[smokeping-users] rtt alert granularity

[G.W. Haywood]

Hi there,

On Tue, 19 Jan 2010, Thomas Klein wrote:

> i have now setup alerts but do not get the needed granularity logged.
> ...
> What can i do to get the measured rtt with microsecond granularity?

I thought I'd explained that to you in my mail of 15 January.  Under
some circumstances, some implementations of the timing functions give
granularities of the order of several milliseconds.  You will need to
check first of all that your 'ping' for example is capable of giving
reasonable results.  On a dual Opteron server here, the Debian kernel
installation was fine until I upgraded the kernel one day and then it
started to give RTTs in integer multiples of 4ms.  After quite a lot
of messing around I found that the good folks at Debian had changed a
part of the kernel configuration.  I had to use a kernel boot command

clock=tsc

to get the gettimeofday() C library function to return more reasonable
values.  Note that I still do not know much about the accuracy of the
values, and there still could be something horribly wrong, but for the
moment if there is something wrong it doesn't seem to be hurting me so
I've moved on.  I have no reason to believe that the problems I've had
with the Opteron machine are the same as the problems you have now.

Here's a random example of the sort of thing you need to worry about
if you take time measurements on computers seriously:

http://www.ntp.org/ntpfaq/NTP-s-sw-clocks-quality.htm
http://lkml.indiana.edu/hypermail/linux/kernel/0411.1/2135.htm

--

73,
Ged.