On Thu, Mar 27, 2025 at 04:42:49PM +0100, Miroslav Lichvar wrote: > Maybe I could simply patch the kernel to force a small clock > multiplier to increase the rate at which the error accumulates.
I tried that and it indeed makes the issue clearly visible. The COARSE fix makes the clock less stable. It's barely visible with the normal multiplier, at least for the clocksource I tested, but a reduced multiplier forces a larger NTP error and raises it above the precision and instability of the system and reference clocks. The test was done on a machine with a TSC clocksource (3GHz CPU with disabled frequency scaling - normal multplier is 5592407) and tried a multiplier reduced by 4, 16, 64 with this COARSE-fixing patch not applied and applied. Each test ran for 1 minute and produced an average value of skew - stability of the clock frequency as reported by chronyd in the tracking log when synchronizing to a free-running PTP clock at 64, 16, and 4 updates per second. It's in parts per million (resolution in the chrony log is limited to 0.001 ppm). Mult reduction Updates/sec Skew before Skew after 1 4 0.000 0.000 1 16 0.001 0.002 1 64 0.002 0.006 4 4 0.001 0.001 4 16 0.003 0.005 4 64 0.005 0.015 16 4 0.004 0.009 16 16 0.011 0.069 16 64 0.020 0.117 64 4 0.013 0.012 64 16 0.030 0.107 64 64 0.058 0.879 -- Miroslav Lichvar
