[GSoc] Timeconter Performance Improvements

[Peter Jeremy]

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 believe we already have a shared page (it holds the signal trampoline now)
>for at least the x86 platform (probably some others as well).

r217151 for amd64 and r217400 for ppc.  It doesn't appear to be
supported on other platforms.  My reading of the code is that there is
a single shared page used by all processes/CPUs.  In order to support
non-synchronised TSCs, this would need to be changed to per-CPU.

-- 
Peter Jeremy
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