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/*
* QEMU timed average computation
*
* Copyright (C) Nodalink, EURL. 2014
* Copyright (C) Igalia, S.L. 2015
*
* Authors:
* Benoît Canet <benoit.canet@nodalink.com>
* Alberto Garcia <berto@igalia.com>
*
* This program is free sofware: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Sofware Foundation, either version 2 of the License, or
* (at your option) version 3 or any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include <string.h>
#include "qemu/timed-average.h"
/* This module computes an average of a set of values within a time
* window.
*
* Algorithm:
*
* - Create two windows with a certain expiration period, and
* offsetted by period / 2.
* - Each time you want to account a new value, do it in both windows.
* - The minimum / maximum / average values are always returned from
* the oldest window.
*
* Example:
*
* t=0 |t=0.5 |t=1 |t=1.5 |t=2
* wnd0: [0,0.5)|wnd0: [0.5,1.5) | |wnd0: [1.5,2.5) |
* wnd1: [0,1) | |wnd1: [1,2) | |
*
* Values are returned from:
*
* wnd0---------|wnd1------------|wnd0---------|wnd1-------------|
*/
/* Update the expiration of a time window
*
* @w: the window used
* @now: the current time in nanoseconds
* @period: the expiration period in nanoseconds
*/
static void update_expiration(TimedAverageWindow *w, int64_t now,
int64_t period)
{
/* time elapsed since the last theoretical expiration */
int64_t elapsed = (now - w->expiration) % period;
/* time remaininging until the next expiration */
int64_t remaining = period - elapsed;
/* compute expiration */
w->expiration = now + remaining;
}
/* Reset a window
*
* @w: the window to reset
*/
static void window_reset(TimedAverageWindow *w)
{
w->min = UINT64_MAX;
w->max = 0;
w->sum = 0;
w->count = 0;
}
/* Get the current window (that is, the one with the earliest
* expiration time).
*
* @ta: the TimedAverage structure
* @ret: a pointer to the current window
*/
static TimedAverageWindow *current_window(TimedAverage *ta)
{
return &ta->windows[ta->current];
}
/* Initialize a TimedAverage structure
*
* @ta: the TimedAverage structure
* @clock_type: the type of clock to use
* @period: the time window period in nanoseconds
*/
void timed_average_init(TimedAverage *ta, QEMUClockType clock_type,
uint64_t period)
{
int64_t now = qemu_clock_get_ns(clock_type);
/* Returned values are from the oldest window, so they belong to
* the interval [ta->period/2,ta->period). By adjusting the
* requested period by 4/3, we guarantee that they're in the
* interval [2/3 period,4/3 period), closer to the requested
* period on average */
ta->period = (uint64_t) period * 4 / 3;
ta->clock_type = clock_type;
ta->current = 0;
window_reset(&ta->windows[0]);
window_reset(&ta->windows[1]);
/* Both windows are offsetted by half a period */
ta->windows[0].expiration = now + ta->period / 2;
ta->windows[1].expiration = now + ta->period;
}
/* Check if the time windows have expired, updating their counters and
* expiration time if that's the case.
*
* @ta: the TimedAverage structure
*/
static void check_expirations(TimedAverage *ta)
{
int64_t now = qemu_clock_get_ns(ta->clock_type);
int i;
assert(ta->period != 0);
/* Check if the windows have expired */
for (i = 0; i < 2; i++) {
TimedAverageWindow *w = &ta->windows[i];
if (w->expiration <= now) {
window_reset(w);
update_expiration(w, now, ta->period);
}
}
/* Make ta->current point to the oldest window */
if (ta->windows[0].expiration < ta->windows[1].expiration) {
ta->current = 0;
} else {
ta->current = 1;
}
}
/* Account a value
*
* @ta: the TimedAverage structure
* @value: the value to account
*/
void timed_average_account(TimedAverage *ta, uint64_t value)
{
int i;
check_expirations(ta);
/* Do the accounting in both windows at the same time */
for (i = 0; i < 2; i++) {
TimedAverageWindow *w = &ta->windows[i];
w->sum += value;
w->count++;
if (value < w->min) {
w->min = value;
}
if (value > w->max) {
w->max = value;
}
}
}
/* Get the minimum value
*
* @ta: the TimedAverage structure
* @ret: the minimum value
*/
uint64_t timed_average_min(TimedAverage *ta)
{
TimedAverageWindow *w;
check_expirations(ta);
w = current_window(ta);
return w->min < UINT64_MAX ? w->min : 0;
}
/* Get the average value
*
* @ta: the TimedAverage structure
* @ret: the average value
*/
uint64_t timed_average_avg(TimedAverage *ta)
{
TimedAverageWindow *w;
check_expirations(ta);
w = current_window(ta);
return w->count > 0 ? w->sum / w->count : 0;
}
/* Get the maximum value
*
* @ta: the TimedAverage structure
* @ret: the maximum value
*/
uint64_t timed_average_max(TimedAverage *ta)
{
check_expirations(ta);
return current_window(ta)->max;
}
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