Re: TSC instead of ACPI: powerd doesn't work anymore (to be expected?)
From: Robert Watson (rwatson_at_FreeBSD.org)
Date: 10/31/05
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Date: Mon, 31 Oct 2005 10:53:45 +0000 (GMT) To: David Xu <davidxu@freebsd.org>
On Mon, 31 Oct 2005, David Xu wrote:
> In thread program, we have to use clock_gettime, for example a thread
> wants to wait for condition variable for two seconds, it has to:
>
> struct timespec ts;
>
> clock_gettime(CLOCK_REALTIME, &ts);
> ts.tv_sec += 2;
> pthread_cond_wait(&cond, &mtx);
>
> problem is who really cares time precise? how many people are really
> handling realtime critical tasks? this is really an unpleasant side
> effect that a simple syscall can stall cpu.
The questions are: how good is good enough? And for what?
The problem here is that while pthread_cond_wait() can afford to accept
low resolution, the same clock retrieval interface is also used for things
like scientific measurement. So it sounds like the real mistake was the
specification for the thread API: while timespec is the New Way for
time-related interfaces, this approach has coalesced precision time
measurement and "how long to sleep for" time measurement. Moving to HZ
incrementing of the clock probably exceeds the quality reduction that
makes sense for clock_gettime(), but moving to a roughly sync'd TSC clock
(such as the Linux approach) probably doesn't.
Robert N M Watson
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