I've now gotten this twice with the release version doing a self compile: Compile SML raised in 218.29 + 115.41 (35% GC) MLton raised in 218.47 + 115.45 (35% GC) mlton: timeMinus: traceBatch: t' - t Tracing back through calls, it seems that successive calls to MLton.Rusage.rusage are returning self,children times such that the sum self+children is not increasing.