On Sat, Mar 26, 2011 at 11:16:46PM +1100, Peter Jeremy wrote: > On 2011-Mar-25 08:18:38 -0400, John Baldwin <j...@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. Not neccessary. If you have a reliable way to access proper private per-CPU page from the array, then you could use the same method to access the array in the single page.
IMO, per-cpu page in process address space at the same address for all pages is too costly. I think we can target a modern hardware for user-mode tsc, this is the kind of machines that are used for benchmarks anyway.
pgpnxlUPO1v61.pgp
Description: PGP signature
