Outline for 3/11/98

• Last time: Distributed Shared Memory
• Administrative:
  • Come with questions next time about exam
  • Remember my offer to accept questions for exam.
• Objective: Harvesting processing cycles across the network.

• Why?
• Which tasks are candidates for remote execution?
• Where to find processing cycles? What does "idle" mean?
• When should a task be moved?
• How?

• Load imbalances. Parallel program completion time determined by slowest thread. Speedup limited.
• Utilization. In trend from shared mainframe to networks of workstations -> scheduled cycles to statically allocated cycles
  • "Ownership" model
  • Heterogeneity

• Explicit submission to a "batch" scheduler (e.g., Condor) or Transparent to user.
• Should be demanding enough to justify overhead of moving elsewhere. Properties?
• Proximity of resources.
  • Example: move query processing to site of database records.
  • Cache affinity

Finding Destination

• Defining "idle" workstations
  • Keyboard/mouse events? CPU load?
• How timely and complete is the load information (given message transit times)?
  • Global view maintained by some central manager with local daemons reporting status.
  • Limited negotiation with a few peers
  • How binding is any offer of free cycles?
• Task requirements must match machine capabilities

When to Move

• At task invocation. Process is created and run at chosen destination.
• Process migration, once task is already running at some node. State must move.
  • For adjusting load balance (generally not done)
  • On arrival of workstation's owner (vacate, when no longer idle)

• Condor example: Central manager with each machine reporting status, properties (e.g. architecture, OS). Regular match of submitted tasks against available resources.
• Decentralized example: select peer and ask if load is below threshold. If agreement to accept work, send task. Otherwise keep asking around (until probe limit reached).

• Issue - Execution environment.
  • File access - possibly without user having account on destination machine or network file system to provide access to user's files.
  • UIDs?
• Remote System Calls (Condor)
  • On original (submitting) machine, run a "shadow" process (runs as user)
  • All system calls done by task at remote site are "caught" and message sent to shadow.

Checkpointing current execution state (both for recovery and for migration)

• Generic representation for heterogeneity?
• Condor has a checkpoint file containing register state, memory image, open file descriptors, etc.
  Checkpoint can be returned to Condor job queue.
• Mach - package up processor state, let memory working set be demand paged into new site.
• Messages in-flight?