WorstCaseResponseTime

WorstCaseResponseTime (WCRT) is a metric used in real-time computing to denote the maximum time from a stimulus, such as a task release, to the completion of its response under a given schedule and hardware configuration.

WCRT is used to analyze and guarantee timing constraints. A task's WCRT must not exceed its deadline for the system to be considered schedulable.

Calculation for fixed-priority preemptive scheduling: The worst-case response time R_i for a task i is the smallest value that satisfies R_i = C_i + sum over higher-priority tasks j of ceil(R_i / T_j) * C_j + B_i, where C_i is the task's worst-case execution time, T_j is the period of task j, and B_i is the blocking time from lower-priority tasks or I/O. R_i is found by fixed-point iteration, starting with R_i^0 = C_i + B_i.

For other scheduling policies, such as earliest-deadline-first (EDF) or non-preemptive scheduling, different interference terms are used and analyses are policy-specific.

In practice, WCRT can be estimated analytically, but may also be measured or simulated. WCRT bounds are generally conservative, accounting for preemption overhead, cache effects, and synchronization delays that may occur in worst-case conditions.

On multi-core systems, WCRT analysis becomes more complex due to inter-core interference, shared resources, and memory hierarchy; dedicated methods exist to bound response times under multi-core scheduling.