Il 08/09/2014 14:18, Benoît Canet ha scritto:
> The algorithm used was defined on the list while discussing the new IO
> accounting
> overhaul.
> See http://lists.nongnu.org/archive/html/qemu-devel/2014-08/msg04954.html
>
> Also the module takes care of computing minimal and maximal values over the
> time
> slice duration.
>
> Signed-off-by: Benoît Canet <benoit.ca...@nodalink.com>
If you add
int64_t cpu_get_clock(void)
{
return my_clock_value;
}
to the test, and use a QEMU_CLOCK_VIRTUAL-based average, you should be
able to advance the clock directly in the test with no need for sleep()
and with 100% deterministic results.
>
> +/* Check if the ta->periods seconds time slice has expired
> + *
> + * If the slice has expired the counters will be reseted
> + *
> + * @ta: the timed average structure used
> + */
> +static void timed_average_check_expiration(TimedAverage *ta)
> +{
> + int64_t now = qemu_clock_get_ns(ta->clock_type);
> +
> + /* if we are still in the period slice do nothing */
> + if (now < ta->expiration) {
> + return;
> + }
> +
> + /* the slice has expired -> create a new slice */
> + ta->min = UINT64_MAX;
> + ta->sum = 0;
> + ta->max = 0;
> + ta->count = 0;
> + timed_average_set_expiration(ta);
> +}
This can produce very noisy results if you invoke min/avg/max at the
wrong time. Some alternatives include:
- create two windows, with twice the suggested expiration period, and
return min/avg/max from the oldest window. Example
t=0 |t=1 |t=2 |t=3 |t=4
wnd0: [0,1) |wnd0: [1,3) | |wnd0: [3,5) |
wnd1: [0,2) | |wnd1: [2,4) | |
Values are returned from:
wnd0---------|wnd1---------|wnd0---------|wnd1---------|
- if you do not need min/max, you can use exponential smoothing, with a
weighted factor that depends on the time since the last sample.
http://www.drdobbs.com/tools/discontiguous-exponential-averaging/184410671
-- for example, giving 90% weight to the last second. Of course the
exponential nature means that, in that case, 1-sqrt(10%)=68.3% weight is
given to the last half second, 21.6% weight is given to the previous
half second, and 10% to the entire previous history. This cannot give
min/max, but can give avg/stdev.
Paolo
