Date: Fri, 28 Oct 2005 23:19:27 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: davidxu@FreeBSD.org Cc: pertti.kosunen@pp.nic.fi, phk@phk.freebsd.dk, rwatson@FreeBSD.org, current@FreeBSD.org, jura@networks.ru Subject: Re: Timers and timing, was: MySQL Performance 6.0rc1 Message-ID: <20051028.231927.71089518.imp@bsdimp.com> In-Reply-To: <43630215.9050700@freebsd.org> References: <4362CBC2.8050602@freebsd.org> <20051028.221825.90826015.imp@bsdimp.com> <43630215.9050700@freebsd.org>
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In message: <43630215.9050700@freebsd.org>
David Xu <davidxu@freebsd.org> writes:
: M. Warner Losh wrote:
:
: >: thread libraries use clock_gettime, this becauses there is
: >: pthread_cond_timedwait and other synchronization objects
: >: like rwlock, and mutex all have a timeout version, I think
: >: pthread_cond_timedwait is mostly used in some applications,
: >: though normally, application is not looking for high accuracy.
: >: they will get benefit from the clock_gettime speed improvement.
: >
: >And unfortuantely, the argument that needs to be passed to abstime is
: >unspecified, at leas tin our man page. Also unfortuantely,
: >CLOCK_REALTIME seems to be what's required here (our man page just
: >says 'if the system time reaches the time specified in abstime'),
: >rather than CLOCK_MONOTONIC so jumps in system time can cause
: >previously short timeouts to become rather large timeouts... This is
: >a flaw in the API. :-(
: >
: >Warner
: >
: >
: I would rather to think it is brokeness of POSIX thread API specification,
: in real world, nobody uses the timeout as an absolute timestamp, almost
: all applications are doing relative timeout sleep, if implementor 100%
: respects POSIX spec, he will break many applications.
: libthr supports pthread_condattr_setclock, you can select CLOCK_MONOTONIC
: for pthread_cond_timedwait, but internally, all absolute timeout waits are
: converted to relative.
Does this mean I can have a 1s wait, jump time back an hour and that
the timeout will happen in a little under 1s?
Warner
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