cvs commit: src/sys/sys time.h src/sys/kern kern_time.c

[Robert Watson]

On Mon, 28 Nov 2005, Bruce Evans wrote:

> % /*
> %  * Small wrapper library to substitute implementations of gettimeofday(2) 
> and
> %  * time(3) with lower resolution variations.  time(3) is unconditionally
> %  * degraded, since it will return a truncated time anyway.  gettimeofday(3)
> %  * checks the TIMEWRAPPER environmental variable, which can be set to 
> either
> %  * "PRECISE" or "FAST".
> %  */
>
> time(3) should use the environment variable too, since the fast version 
> gives a value that is both imprecise and wrong.  It inherits bugs from 
> the kernel's time_second variable.  time_second is not the current time 
> truncated, but is the (current time less up to about tc_tick/HZ) 
> truncated.  It lags the current time by more than 1 second for up to 
> about tc_tick/HZ seconds before every rollover of the correct truncated 
> time.

Yes -- this is a mistake in the current library wrapper.  All interfaces 
modified to be "fast" or "precise" should be controlled by the 
environmental variable so that testing is more consistent (i.e., other 
than wrapping costs, the non-configured version should behave identically 
to the non-wrapped version).

As mentioned in my previous e-mail, I have no particular commitment to any 
particular implementation of "fast", but feel that to understand the 
implications of time measurement costs on applications, we need some way 
to express this.  The POSIX APIs seem to lack any way for an application 
to express its requirements for time quality vs performance, meaning that 
if a system errs on the side of quality (FreeBSD), applications will 
perform much more slowly and potentially no better than on a system which 
errs on the side of performance (Linux).  We work very hard to provide 
very accurate time stamps for applications that sometimes don't need them, 
and while uniformly degrading quality doesn't make sense, a bit more 
expressiveness appears to be required to do better.

Robert N M Watson