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

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

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)

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

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

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

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

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