Lockstep (computing)

Lockstep systems are fault-tolerant computer systems that run the same set of operations at the same time in parallel. The redundancy allows error detection and error correction: the output from lockstep operations can be compared to determine if there has been a fault if there are at least two systems (dual modular redundancy), and the error can be automatically corrected if there are at least three systems (triple modular redundancy), via majority vote. The term "lockstep" originates in the army usage, where it refers to the synchronized walking, in which the marchers walk as closely together as physically practical.

To run in lockstep, each system is set up to progress from one well-defined state to the next well-defined state. When a new set of inputs reaches the system, it processes them, generates new outputs and updates its state. This set of changes (new inputs, new outputs, new state) is considered to define that step, and must be treated as an atomic transaction; in other words, either all of it happens, or none of it happens, but not something in between. Sometimes a timeshift (delay) is set between systems, which increases the detection probability of errors induced by external influences (e.g. voltage spikes, ionizing radiation, or in situ reverse engineering).

Some vendors, including Intel, use the term lockstep memory to describe a multi-channel memory layout in which cache lines are distributed between two memory channels, so one half of the cache line is stored in a DIMM on the first channel, while the second half goes to a DIMM on the second channel. By combining the single error correction and double error detection (SECDED) capabilities of two ECC-enabled DIMMs in a lockstep layout, their single-device data correction (SDDC) nature can be extended into double-device data correction (DDDC), providing protection against the failure of any single memory chip.

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