Dynamic Processor Sparing

1
Dynamic Processor Sparing

• IBM Academy of Research study on Proactive
  Problem Prediction, Avoidance and Diagnosis
• 04-28-2003

2
Redundancy

• Redundancy always provides a theoretical
  mechanism for avoiding failures
• Requires precise fault detection and containment,
  and operation retry capability
• Non-retry mechanisms possible, but require more
  redundancy
• In a multi-processor environment, each processor
  is inherently redundant

3
Processor Recovery

• Several different recovery techniques used
  throughout the industry
• Most focused on array errors
• ECC, miss/purge/re-fetch,
• Few focused on logic errors
• Z-series uses Instruction Checkpoint Retry

4
Instruction Checkpoint Retry

• Processor micro-architectural checkpoint
  maintained on instruction boundaries
• Protected with robust error detection
• When error detected, processor micro-architected
  state restored from checkpoint
• Operation resumed from last known-good checkpoint
• Checkpoint may be restored on the same processor,
  or an alternate (redundant) processor

5
Dynamic Processor Sparing

• Dynamically move workload from a defective
  processor to a healthy, redundant processor
• Wide range of implementations
• Defective processor still able to make forward
  progress
• Gracefully de-allocate workload from defective
  processor and dispatch to alternate processor
• Defective processor unable to make forward
  progress
• Defective processor abruptly shut-down
• Checkpoint extracted from defective processor and
  transplanted into alternate processor
• Redundant processor could be active or dormant

6
Proactive Implementation of Dynamic Processor
Sparing

• Implement thresholds for recoverable errors
• Take action to remove defective processor from
  configuration
• Guarantee spare (dormant) processors in every
  machine configuration
• Use of spare processors does not reduce overall
  capacity
• Potentially transparent to operating system
• No impact to customer, no parts need to be
  replaced
• Align number of available spares with concurrent
  repair capabilities
• Still support sparing to active processors
• Effective for continuous reliable operation
• More difficult to hide from operating system
• Usually does not avoid parts needing replaced