clock_gettime_mono in the VFS.

[Jeremy Allison]

On Fri, Mar 04, 2016 at 01:18:20PM -0500, Ira Cooper wrote:
> I'm sorry that I'm coming in late on this one.  I've been a bit head down
> recently.
> 
> On some platforms clock_gettime_mono has a real measurable cost, alas.  I
> discovered this on Solaris/Illumos when I compiled in the timing code in
> the samba 3.6 profiling framework.

Just to follow up, I was OK with this in the review as at least on x86-64-linux, the
gettime calls aren't even real system calls - just a memcpy. It's
different on ARM I know.

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_MRG/1.3/html/Realtime_Tuning_Guide/sect-Realtime_Tuning_Guide-General_System_Tuning-gettimeofday_speedup.html

> I can understand why you'd want this on Linux, getting this data is pretty
> painful, especially on older kernels.
> 
> But it should be a compile time option, so we don't impose a cost on
> platforms where it has real cost, and on people who don't want it.
> 
> Hopefully this is something we can add. :)
> 
> Note: I expect over-time, this type of profiling will go down in
> usefulness.  Linux tools to do this type of measurement are improving
> quickly, and dtrace can do it fairly well on other platforms.
> 
> But today.... I totally get it, I just don't want us to regress on other
> platforms.

Fair enough - if you want to add something that allows this
as a configure-time option I'm happy to review. I'd suggest
it's on by default - that allows vendors who really
don't want it to disable. Ralph, do you agree ?