Numerator: λ T

The passage around this formula

…than to reproduce them exactly. Model checkpoint writes as occurring every T hours, each costing $\tau$ hours of write time, with failures arriving as a Poisson process at fleet-wide rate $\lambda$ (failures per hour). For $\lambda$ T $\ll$ 1 , a failure inside an interval loses on average T/2 hours of recomputation, so the expected fraction of wall-clock time lost to the combination of checkpoint overhead and lost recompute is approximately $f(T) \approx \frac{\tau}{T} + \frac{\lambda T}{2}$. Minimizing over T gives an optimal interval $T^{*}$ =…

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