Fault-Tolerant Systems Design Part 1

1. Introduction: Basic Definitions Fault-Tolerance is the ability of a system to continuously perform correctly its tasks after the occurrence of a fault.

Reliability of a system is the function, R(t), defined as the probability of the system to perform correctly through the time interval [t 0, t], given that the system was performing correctly at t Introduction: Basic Definitions

Availability is the function, A(t), defined as the probability of the system to operate correctly and to be available to perform its tasks through the interval [t 0, t]. 1. Introduction: Basic Definitions

Fault-Tolerant Systems can be designed by means of two basic approaches: Fault Masking Detection, localization and recovery, (via reconfiguration) of the system to remove the defective part. 2. Design of FT Systems

If the option is reconfiguration, then... before... Fault detection techniques Fault location techniques after... Fault recovery techniques 2. Design of FT Systems

Fault Recovery Techniques... Rollback Recovery Forward Recovery 2. Design of FT Systems

All techniques to design FT systems are based on some type and degree of redundancy. 2. Design of FT Systems

Redundancy is implemented through the use of HW, SW, information, or time beyond that necessary to system normal operation.  Results in a not negligible impact in the system in terms of performance, size, weight, power consumption, and reliability. 2. Design of FT Systems

Active Passive Hybrid Redundancy at the HW Level: 2. Design of FT Systems

1.  Based on the concept of fault masking to hide the occurrence of faults and prevent the faults from resulting in errors (developed around the concept of majority voting)  Do not provide for faults detection, but simply mask them HW Redundancy: 1. Passive 2. Design of FT Systems

Module 1 Module 2 Module 3 Voter Output Basic concept of Triple Modular Replication (TMR) Proc 1 Proc 2 Proc 3 Voter The use of triplicated voters in a TMR configuration Voter Mem 1 Mem 2 Mem 3 HW Redundancy: 1. Passive 2. Design of FT Systems

Example of SW voting VoterTask Task A Task B Task A Proc 1 Proc 3 Proc 2 HW Voting x SW Voting ? 1. The availability of processor to perform the voting 2. The speed at which voting must be performed 3. The criticality of space, power, and weight limitations 4. The # of different voters that must be provided 5. The flexibility required of the voter with respect to future changes in the system HW Redundancy: 1. Passive 2. Design of FT Systems

n In practical applications of voting, 3 results in a TMR system may not completely agree, even in a fault-free environment: e.g., A/D converters in sensors may produce quantities that disagree in the least-significant bits. This disagreement can propagate into larger discrepancies after computation, which can significantly affect the voting process. HW Redundancy: 1. Passive 2. Design of FT Systems

 Solution  Mid-Value Select Technique A TMR system selects the value that lies in the middle of the others : Corrupted signal Uncorrupted signals Selected signals HW Redundancy: 1. Passive 2. Design of FT Systems

 Attempts to achieve fault tolerance by means of fault detection, fault location, reconfiguration, and recovery (property of fault masking is not obtained: there is no attempt to prevent faults from producing errors within the system)  More suitable for applications where temporary, erroneous results are acceptable, as long as the system reconfigures and regains its operational status in a satisfactory length of time HW Redundancy: 2. Design of FT Systems 2. Active (or Dynamic)

Duplication of Functional Units Standby Blocks  Hot Standby Sparing  Cold Standby Sparing HW Redundancy: 2. Design of FT Systems 2. Active (or Dynamic)

Comparison Task Processor A Comparison Task Processor B Error Signals AB Processor A’s Result Processor B’s Result Shared Memory Processor A’s Private Memory Processor A’s Result Processor B’s Private Memory Processor B’s Result A software implementation of duplication with comparison 2. Active (or Dynamic) HW Redundancy: 2. Design of FT Systems

3. Hybrid HW Redundancy: 2. Design of FT Systems  Combines the attractive features of both the Active and the Passive approaches

Consistency Checks Capacity Checks N-Auto testable Programming N-Version Programming Recovery Blocks SW Redundancy: 2. Design of FT Systems

Consistency Checks SW Redundancy: 2. Design of FT Systems Use the previous knowledge about the chacacteristics of a given information to check the information correctness. Typically, for most applications, it is well known that a certain quantity of a given operand cannot assume values beyond predefined limits.

Consistency Checks SW Redundancy: 2. Design of FT Systems Examples...  A processing system can sample and store many sensor readings in a typical control application.  The amount of cash requested by a patron at a bank’s teller machine should never exceed the maximum withdrawal allowed.

Consistency Checks SW Redundancy: 2. Design of FT Systems Examples...  The address generated by a computer should never lie outside the address range of the available memory.  In a computer, each instruction code can be checked to verify that it is not one the illegal codes.

Capability Checks SW Redundancy: 2. Design of FT Systems Capability checks are performed to verify that a system possesses the capability expected.

Capability Checks SW Redundancy: 2. Design of FT Systems Examples...  Check whether a computer has the complete memory available.  Check whether the processors in a multiprocessor system are working properly.  Periodically, a processor can execute specific instrutions on specific data and compare the results to known results stored in a ROM: check for ALU and Memory

Program Version 1 Program Version n Acceptance Tests Selection Logic Program Outputs Program Inputs Program Inputs The N-Self-Checking Programming Approach to software fault tolerance SW Redundancy: N-Auto testable Programming 2. Design of FT Systems

Parity, Berger, and m-of-n Codes Arithmetic Codes Hamming Codes Checksum Code CRC ( Cyclic Redundancy Checking ) Code Information Redundancy: 2. Design of FT Systems

Transient Fault Detection Permanent Fault Detection Re-computation for Error Correction Time Redundancy: 2. Design of FT Systems

Transient Faults Detection Time Redundancy: 2. Design of FT Systems The fundamental concept is to perform the same computation two or more times and compare the results to determine if a discrepancy exists.

Time Redundancy: 2. Design of FT Systems Permanent Faults Detection Computation Encode Data Decode Result Store Result Store Result Compare Results Data Time t 0 Data Time t 1 Error

Time Redundancy: 2. Design of FT Systems Re-computation for Error Correction Time redundancy approach can also provide for error correction if the computations are repeated three or more times. AND Consider the example of a logical AND operation. Suppose the operation is performed three times: first, without shifting the operands; second, with a one-bit logical shift of the operands; and third, with a two-bit logical shift of the operands.