incorrect usleep/select delays with HZ > 2500

Luigi Rizzo rizzo at
Sun Sep 6 15:46:04 UTC 2009

(this problem seems to affect both current and -stable,
so let's see if here i have better luck)

I just noticed [Note 1,2] that when setting HZ > 2500 (even if it is
an exact divisor of the APIC/CPU clock) there is a significant
drift between the delays generated by usleep()/select() and those
computed by gettimeofday().  In other words, the error grows with
the amount of delay requested.

To show the problem, try this function

        int f(int wait_time) {  // wait_time in usec
                struct timeval start, end;
                usleep(w);      // or try select
                timersub(&end, &start, &x);
                return = x.tv_usec + 1000000*x.tv_sec - wait_time;

for various HZ (kern.hz=NNNN in /boot/loader.conf) and wait times.
Ideally, we would expect the timings to be in error by something
between 0 and 1 (or 2) ticks, irrespective of the value of wait_time.
In fact, this is what you see with HZ=1000, 2000 and 2500.
But larger values of HZ (e.g. 4000, 5000, 10k, 40k) create
a drift of 0.5% and above (i.e. with HZ=5000, a 1-second delay
lasts 1.0064s and a 10s delay lasts 10.062s; with HZ=10k the
error becomes 1% and at HZ=40k the error becomes even bigger.

Note that with the fixes described below, even HZ=40k works perfectly well.

Turns out that the error has three components (described with
possible fixes):

1.  CAUSE: Errors in the measurement of the TSC (and APIC) frequencies,
        see [Note 3] for more details. This is responsible for the drift.
    FIX: It can be removed by rounding the measurement to the closest
        nominal values (e.g. my APIC runs at 100 MHz; we can use a
        table of supported values). Otherwise, see [Note 4]
    PROBLEM: is this general enough ?

2.  CAUSE: Use of approximate kernel time functions (getnanotime/getmicrotime)
        in nanosleep() and select(). This imposes an error of max(1tick, 1ms)
        in the computation of delays, irrespective of HZ values.
        BTW For reasons I don't understand this seems to affect
        nanosleep() more than select().
    FIX: It can be reduced to just 1 tick making kern.timecounter.tick writable
        and letting the user set it to 1 if high precision is required.
    PROBLEM: none that i see.

3.  CAUSE an error in tvtohz(), reported long ago in
        which causes a systematic error of an extra tick in the computation
        of the sleep times.
    FIX: the above link also contains a proposed fix (which in fact
	reverts a bug introduced in some old commit on FreeBSD)
    PROBLEM: none that i see.

Applying these three fixes i was able to run a kernel with HZ=40000
and see timing errors within 80-90us even with ten second delays.
This would put us on par with Linux [Note 5].
This is a significant improvement over the current situation
and the reason why I would like to explore the possibility of applying
some of these fixes.

I know there are open problems -- e.g. when the timer source used
by gettimeofday() gets corrected by ntp or other things, hardclock()
still ticks at the same rate so you'll see a drift if you don't apply
corrections there as well. Similarly, if HZ is not an exact
divisor of the clock source used by gettimeofday(), i suppose
errors will accumulate as well. However fixing these other
drift seems reasonably orthogonal at least to #2 and #3 above, and
a lot more difficult so we could at least start from these simple

Would anyone be interested in reproducing the experiment (test program
attached -- run it with 'lat -p -i wait_time_in_microseconds')
and try to explain me what changes the system's behaviour above HZ=2500 ?



[Note 1] I have some interest in running machines with high HZ values
    because this gives better precision to dummynet and various
    other tasks with soft timing constraints.

[Note 2] I have seen the same phenomenon on the following platform:
        RELENG_8/amd64 with AMD BE-2400 dual core cpu
        RELENG_7/i386 with AMD BE-2400 dual core cpu
        RELENG_7/i386 with Intel Centrino single core (Dell X1 Laptop)

[Note 3] the TSC frequency is computed reading the tsc around a
        call to DELAY(1000000) and assuming that the i8254 runs
        at the nominal rate, 1.193182 MHz.
        From tests I have made, the measurement in init_TSC() returns
        a large error when HZ is large, whereas repeating the measurement
        at a later time returns a much more reliable value.
        As an example, see the following:

    Sep  6 14:21:59 lr kernel: TSC clock: at init_TSC 2323045616 Hz
    Sep  6 14:21:59 lr kernel: AMD Features=0xea500800<SYSCALL,NX,MMX+,FFXSR,RDTSCP,LM,3DNow!+,3DNow!>
    Sep  6 14:21:59 lr kernel: TSC: P-state invariant
    Sep  6 14:21:59 lr kernel: TSC clock: at cpu_startup_end 2323056910 Hz
    Sep  6 14:21:59 lr kernel: TSC clock: at acpi_timer_probe 2311254060 Hz
    Sep  6 14:21:59 lr kernel: TSC clock: at acpi_timer_probe_2 2311191310 Hz
    Sep  6 14:21:59 lr kernel: TSC clock: at pn_probe_start 2300822648 Hz
    Sep  6 14:21:59 lr kernel: TSC clock: at pn_attach_start 2300830946 Hz
    Sep  6 14:21:59 lr kernel: TSC clock: at pn_probe_start 2300840133 Hz
    Sep  6 14:21:59 lr kernel: TSC clock: at pn_attach_start 2300835253 Hz
    Sep  6 14:21:59 lr kernel: TSC clock: at lapic_setup_clock 2300868376 Hz

        The latter values are close to what is reported when HZ=1000.

[Note 4] DELAY() uses the TSC when available, but perhaps for
        larger intervals (say above 100ms) it might be better
        to always use the 8254 which at least does not change frequency
        over time ?

[Note 5] Linux has some high precision timers which one could expect
    to use for high precision delays. In fact, I ran the test
    program on a recent Linux 2.6.30 (using HZ=1000), and the
    usleep() version has an error between 80 and 90us irrespective
    of the delay. However the select() version is much worse,
    and loses approx 1ms per second.


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