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| -rw-r--r-- | docs/gettingstarted/developers/infrastructure.md | 20 |
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diff --git a/docs/gettingstarted/developers/infrastructure.md b/docs/gettingstarted/developers/infrastructure.md index 55b01e1c2ef..952be26cacf 100644 --- a/docs/gettingstarted/developers/infrastructure.md +++ b/docs/gettingstarted/developers/infrastructure.md @@ -239,6 +239,26 @@ reference time, at the current clock tick rate: * c->clocks_per_second; ``` +### Timebase precision + +Cognoscenti may notice that vlib/clib\_time\_now(...) return a 64-bit +floating-point value; the number of seconds since vpp started. + +Please see [this Wikipedia +article](https://en.wikipedia.org/wiki/Double-precision_floating-point_format) +for more information. C double-precision floating point numbers +(called f64 in the vpp code base) have a 53-bit effective mantissa, +and can accurately represent 15 decimal digits' worth of precision. + +There are 315,360,000.000001 seconds in ten years plus one +microsecond. That string has exactly 15 decimal digits. The vpp time +base retains 1us precision for roughly 30 years. + +vlib/clib\_time\_now do *not* provide precision in excess of 1e-6 +seconds. If necessary, please use clib_cpu_time_now(...) for direct +access to the CPU clock-cycle counter. Note that the number of CPU +clock cycles per second varies significantly across CPU architectures. + Format ------ |