Re: PSA: Clock drift and pkgin

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

In a discussion of timekeeping on emulated VAXen, over on port-vax@, I
mentioned that I've found that, on 4.0.1, 5.2, and 9.1, and, based on a
report on port-vax@, apparently 9.3 as well, there's a bug in
ITIMER_REAL signals (possibly on only some hardware - I've seen it on
amd64 and i386, and, if my guess below is correct, it should manifest
on various others as well).

Specifically, under a kernel built with HZ=100, requesting signals at
100Hz actually delivers them at 50Hz.  This is behind the clock running
at half speed on my VAX emulator, and quite likely behind similar
behaviour from simh (which emulates VAXen, among other things) on 9.3.
I suspect it will happen on any port when requesting signals one timer
tick apart (ie, at HZ Hz).

In case anyone wants it, I wrote a small test program.  It requests
100Hz signals, then, in the signal handler, takes a gettimeofday()
timestamp.  After taking 6000 timestamps (which ideally should take
60.00 seconds), it then prints out all the timestamps, thus indicating
the actual rate signals were delivered at.  It's on
ftp.rodents-montreal.org (which supports HTTP fetches as well as FTP)
in /mouse/misc/test-alrm.c for anyone interested.  On machines with the
half-speed bug, it takes two minutes rather that one, and the
timestamps average about 20ms apart, instead of 10ms.  ("About" because
in most of my tests there is usually at least one interval that is
slightly longer than it should be.)

I don't _know_ what's behind it.  But today I went looking, and, in
5.2, there is code which looks suspicious.  I don't know where the
analogous code is in 9.x, but presumably plenty of people here do.
Speaking of 5.2, then: in kern/subr_time.c, there is tvtohz(), which
has code

        } else if (sec <= (LONG_MAX / 1000000))
                ticks = (((sec * 1000000) + (unsigned long)usec + (tick - 1))
                    / tick) + 1;

which looks suspicious.  If sec is zero and usec is tick, that
expression will return 2 instead of the 1 I suspect it needs to return.

I haven't yet actually tried changing that.  Holiday preparations and
observations are likely to occupy much of my time for the next week or
so, but I'll try to fit in the time to change that and see if it helps
any.

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