2 docs tagged with "wcet"

Almost every mechanism that makes a computer fast makes it less predictable, and the trade is usually invisible in source code. Caches, prefetchers, branch speculation, bus arbitration and dynamic memory allocation are all bets on history repeating: they are fast when the recent past resembles the present and slow when it does not. In a throughput-oriented system that is an unambiguously good bargain. In a system with deadlines it means the number you have to defend — the worst case — is set by the unlucky path through every one of them, while every measurement you take is dominated by the lucky one.

Every piece of real-time analysis needs one number per task: how long it takes to run. Not how long it usually takes — how long it can possibly take. That number, the worst-case execution time, is the input to response-time analysis, to utilisation budgets, and to every claim that a deadline is met. It is also the hardest number in embedded engineering to obtain honestly, because the obvious way to get it — run the code and time it — cannot produce it even in principle.