[Bug 216833] TSC as timecounter makes system lag on Core 2 generation hardware

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=216833

            Bug ID: 216833
           Summary: TSC as timecounter makes system lag on Core 2
                    generation hardware
           Product: Base System
           Version: CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: jiashiun at gmail.com
                CC: freebsd-amd64 at FreeBSD.org
                CC: freebsd-amd64 at FreeBSD.org

Originally discussed on freebsd-current@
https://lists.freebsd.org/pipermail/freebsd-current/2017-January/064419.html

Since r310177 enablement of EARLY_AP_STARTUP, system time lags behind on Intel
Core 2 generation hardware (family 6 model 17). It happens to me on Intel
Pentium T4200 and Core 2 E7400. Larry Rosenman (ler@) also confirmed it
happening on Xeon E5410.

Workaround:
- Disable EARLY_AP_STARTUP, or
- sysctl kern.timecounter.hardware=HPET

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