mirror of
http://git.haproxy.org/git/haproxy.git/
synced 2024-12-13 06:54:37 +00:00
627505d36a
Some samples representing time will cover more than one sample at once if they are units of time per time. For this we'd need to have the ability to loop over swrate_add() multiple times but that would be inefficient. By developing the function elevated to power N, it's visible that some coefficients quickly disappear and that those which remain at the first order more or less compensate each other. Thus a simplified version of this function was added to provide a single value for a given number of samples. Tests with multiple values, window sizes and sample sizes have shown that it is possible to make it remain surprisingly accurate (typical error < 0.2% over various large window and sample sizes, even samples representing up to 1/4 of the window).
291 lines
10 KiB
C
291 lines
10 KiB
C
/*
|
|
* include/proto/freq_ctr.h
|
|
* This file contains macros and inline functions for frequency counters.
|
|
*
|
|
* Copyright (C) 2000-2014 Willy Tarreau - w@1wt.eu
|
|
*
|
|
* This library is free software; you can redistribute it and/or
|
|
* modify it under the terms of the GNU Lesser General Public
|
|
* License as published by the Free Software Foundation, version 2.1
|
|
* exclusively.
|
|
*
|
|
* This library 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
|
|
* Lesser General Public License for more details.
|
|
*
|
|
* You should have received a copy of the GNU Lesser General Public
|
|
* License along with this library; if not, write to the Free Software
|
|
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
|
|
*/
|
|
|
|
#ifndef _PROTO_FREQ_CTR_H
|
|
#define _PROTO_FREQ_CTR_H
|
|
|
|
#include <common/config.h>
|
|
#include <common/time.h>
|
|
#include <common/hathreads.h>
|
|
#include <types/freq_ctr.h>
|
|
|
|
|
|
/* Update a frequency counter by <inc> incremental units. It is automatically
|
|
* rotated if the period is over. It is important that it correctly initializes
|
|
* a null area.
|
|
*/
|
|
static inline unsigned int update_freq_ctr(struct freq_ctr *ctr, unsigned int inc)
|
|
{
|
|
int elapsed;
|
|
unsigned int tot_inc;
|
|
unsigned int curr_sec;
|
|
|
|
do {
|
|
/* remove the bit, used for the lock */
|
|
curr_sec = ctr->curr_sec & 0x7fffffff;
|
|
}
|
|
while (!HA_ATOMIC_CAS(&ctr->curr_sec, &curr_sec, curr_sec | 0x80000000));
|
|
|
|
elapsed = (now.tv_sec & 0x7fffffff)- curr_sec;
|
|
if (unlikely(elapsed > 0)) {
|
|
ctr->prev_ctr = ctr->curr_ctr;
|
|
ctr->curr_ctr = 0;
|
|
if (likely(elapsed != 1)) {
|
|
/* we missed more than one second */
|
|
ctr->prev_ctr = 0;
|
|
}
|
|
curr_sec = now.tv_sec;
|
|
}
|
|
|
|
ctr->curr_ctr += inc;
|
|
tot_inc = ctr->curr_ctr;
|
|
|
|
/* release the lock and update the time in case of rotate. */
|
|
HA_ATOMIC_STORE(&ctr->curr_sec, curr_sec & 0x7fffffff);
|
|
return tot_inc;
|
|
/* Note: later we may want to propagate the update to other counters */
|
|
}
|
|
|
|
/* Update a frequency counter by <inc> incremental units. It is automatically
|
|
* rotated if the period is over. It is important that it correctly initializes
|
|
* a null area. This one works on frequency counters which have a period
|
|
* different from one second.
|
|
*/
|
|
static inline unsigned int update_freq_ctr_period(struct freq_ctr_period *ctr,
|
|
unsigned int period, unsigned int inc)
|
|
{
|
|
unsigned int tot_inc;
|
|
unsigned int curr_tick;
|
|
|
|
do {
|
|
/* remove the bit, used for the lock */
|
|
curr_tick = (ctr->curr_tick >> 1) << 1;
|
|
}
|
|
while (!HA_ATOMIC_CAS(&ctr->curr_tick, &curr_tick, curr_tick | 0x1));
|
|
|
|
if (now_ms - curr_tick >= period) {
|
|
ctr->prev_ctr = ctr->curr_ctr;
|
|
ctr->curr_ctr = 0;
|
|
curr_tick += period;
|
|
if (likely(now_ms - curr_tick >= period)) {
|
|
/* we missed at least two periods */
|
|
ctr->prev_ctr = 0;
|
|
curr_tick = now_ms;
|
|
}
|
|
}
|
|
|
|
ctr->curr_ctr += inc;
|
|
tot_inc = ctr->curr_ctr;
|
|
/* release the lock and update the time in case of rotate. */
|
|
HA_ATOMIC_STORE(&ctr->curr_tick, (curr_tick >> 1) << 1);
|
|
return tot_inc;
|
|
/* Note: later we may want to propagate the update to other counters */
|
|
}
|
|
|
|
/* Read a frequency counter taking history into account for missing time in
|
|
* current period.
