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From: Peter Chubb <peter@chubb.wattle.id.au>
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Date: Fri, 31 Oct 2003 10:15:05 +1100
To: root@chaos.analogic.com
Cc: George Anzinger <george@mvista.com>,
       Peter Chubb <peter@chubb.wattle.id.au>,
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       Joe Korty <joe.korty@ccur.com>, Linus Torvalds <torvalds@osdl.org>,
       lkml <linux-kernel@vger.kernel.org>, Andrew Morton <akpm@osdl.org>
Subject: Re: gettimeofday resolution seriously degraded in test9
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>>>>> "Richard" == Richard B Johnson <root@chaos.analogic.com> writes:

Richard> There isn't any magic that can solve this problem. It turns
Richard> out that with later Intel CPUs, one can get CPU-clock
Richard> resolution from rdtsc. However, this is hardware-specific. If
Richard> somebody modifies the gettimeofday() and the POSIX clock
Richard> routines to use rdtsc when available, a lot of problems will
Richard> go away.

gettimofday() and the posix clock routines (which use gettimeofday())
*do* use rdtsc if the processor has a reliable one --- do_gettimeofday() calls
cur_timer->get_offset(), which is essentially a scaled rdtsc if you're
using timers_tsc.c.

But when you have power management turned on, TSC doesn't run at a
constant rate.   If it gets *too* slow, the timer switches to use the
PIT instead, and one loses the cycle-resolution one would otherwise have.

--
Dr Peter Chubb  http://www.gelato.unsw.edu.au  peterc AT gelato.unsw.edu.au
You are lost in a maze of BitKeeper repositories,   all slightly different.