Fault tolerance techniques

Fault tolerance techniques

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  Fault Tolerance Techniques Dr.J.Nandhini Professor /ECE Jai Shriram Engineering College Tirupur
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  Fault Tolerance • FaultTolerant – able to continue operating despite the failure of a limited subset of their hardware or software. • Robust – reduce the number of faults that the system will encounter • Careful specification and design • Use high grade components to reduce the manufacturing errors • Extensive system wide testing • Short term failure – quickly correcting for a failure to allow immediate deadlines to be met. • Long term response – locating the failure, determining the best response to it and initiating a recovery and reconfiguration procedure.
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  • Fault tolerance-ability of a system to maintain its functionality, even in the presence of faults. • Fault – defect or flow that occurs in some hardware or software component • Error – manifestation of a fault • Failure – departure of a system from the service required.
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  Types of Fault Canbe classified as 1) Hardware fault 2) Software fault Hardware Fault – some physical defect that can cause a component to malfunction. Eg: A broken wire or the output of a logic gate Software Fault – software fault is a bug that can cause the program to fail for a given set of inputs
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  • Fault Latency– duration between the onset of a fault and its manifestation as an error. • Error Recovery – is the process by which the system attempts to recover from the effects of an error. • There are 2 forms of error recovery • Forward error recovery – the system is rolled back to a moment in time before the error is believed to have occurred and the computation is carried out again. • Backward error recovery – uses time redundancy since it consumes additional time to mask the effects of failure
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  Cause of Failure •Errors in the specification or design • Defects in the components • Environmental effects
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  Errors in thespecification or design • Mistake in the specification and design are very difficult to guard against • Hardware and software failures • Very difficult to ensure the specification is right • Specifications can be reviewed by persons unconnected with writing them
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  Defects in theComponents • Hardware components can develop defects • These may arise from manufacturing defects or from defects caused by the wear and tear of use.
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  Environmental effects • Poorventilation or excessively high ambient temperatures can melt components or otherwise damage them • If computer is in a missile, it can undergo high g-forces and vibrational stress
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  Fault types • Faultsare classified according to their temporal behavior and output behavior • Temporal behavior classification 1) Permanent 2) Intermittent 3) Transient
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  • Permanent fault: does not die away with time but remains until it is repaired or the affected unit is replaced • Intermittent fault: cycles between the fault active and fault benign states (loose wires) • Transient fault: dies away after some time. Hard to catch. Temporal behavior
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  Output behavior • Afault is characterized by the nature of the errors that it generates. • Two categories of output behavior: • Non malicious • Malicious
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  Fault detection • Twoways to determine that a processor is malfunctioning i) Online detection ii) Offline detection
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  • Online detection:goes on in parallel with normal system operation • One way of doing this is to check for any behavior that is inconsistent with correct option. • Eg: branching to an invalid destination • Fetching an illegal opcode • Inactive for more than a prescribed period • Watch dog and multiple processors
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  • Offline detection •Consists of running diagnostic tests. When a processor is running such a test, it obviously cannot be executing the applications software. • Diagnostic tests can be scheduled just like ordinary tests.
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  Redundancy • Fault toleranceconsists of using and properly managing redundancy • Hardware redundancy • Software redundancy • Time redundancy • Information redundancy