|
|
*/
|
|
unsigned int read_freq_ctr(struct freq_ctr *ctr);
|
|
|
|
/* returns the number of remaining events that can occur on this freq counter
|
|
* while respecting <freq> and taking into account that <pend> events are
|
|
* already known to be pending. Returns 0 if limit was reached.
|
|
*/
|
|
unsigned int freq_ctr_remain(struct freq_ctr *ctr, unsigned int freq, unsigned int pend);
|
|
|
|
/* return the expected wait time in ms before the next event may occur,
|
|
* respecting frequency <freq>, and assuming there may already be some pending
|
|
* events. It returns zero if we can proceed immediately, otherwise the wait
|
|
* time, which will be rounded down 1ms for better accuracy, with a minimum
|
|
* of one ms.
|
|
*/
|
|
unsigned int next_event_delay(struct freq_ctr *ctr, unsigned int freq, unsigned int pend);
|
|
|
|
/* process freq counters over configurable periods */
|
|
unsigned int read_freq_ctr_period(struct freq_ctr_period *ctr, unsigned int period);
|
|
unsigned int freq_ctr_remain_period(struct freq_ctr_period *ctr, unsigned int period,
|
|
unsigned int freq, unsigned int pend);
|
|
|
|
/* While the functions above report average event counts per period, we are
|
|
* also interested in average values per event. For this we use a different
|
|
* method. The principle is to rely on a long tail which sums the new value
|
|
* with a fraction of the previous value, resulting in a sliding window of
|
|
* infinite length depending on the precision we're interested in.
|
|
*
|
|
* The idea is that we always keep (N-1)/N of the sum and add the new sampled
|
|
* value. The sum over N values can be computed with a simple program for a
|
|
* constant value 1 at each iteration :
|
|
*
|
|
* N
|
|
* ,---
|
|
* \ N - 1 e - 1
|
|
* > ( --------- )^x ~= N * -----
|
|
* / N e
|
|
* '---
|
|
* x = 1
|
|
*
|
|
* Note: I'm not sure how to demonstrate this but at least this is easily
|
|
* verified with a simple program, the sum equals N * 0.632120 for any N
|
|
* moderately large (tens to hundreds).
|
|
*
|
|
* Inserting a constant sample value V here simply results in :
|
|
*
|
|
* sum = V * N * (e - 1) / e
|
|
*
|
|
* But we don't want to integrate over a small period, but infinitely. Let's
|
|
* cut the infinity in P periods of N values. Each period M is exactly the same
|
|
* as period M-1 with a factor of ((N-1)/N)^N applied. A test shows that given a
|
|
* large N :
|
|
*
|
|
* N - 1 1
|
|
* ( ------- )^N ~= ---
|
|
* N e
|
|
*
|
|
* Our sum is now a sum of each factor times :
|
|
*
|
|
* N*P P
|
|
* ,--- ,---
|
|
* \ N - 1 e - 1 \ 1
|
|
* > v ( --------- )^x ~= VN * ----- * > ---
|
|
* / N e / e^x
|
|
* '--- '---
|
|
* x = 1 x = 0
|
|
*
|
|
* For P "large enough", in tests we get this :
|
|
*
|
|
* P
|
|
* ,---
|
|
* \ 1 e
|
|
* > --- ~= -----
|
|
* / e^x e - 1
|
|
* '---
|
|
* x = 0
|
|
*
|
|
* This simplifies the sum above :
|
|
*
|
|
* N*P
|
|
* ,---
|
|
* \ N - 1
|
|
* > v ( --------- )^x = VN
|
|
* / N
|
|
* '---
|
|
* x = 1
|
|
*
|
|
* So basically by summing values and applying the last result an (N-1)/N factor
|
|
* we just get N times the values over the long term, so we can recover the
|
|
* constant value V by dividing by N. In order to limit the impact of integer
|
|
* overflows, we'll use this equivalence which saves us one multiply :
|
|
*
|
|
* N - 1 1 x0
|
|
* x1 = x0 * ------- = x0 * ( 1 - --- ) = x0 - ----
|
|
* N N N
|
|
*
|
|
* And given that x0 is discrete here we'll have to saturate the values before
|
|
* performing the divide, so the value insertion will become :
