On 31.12.2012 17:02, Ian Lepore wrote:
On Mon, 2012-12-31 at 12:17 +0200, Alexander Motin wrote:
On 31.12.2012 08:17, Luigi Rizzo wrote:
On Sun, Dec 30, 2012 at 04:13:43PM -0700, Ian Lepore wrote:
...
I grabbed testsleep.c to test an arm event timer implementation, and had
to fix a couple nits... kqueueto was missing from the names[] array, and
I had to add a "* 1000" to a couple places where usec was stuffed into a
timespec's tv_nsec.
I also tested the calloutng_12_17 patches and the kqueue stuff behaved
very strangely.
I've rewritten kqueue timeouts at the calloutng_12_26.patch.
Then I noticed you had a 12_26 patchset so I tested
that (after crudely fixing a couple uninitialized var warnings), and it
all looks good on this arm (Raspberry Pi). I'll attach the results.
It's so sweet to be able to do precision sleeps.
Thank you for testing, Ian.
interesting numbers, but there seems to be some problem in computing
the exact interval; delays are much larger than expected.
In this test, the original timer code used to round to the next multiple
of 1 tick and then add another tick (except for the kqueue case),
which is exactly what you see in the second set of measurements.
The calloutng code however seems to do something odd:
in addition to fixed overhead (some 50us, which you can see in
the tests for 1us and 300us), all delay seem to be ~10% larger
than what is requested, upper bounded to 10ms (note, the
numbers are averages so i cannot tell whether all samples are
the same or there is some distribution of values).
I am not sure if this error is peculiar of the ARM version or also
appears on x86/amd64 but I believe it should be fixed.
If you look at the results below:
1us possily ok:
for very short intervals i would expect some kind
of 'reschedule' without actually firing a timer; maybe
50us are what it takes to do a round through the scheduler ?
300us probably ok
i guess the extra 50-90us are what it takes to do a round
through the scheduler
1000us borderline (this is the case for poll and kqueue, which are
rounded to 1ms)
here intervals seem to be increased by 10%, and i cannot see
a good reason for this (more below).
3000us and above: wrong
here again, the intervals seem to be 10% larger than what is
requested, perhaps limiting the error to 10-20ms.
Maybe the 10% extension results from creating a default 'precision'
for legacy calls, but i do not think this is done correctly.
First of all, if users do not specify a precision themselves, the
automatically generated value should never exceed one tick.
Second, the only point of a 'precision' parameter is to merge
requests that may be close in time, so if there is already a
timer scheduled within [Treq, Treq+precision] i will get it;
but if there no pending timer, then one should schedule it
for the requested interval.
Davide/Alexander, any ideas ?
All mentioned effects could be explained with implemented logic. 50us at
1us is probably sum of minimal latency of the hardware eventtimer on the
specific platform and some software processing overhead (syscall,
callout, timecouters, scheduler, etc). At later points system starts to
noticeably use precision specified by kern.timecounter.alloweddeviation
sysctl. It affects results from two sides: 1) extending intervals for
specified percent of time to allow event aggregation, and 2) choosing
time base between fast getbinuptime() and precise binuptime(). Extending
interval is needed to aggregate not only callouts with each other, but
also callouts with other system events, which are impossible to schedule
in advance. It gives specified relative error, but no more then one CPU
wakeup period in absolute: for busy CPU (not skipping hardclock() ticks)
it is 1/hz, for completely idle one it can be up to 0.5s. Second point
allows to reduce processing overhead by the cost of error up to 1/hz for
long periods (>(100/allowed)*(1/hz)), when it is used.
To get best possible precision kern.timecounter.alloweddeviation sysctl
can be set to smaller value. Setting it to 0 will effectively disable
all optimizations, but should give 50us precision in all cases.
for t in 1 300 3000 30000 300000 ; do
for m in select poll usleep nanosleep kqueue kqueueto syscall ; do
./testsleep $t $m
done
done
[test results snipped]
I should have posted some information about the test platform... It's a
single-processor 700mhz arm chip. There was essentially nothing else
running during the tests other than mostly-idle daemons (sshd, ntpd, the
usual suspects). Kernel debugging options off (INVARIANTS[_SUPPORT],
DIAGNOSTIC, and WITNESS).
Some sysctl values of interest...
rpi# sysctl kern.timecounter
kern.timecounter.fast_gettime: 1
kern.timecounter.tick: 1
kern.timecounter.choice: BCM2835 Timecounter(1000) dummy(-1000000)
kern.timecounter.hardware: BCM2835 Timecounter
kern.timecounter.alloweddeviation: 5
kern.timecounter.stepwarnings: 1
kern.timecounter.tc.BCM2835 Timecounter.mask: 4294967295
kern.timecounter.tc.BCM2835 Timecounter.counter: 734706756
kern.timecounter.tc.BCM2835 Timecounter.frequency: 1000000
kern.timecounter.tc.BCM2835 Timecounter.quality: 1000
rpi# sysctl kern.eventtimer
kern.eventtimer.choice: BCM2835 Event Timer 3(1000)
kern.eventtimer.et.BCM2835 Event Timer 3.flags: 2
kern.eventtimer.et.BCM2835 Event Timer 3.frequency: 1000000
kern.eventtimer.et.BCM2835 Event Timer 3.quality: 1000
kern.eventtimer.periodic: 0
kern.eventtimer.timer: BCM2835 Event Timer 3
kern.eventtimer.activetick: 1
kern.eventtimer.idletick: 0
kern.eventtimer.singlemul: 4
BTW, is there any advantage to implementing periodic mode for an
eventtimer? It would be easy enough to do for this hardware, but it
looks like all this new event timer code is pretty much a stake through
the heart of periodic timer ticks.
Periodic-mode-only hardware is still supported, but present code takes
almost no advantage from periodic mode if one-shot mode is supported. It
can't use interrupts as time source to run events (as legacy code did)
because of possible drift from system timecounter that makes impossible
to specify absolute event time. The only benefit is that timer hardware
is not reprogrammed each time, and I don't think that this economy worth
result. But for all hardware supporting periodic mode I've implemented
respective support at least for completeness and testing purposes.
--
Alexander Motin
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