clock_gettime.2: _COARSE clocks are not always faster...

[Michael Kerrisk (man-pages)]

Hello Rasmus,

On 04/09/2016 05:50 PM, Rasmus Villemoes wrote:
> Hi Michael
> 
> The other day, I was curious how the vdso was implemented on ppc, and I
> noted that neither ppc32 or ppc64 handle the _COARSE versions of
> CLOCK_{REALTIME,MONOTONIC} in the vdso, so they fall back to an actual
> syscall. And sure enough, measuring CLOCK_MONOTONIC
> vs. CLOCK_MONOTONIC_COARSE shows that the latter has three times as much
> overhead as the former.
> 
> Whether it's worth adding a note to the man page is up to you.

I think it's useful to do so. For both *COARSE flags, I added:

[[
Requires per-architecture support, and probably also architecture support
for this flag in the vdso(7).
]]

Thanks!

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/