Re: [Linux-ia64] provide /proc/sal/itc_drift through AUX?

From: David Mosberger <davidm_at_napali.hpl.hp.com>
Date: 2003-03-20 10:54:50
>>>>> On Wed, 19 Mar 2003 16:39:31 -0500, Jes Sorensen <jes@wildopensource.com> said:

  Jes> Hi I was wondering what people think about providing the
  Jes> information of /proc/sal/itc_drift as an AUX vector?

  Jes> The problem is that on some NUMA boxen (such as the SGI boxes),
  Jes> the ITC isn't synchronized across nodes and we can't rely on
  Jes> ar.itc in userland for implementing the high-precision
  Jes> timing. I believe the IBM NUMA-Q team has a similar problem
  Jes> that could be solved in a similar way on ia32?

  Jes> Instead one can switch to using gettimeofday() for the timing,
  Jes> which with the new fast syscalls should be quite pleasant.

  Jes> I have a patch which implements this for glibc-2.2 (will do 2.3
  Jes> later), however what I don't like about it is that one ends up
  Jes> opening and reading /proc/sal/itc_drift in every single binary
  Jes> executed. To avoid the overhead of this it seems a good idea to
  Jes> me to provide this information via an AUX vector.

  Jes> If anybody is interested in the glibc patch, feel free to grab
  Jes> it from
  Jes> http://www.wildopensource.com/~jes/glibc/itc-drift-patch.diff
  Jes> For now it's a test patch, though it does seem to behave as
  Jes> expected.

  Jes> Thoughts?

Why do you need to export it if you just use a light-weight
gettimeofday()?  The light-weight syscall can readily access the
ITC-drift info via kernel memory.

BTW: I think someone should explore using an NTP-like approach to keep
ITC drift small enough that it's still usable for ITC-based
interpolation.  Granted, this assumes that hw drifts are reasonably
small and you'd still need to worry about different clock-frequencies,
but I suspect that in practice, this would work extremely well.  It
would be nice because it would be much faster than an HPET-based
approach, would work on all ia64 machines, and would provide better
resolution.  Furthermore, we already have all the logic in the
time-interpolation to ensure that there is never an observable time
discontinuity.

	--david
Received on Wed Mar 19 15:56:44 2003

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