[time-nuts] FreeBSD, NetBSD, or Minix-III?

[Poul-Henning Kamp]

In message <4A0F2581.7000303 at erols.com>, Chuck Harris writes:

>I don't believe that will be happening in a message passing microkernel
>(like minix) anytime soon... unless you build all of the timekeeping
>software into the kernel, and then you are in the process of becoming
>a monolithic kernel ;-)

Well, this brings us into a philosophical area.


If a piece of code calls a function to figure out what time it is,
the tacit assumption is that the function takes zero time to execute
and returns the timestamp of the time where it didn't take any time.

In practice, the call will always occupy some duration of time, and
the timestamp returned represents some instant during that interval.

Depending on the function called, and the CPU hardware we execute
it on, the timestamp returned may have more or less jitter relative
to the call instruction and the first instrucation executed upon
return.

I call this jitter the "software-jitter" because it has nothing to
do with the time keeping hardware, it is jitter introduced because
we operate in software.

In addition to the software jitte, we have whatever jitter the
function runs into, trying to actually produce the timestamp: Bus
contentions, hardware locking, page faults etc. etc.

On the other side, if a piece of hardware yanks a signal line to
record a timestamp, we can reasonably expect the timestamp will
be the exact time of the signals reception +/-1 count on whatever
hardware counter is used to keep track of time.


These observations hold in general, even for the case where you
have no operating system of any kind.


If you look at this prism with your MACH-coloured glasses, what you
see is that the access to the timekeeping hardware counter is likely
to be restricted to one semi-priviledged process.  Otherwise,
why bother with microkernels in the first place ?

It follows that all software calls to get a timestamp consequently
becomes IPC calls.

And we have 30 years experience telling us, that IPC calls are
even more smeared out over time than direct calls and therefore
the software jitter becomes worse.

But other than that, and that is bad enough because of the scheduling
latency/jitter typically present, there are no ill effects from
running on a microkernel.

If you feed your PPS signal in and do hardware timestamps like we
FreeBSD's timecounters allows you to, for instance on the Soekris
net4501[1], then your basic clock can be precise, but your microkernel
will only allow your software to read fuzzy timestamps from it.

If you hook the PPS signal to an interrupt line, and timestamp it
in software, then your basic clock can never be precise, because
your PPS timestamps are fuzzed by the microkernel.

Poul-Henning

[1] This is why I say that FreeBSD is a generation ahead, I have
yet to see any other operating system support PPS-API on hardware
captured signals.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.