[GSoc] Timeconter Performance Improvements

[John Baldwin]

On Saturday, March 26, 2011 08:16:46 am Peter Jeremy wrote:
> On 2011-Mar-25 08:18:38 -0400, John Baldwin <jhb at freebsd.org> wrote:
> >For modern Intel CPUs you can just assume that the TSCs are in sync across
> >packages.  They also have invariant TSC's meaning that the frequency
> >doesn't change.
> 
> Synchronised P-state invariant TSCs vastly simplify the problem but
> not everyone has them.  Should the fallback be more complexity to
> support per-CPU TSC counts and varying frequencies or a fallback to
> reading the time via a syscall?

I think we should just fallback to a syscall in that case.  We will also need 
to do that if the TSC is not used as the timecounter (or always duplicate the 
ntp_adjtime() work we do for the current timecounter for the TSC timecounter).

Doing this easy case may give us the most bang for the buck, and it is also a 
good first milestone.  Once that is in place we can decide what the value is 
in extending it to support harder variations.

One thing we do need to think about is if the shared page should just export a
fixed set of global data, or if it should export routines.  The latter 
approach is more complex, but it makes the ABI boundary between userland and 
the kernel more friendly to future changes.  I believe Linux does the latter 
approach?

-- 
John Baldwin