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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