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(Look ma, I might have broken the tree)
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From NetBSD.
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The older code actually ensured that no timeout would be too early, but
it violated the principle of least surprise by making it seem (when you
looked at the time variable) that every timeout was one tick late.
Also periodic timeouts (that readd themselves in the timeout function),
will now happen with the frequency you expect.
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From FreeBSD: eventually, we should replace hzto() with something
like tvtohz() as well.
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makes it the callers responsibility to allocate resources for the
timeouts.
This is a KISS implementation and does _not_ solve the problems of slow
handling of a large number of pending timeouts (this will be solved in
future work) (although hardclock is now guarateed to take constant time
for handling of timeouts).
Old timeout() and untimeout() are implemented as wrappers around the new
API and kept for compatibility. They will be removed as soon as all
subsystems are converted to use the new API.
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commit messages:
Scheduler bug fixes and reorganization
* fix the ancient nice(1) bug, where nice +20 processes incorrectly
steal 10 - 20% of the CPU, (or even more depending on load average)
* provide a new schedclock() mechanism at a new clock at schedhz, so high
platform hz values don't cause nice +0 processes to look like they are
niced
* change the algorithm slightly, and reorganize the code a lot
* fix percent-CPU calculation bugs, and eliminate some no-op code
=== nice bug === Correctly divide the scheduler queues between niced and
compute-bound processes. The current nice weight of two (sort of, see
`algorithm change' below) neatly divides the USRPRI queues in half; this
should have been used to clip p_estcpu, instead of UCHAR_MAX. Besides
being the wrong amount, clipping an unsigned char to UCHAR_MAX is a no-op,
and it was done after decay_cpu() which can only _reduce_ the value. It
has to be kept <= NICE_WEIGHT * PRIO_MAX - PPQ or processes can
scheduler-penalize themselves onto the same queue as nice +20 processes.
(Or even a higher one.)
=== New schedclock() mechanism === Some platforms should be cutting down
stathz before hitting the scheduler, since the scheduler algorithm only
works right in the vicinity of 64 Hz. Rather than prescale hz, then scale
back and forth by 4 every time p_estcpu is touched (each occurance an
abstraction violation), use p_estcpu without scaling and require schedhz
to be generated directly at the right frequency. Use a default stathz (well,
actually, profhz) / 4, so nothing changes unless a platform defines schedhz
and a new clock.
[ To do: Define these for alpha, where hz==1024, and nice was totally broke.]
=== Algorithm change === The nice value used to be added to the
exponentially-decayed scheduler history value p_estcpu, in _addition_ to
be incorporated directly (with greater weight) into the priority calculation.
At first glance, it appears to be a pointless increase of 1/8 the nice
effect (pri = p_estcpu/4 + nice*2), but it's actually at least 3x that
because it will ramp up linearly but be decayed only exponentially, thus
converging to an additional .75 nice for a loadaverage of one. I killed
this: it makes the behavior hard to control, almost impossible to analyze,
and the effect (~~nothing at for the first second, then somewhat increased
niceness after three seconds or more, depending on load average) pointless.
=== Other bugs === hz -> profhz in the p_pctcpu = f(p_cpticks) calcuation.
Collect scheduler functionality. Try to put each abstraction in just one
place.
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Dennis Ferguson (NetBSD PR #2788)
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Add ntp_adjtime() and ntp_gettime() system calls
Mostly stolen from FreeBSD
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fix off-by-one error in tickfix code. (should increment when count >=
interval, because count goes from 0->(interval-1) to count interval
ticks.)
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Move the old-style disk instrumentation "structures" to a central
location (sys/kern/subr_disk.c) and note that they should/will be
deprecated.
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