartmouth.edu