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Fault Tolerance BOF

Fault Tolerance BOF. Possible CBHPC paper Co-authors wanted Tammy, Rob, Bruce, Daniel, Nanbor, Sameer, Jim, Doug, David What infrastructure is needed to enable application-level FT in (component) applications? Little experience with anything beyond checkpoint/restart (CR) in general

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Fault Tolerance BOF

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  1. Fault Tolerance BOF • Possible CBHPC paper • Co-authors wanted • Tammy, Rob, Bruce, Daniel, Nanbor, Sameer, Jim, Doug, David • What infrastructure is needed to enable application-level FT in (component) applications? • Little experience with anything beyond checkpoint/restart (CR) in general • Assume an FT-friendly lower-level environment • Event service for awareness of faults • Ability to request certain behavior from lower-level software • Example: scheduler shouldn’t automatically kill an FT job

  2. Use Cases • MCMD app, 1 node fails • Recovery restarting failed task or ignore failure and go on (self-healing) • MCMD C/R

  3. Checkpoint/Restart Taxonomy • System-level • Eg: BLCR, Cray XT (site option?), but not universally available • Store (complete) memory image to stable storage • Daemon schedules checkpoints • No application (or framework) involvement • Possibly app can request checkpoint • Potential problems: open files, driver state, in-flight messages, etc. • Component i/f to system c/r API • Component support for intelligent reduced checkpointing (MyState interface)

  4. Application-level • Coordinated • Uncoordinated • Causal, Message Logging, etc. • Incremental checkpointing support • Capture component assembly • Checkpoint data component • In-memory (copy or RAID), disk, write-behind, etc., special system services • Quality of Fault Tolerance • What does interface look like? Like RMI • Components to detect faults • Reduced storage (satisfying stability criteria, but not all available data) • Checkpoint-free FT data holders

  5. How to capture/restore state of blackbox components? • Components implement SaveYourself method • Central service invokes SaveYourself on all components that implement it • Serialized data sent to central service for storage • How to restart and restore state? • Not all components will implement SaveYourself • May not have state to store • May be error • Check at start of execution and notify user • RestoreYourself • Specify state (in SIDL file?) and auto-gen serialize/unserialize methods

  6. Another idea • Components register their state data with a central service • Breaks encapsulation and OOness, but not a major violation • Could be higher performance

  7. Recovery • Local restore vs global restore? • Rollback • Need to save execution path at which checkpoint was taken? • Put responsibility on app components? • Extend GoPort abstraction to include save/restore/restart? • Framework tells components to restart • What order? Shouldn’t matter

  8. Paper themes • How components can help with FT • Abstracting FT services into reusable components • Abstracting FT requirements into ports • How components make FT more complicated • Don’t have monolithic view of application (state) • Going beyond CR

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