""The CPU number in Task Manager is a moving little obituary for the immediate past," explained former Microsoft engineer Dave Plummer, "Not what happened at the moment that your eyeballs landed on the row.""
""For an individual process, the math essentially is the cumulative current CPU time minus the previous cumulative CPU time. And that gives you how much CPU has been consumed by that process during the interval between the samples," he said."
""Now, if that looks like the sort of thing you write when you've been locked in an office too long with a copy of Petzold and a lot of coffee, that's because it basically is.""
Task Manager in Windows provides a CPU usage meter that reflects past performance rather than real-time data. The original version, created by Dave Plummer, was timer-driven, calculating CPU usage by comparing cumulative execution times at intervals. The per-process percentage is derived from the difference in CPU time over the sampling period. While the method was effective, it had some quirks due to the Windows kernel's reporting mechanisms, which could lead to occasional inaccuracies in the displayed CPU usage.
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