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haproxy/include/proto/freq_ctr.h

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/*
* 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 <types/freq_ctr.h>
/* Rotate a frequency counter when current period is over. Must not be called
* during a valid period. It is important that it correctly initializes a null
* area.
*/
static inline void rotate_freq_ctr(struct freq_ctr *ctr)
{
ctr->prev_ctr = ctr->curr_ctr;
if (likely(now.tv_sec - ctr->curr_sec != 1)) {
/* we missed more than one second */
ctr->prev_ctr = 0;
}
ctr->curr_sec = now.tv_sec;
ctr->curr_ctr = 0; /* leave it at the end to help gcc optimize it away */
}
/* 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 void update_freq_ctr(struct freq_ctr *ctr, unsigned int inc)
{
if (likely(ctr->curr_sec == now.tv_sec)) {
ctr->curr_ctr += inc;
return;
}
rotate_freq_ctr(ctr);
ctr->curr_ctr = inc;
/* Note: later we may want to propagate the update to other counters */
}
/* Rotate a frequency counter when current period is over. Must not be called
* during a valid period. 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 void rotate_freq_ctr_period(struct freq_ctr_period *ctr,
unsigned int period)
{
ctr->prev_ctr = ctr->curr_ctr;
ctr->curr_tick += period;
if (likely(now_ms - ctr->curr_tick >= period)) {
/* we missed at least two periods */
ctr->prev_ctr = 0;
ctr->curr_tick = now_ms;
}
ctr->curr_ctr = 0; /* leave it at the end to help gcc optimize it away */
}
/* 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 void update_freq_ctr_period(struct freq_ctr_period *ctr,
unsigned int period, unsigned int inc)
{
if (likely(now_ms - ctr->curr_tick < period)) {
ctr->curr_ctr += inc;
return;
}
rotate_freq_ctr_period(ctr, period);
ctr->curr_ctr = 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.
*
* 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 * (n - 1) / n + v;
}
/* 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:
*/
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