Date: Thu, 6 Mar 2008 10:29:24 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Peter Jeremy <peterjeremy@optushome.com.au> Cc: cvs-src@freebsd.org, Roman Divacky <rdivacky@freebsd.org>, src-committers@freebsd.org, cvs-all@freebsd.org, Jung-uk Kim <jkim@freebsd.org> Subject: Re: cvs commit: src/usr.sbin/moused moused.c Message-ID: <20080306102751.J21309@fledge.watson.org> In-Reply-To: <20080306095407.GS68971@server.vk2pj.dyndns.org> References: <200803060022.m260MHSx017085@repoman.freebsd.org> <20080306092045.GA54220@freebsd.org> <20080306095407.GS68971@server.vk2pj.dyndns.org>
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On Thu, 6 Mar 2008, Peter Jeremy wrote: > On Thu, Mar 06, 2008 at 10:20:45AM +0100, Roman Divacky wrote: >> On Thu, Mar 06, 2008 at 12:22:17AM +0000, Jung-uk Kim wrote: >>> Prefer clock_gettime(2) over gettimeofday(2) and use CLOCK_MONOTONIC_FAST. >>> It is only used to track elapsed time and it does not have to be >>> precise. >> >> what is the rationale behind this? what is the expected improvement? I am >> not questioning the change I am just curious.. > > There's been a recent extensive thread about poor X11 performance and it > turns out the the Xserver can issue multiple gettimeofday(2) calls every > time it sees a mouse movement. gettimeofday(2) on FreeBSD is relatively > expensive (the Project chose accuracy over speed) and clock_gettime(2) using > CLOCK_MONOTONIC_FAST is a much cheaper alternative when accuracy isn't > critical. > > The Xserver should be using clock_gettime(2) but there's a bug in its > POSIX-compatibility selection. Applying a similar fix to moused is a > logical extension. Many applications also assume that gettimeofday(2) returns a monotonically, and possibly consistently, increasing value. Sometimes this is fine (log timestamps) and other times this is not (time deltas used to calculate mouse acceleration, etc). Robert N M Watson Computer Laboratory University of Cambridge
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