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    SubjectRe: + stupid-hack-to-make-mainline-build.patch added to -mm tree
    On Tue, 2007-03-06 at 16:42 -0800, Dan Hecht wrote:
    > >> accounting would be wrong. Instead, we should allow the
    > >> tick_sched_timer in cases (c) and (d) to have runtime configurable
    > >> period, and then scale the time value accordingly before passing to
    > >> account_system_time. This is probably something the Xen folks will want
    > >> also, since I think Xen itself only gets 100hz hard timer, and so it can
    > >> implement at best a oneshot virtual timer with 100hz resolution. Any
    > >> objections to us doing something like this?
    > >
    > > Yes. It's gross hackery.
    > >
    > > 1) We want to have a cleanup of the tick assumptions _all_ over the
    > > place and this is going to be real hard work.
    > >
    > > 2) As I said above. The time accounting for virtualization needs to be
    > > fixed in a generic way.
    > >
    > > I'm not going to accept some weird hackery for virtualization, which is
    > > of exactly ZERO value for the kernel itself. Quite the contrary it will
    > > make the cleanup harder and introduce another hard to remove thing,
    > > which will in the worst case last for ever.
    > >
    > Okay, to confirm I'm on the same page as you, you want to move process
    > time accounting from being periodic sampled based to being trace based?
    > i.e. at the system-call/interrupt boundaries, read clocksource and
    > compute directly the amount of system/user/process time?

    At least for the paravirt guests this is the correct approach. Once the
    CPU vendors come up with a sane solution for a reliable and fast clock
    source we might use that on real hardware as well.

    > Do you know if anyone has explored this? I thought there was a
    > discussion about this a while back but it was rejected due to the
    > sample-based approach having much lower overheads on high system call
    > rate workloads.

    Yes, with todays hardware it is simply a PITA. PowerPC has some basic
    support for this though, IIRC.


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