SLES 11 SP2上的时钟源

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Author: Tony

1.查看系统可用是时钟源

从代码来看，tsc的rating要大于hpet，的确是应该选择tsc的。那ULK3中提到的问题是怎么回事？

4. About tsc.
  TSC的性能要好于HPET:()
    TSC access is extremly fast as it is a per CPU register. HPET is a chipset device and scales bad when multiple CPUs try to access it simultanously as the access is serialized in hardware. Even on a UP system the access overhead is somewhere in the range of factor 100. 
  TSC主要有4种：
    Constant. Constant TSC means that the TSC does not change with CPU frequency changes, however it does change on C state transitions.
    Invariant. As described in the Intel manual: “The invariant TSC will run at a constant rate in all ACPI P-, C- and T-states”
    Non-stop. The Non-stop TSC has the properties of both Constant and Invariant TSC.
    None of the above. The TSC changes with the C, P and S state transitions
  并不是所有的TSC都可用：
    we only use the TSC, when: 
        - the TSC is known to be stable (not affected by CPU frequency changes) 
        - the TSC is sychronized accross CPUs 
    We also check the TSC with a watchdog mechanism, which verifies that is is keeping accurate time. When we detect that TSC does not, we replace it by the next available clock source.