Re: Patch: add timing of buffer I/O requests

On Sun, Nov 27, 2011 at 11:54:38PM -0800, Greg Smith wrote:
> On 11/27/2011 04:39 PM, Ants Aasma wrote:
> >On the AMD I saw about 3% performance drop with timing enabled. On the
> >Intel machine I couldn't measure any statistically significant change.
>
> Oh no, it's party pooper time again. Sorry I have to be the one to
> do it this round. The real problem with this whole area is that we
> know there are systems floating around where the amount of time
> taken to grab timestamps like this is just terrible. I've been
> annoyed enough by that problem to spend some time digging into why
> that is--seems to be a bunch of trivia around the multiple ways to
> collect time info on x86 systems--and after this CommitFest is over

Something good to know: in Linux the file
/sys/devices/system/clocksource/clocksource0/current_clocksource
lists the current clock source, and
/sys/devices/system/clocksource/clocksource0/available_clocksource
lists the available clock sources. With cat you can switch them. That
way you may be able to quantify the effects on a single machine.

Learned the hard way while tracking clock-skew on a multicore system.
The hpet may not be the fastest (that would be the cpu timer), but it's
the fastest (IME) that gives guarenteed monotonic time.

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> He who writes carelessly confesses thereby at the very outset that he does
> not attach much importance to his own thoughts.
-- Arthur Schopenhauer