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Subject: Re: Clock monotonic  a suggestion
From: john stultz <johnstul@us.ibm.com>
To: george anzinger <george@mvista.com>
Cc: Joel Becker <Joel.Becker@oracle.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Fri, 2003-03-21 at 00:01, george anzinger wrote:
> Joel Becker wrote:
> > 	If the system is delayed (udelay() or such) by a driver or=20
> > something for 10 seconds, then you have this (assume gettimeofday is
> > in seconds for simplicity):
> >=20
> > 1    gettimeofday =3D 1000000000
> > 2    driver delays 10s
> > 3    gettimeofday =3D 1000000000
> > 4    timer notices lag and adjusts
>=20
> Uh, how is this done?  At this time there IS correction for delays up=20
> to about a second built into the gettimeofday() code.  You seem to be=20
> assuming that we can do better than this with clock monotonic.  Given=20
> the right hardware, this may even be possible, but why not correct=20
> gettimeofday in the same way?

Because to to it properly is slow. Right now gettimeofday is all done
with 32bit math. However this bounds us to ~2 seconds of counting time
before we overflow the low 32bits of the TSC on a 2GHz cpu.  Rather then
slowing down gettimeofday with 64bit math to be able to handle the crazy
cases where timer interrupts are not handled for more then 2 seconds, we
propose a new interface (monotonic_clock) that provides increased
corner-case accuracy at increased cost.=20

thanks
-john

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