[LinuxPPS] rt?

[Hal V. Engel]

On Wednesday 25 March 2009 11:05:07 am Hal V. Engel wrote:
> I have been running with an rt kernel now for about a day and a half and I
> have learned some things about how this affects time keeping.
>
> When the machine is idle I don't notice any significant differences. 
> Jitter may be a little lower (IE. it is almost always 0.001 with the rt
> kernel but was almost always 0.002 or 0.003 with a normal kernel) but
> typical offsets, rootdispersion, noise and stability values are very
> similar or at best only slightly better.  So as Bernhard predicted with no
> load on the system there is not a significant difference when using the rt
> kernel.  So for a lightly loaded machine like a network time server
> appliance with only a few clients there may not be any reason to use an rt
> kernel but it may still be worth while for someone to do a more extensive
> tests of a system like this.
>
> I tested loading up the machine by running a long multi-threaded build (IE.
> make -j3 on a dual core machine).  With a normal kernel the time offset
> would drift when the machine was loaded and would increase to perhaps
> 0.0050 (absolute) in a fairly short time (about 15 minutes) and would
   ^^^^^
 Is a typo.  It should have read 0.050

> continue increasing over longer high load periods at times being > 0.100
> (absolute). With the rt kernel this tendency is almost eliminated and the
> offsets under load were about the same as an unload machine!  Although it
> did drift some after the machine was idled with the offset going from 0.005
> to -0.020 but it quickly recovered to -0.008 about 5 minutes after being
> idled.  At about an hour into the load test my offset was 0.006278 and
> rootdispersion, jitter, noise and stability values have all stayed very
> close to their respective values when the machine is idle.  All of these
> values would have increased significantly on a loaded non-rt kernel system.
>  So this clearly helps time keeping when the machine is loaded or under
> variable loads if things are correctly setup.
>
> It appears that when using a non-rt kernel that there is a significant
> amount of variation in the latency of the PPS interrupt handler when the
> machine is under varying loads.  Using the rt kernel with the serial device
> running at high priority seems to improve the consistency of the PPS
> interrupt handler and this does result in a very noticeable improvement in
> the consistency of the time keeping on a machine with a varying load. 
> Frankly I am very surprised that I saw this much improvement.   I will
> definitely continue to run an rt kernel on my workstation and I think it is
> worthwhile for others to test this type of setup although it may only be of
> benefit under some circumstances.
>
> Hal
>
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