|
|
*
|
|
* x0 + N - 1
|
|
* x1 = x0 - ------------
|
|
* N
|
|
*
|
|
* A value added at the entry of the sliding window of N values will thus be
|
|
* reduced to 1/e or 36.7% after N terms have been added. After a second batch,
|
|
* it will only be 1/e^2, or 13.5%, and so on. So practically speaking, each
|
|
* old period of N values represents only a quickly fading ratio of the global
|
|
* sum :
|
|
*
|
|
* period ratio
|
|
* 1 36.7%
|
|
* 2 13.5%
|
|
* 3 4.98%
|
|
* 4 1.83%
|
|
* 5 0.67%
|
|
* 6 0.25%
|
|
* 7 0.09%
|
|
* 8 0.033%
|
|
* 9 0.012%
|
|
* 10 0.0045%
|
|
*
|
|
* So after 10N samples, the initial value has already faded out by a factor of
|
|
* 22026, which is quite fast. If the sliding window is 1024 samples wide, it
|
|
* means that a sample will only count for 1/22k of its initial value after 10k
|
|
* samples went after it, which results in half of the value it would represent
|
|
* using an arithmetic mean. The benefit of this method is that it's very cheap
|
|
* in terms of computations when N is a power of two. This is very well suited
|
|
* to record response times as large values will fade out faster than with an
|
|
* arithmetic mean and will depend on sample count and not time.
|
|
*
|
|
* Demonstrating all the above assumptions with maths instead of a program is
|
|
* left as an exercise for the reader.
|
|
*/
|
|
|
|
/* Adds sample value <v> to sliding window sum <sum> configured for <n> samples.
|
|
* The sample is returned. Better if <n> is a power of two.
|
|
*/
|
|
static inline unsigned int swrate_add(unsigned int *sum, unsigned int n, unsigned int v)
|
|
{
|
|
return *sum = *sum - (*sum + n - 1) / n + v;
|
|
}
|
|
|
|
/* Adds sample value <v> spanning <s> samples to sliding window sum <sum>
|
|
* configured for <n> samples, where <n> is supposed to be "much larger" than
|
|
* <s>. The sample is returned. Better if <n> is a power of two. Note that this
|
|
* is only an approximate. Indeed, as can be seen with two samples only over a
|
|
* 8-sample window, the original function would return :
|
|
* sum1 = sum - (sum + 7) / 8 + v
|
|
* sum2 = sum1 - (sum1 + 7) / 8 + v
|
|
* = (sum - (sum + 7) / 8 + v) - (sum - (sum + 7) / 8 + v + 7) / 8 + v
|
|
* ~= 7sum/8 - 7/8 + v - sum/8 + sum/64 - 7/64 - v/8 - 7/8 + v
|
|
* ~= (3sum/4 + sum/64) - (7/4 + 7/64) + 15v/8
|
|
*
|
|
* while the function below would return :
|
|
* sum = sum + 2*v - (sum + 8) * 2 / 8
|
|
* = 3sum/4 + 2v - 2
|
|
*
|
|
* this presents an error of ~ (sum/64 + 9/64 + v/8) = (sum+n+1)/(n^s) + v/n
|
|
*
|
|
* Thus the simplified function effectively replaces a part of the history with
|
|
* a linear sum instead of applying the exponential one. But as long as s/n is
|
|
* "small enough", the error fades away and remains small for both small and
|
|
* large values of n and s (typically < 0.2% measured).
|
|
*/
|
|
static inline unsigned int swrate_add_scaled(unsigned int *sum, unsigned int n, unsigned int v, unsigned int s)
|
|
{
|
|
return *sum = *sum + v * s - div64_32((unsigned long long)(*sum + n) * s, n);
|
|
}
|
|
|
|
/* Returns the average sample value for the sum <sum> over a sliding window of
|
|
* <n> samples. Better if <n> is a power of two. It must be the same <n> as the
|
|
* one used above in all additions.
|
|
*/
|
|
static inline unsigned int swrate_avg(unsigned int sum, unsigned int n)
|
|
{
|
|
return (sum + n - 1) / n;
|
|
}
|
|
|
|
#endif /* _PROTO_FREQ_CTR_H */
|
|
|
|
/*
|
|
* Local variables:
|
|
* c-indent-level: 8
|
|
* c-basic-offset: 8
|
|
* End:
|
|
*/
|