On Feb 12, 4:40 pm, David Woolley <[EMAIL PROTECTED]> wrote: > > tsc refers to the counter in recent Intel Architecture chips that counts > the processor clock cycles. It is somewhat vulnerable to power management.
Not only PM, but also differences between processor cores. Some models of multi-core processors from AMD or Intel provide one TSC per core, which may cause discrepancies when the TSC is read on one core at a time tick and on another core at the next time tick. This is also true for multi-processor systems: TSCs are not guaranteed to be correlated among processors, not even their relative monotonicity. The Linux kernel goes through hoops to account for such discrepancies among the several TSC instances, but it's never 100% successful neither 100% accurate. Using the TSC as a time reference is tempting because reading it takes just a few CPU cycles to read it. Other time references, such as ACPI- PM and HPET, take much longer to read, even 1000X longer, though they provide consistent reads and accuracy no matter from which processor or core. The speed to get the time-of-day from the kernel may be important when running some applications, such as data-base servers, when every transaction is time-tagged and the time it takes to get the time-of- day can contribute significantly to its performance. Therefore, for time-keeping purposes, it's better to avoid the kernel from using the TSC as its time-reference, in favor of other timers available in the system, unless running an application whose performance may be gated by getting the time-of-day. HTH _______________________________________________ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
