Module Name: src
Committed By: maxv
Date: Sat Jul 8 15:15:43 UTC 2017
Modified Files:
src/sys/kern: sched_4bsd.c
Log Message:
explain a bit
To generate a diff of this commit:
cvs rdiff -u -r1.30 -r1.31 src/sys/kern/sched_4bsd.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/kern/sched_4bsd.c
diff -u src/sys/kern/sched_4bsd.c:1.30 src/sys/kern/sched_4bsd.c:1.31
--- src/sys/kern/sched_4bsd.c:1.30 Tue Jun 24 10:08:45 2014
+++ src/sys/kern/sched_4bsd.c Sat Jul 8 15:15:43 2017
@@ -1,6 +1,6 @@
-/* $NetBSD: sched_4bsd.c,v 1.30 2014/06/24 10:08:45 maxv Exp $ */
+/* $NetBSD: sched_4bsd.c,v 1.31 2017/07/08 15:15:43 maxv Exp $ */
-/*-
+/*
* Copyright (c) 1999, 2000, 2004, 2006, 2007, 2008 The NetBSD Foundation, Inc.
* All rights reserved.
*
@@ -31,7 +31,7 @@
* POSSIBILITY OF SUCH DAMAGE.
*/
-/*-
+/*
* Copyright (c) 1982, 1986, 1990, 1991, 1993
* The Regents of the University of California. All rights reserved.
* (c) UNIX System Laboratories, Inc.
@@ -68,7 +68,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: sched_4bsd.c,v 1.30 2014/06/24 10:08:45 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: sched_4bsd.c,v 1.31 2017/07/08 15:15:43 maxv Exp $");
#include "opt_ddb.h"
#include "opt_lockdebug.h"
@@ -80,13 +80,10 @@ __KERNEL_RCSID(0, "$NetBSD: sched_4bsd.c
#include <sys/cpu.h>
#include <sys/proc.h>
#include <sys/kernel.h>
-#include <sys/signalvar.h>
#include <sys/resourcevar.h>
#include <sys/sched.h>
#include <sys/sysctl.h>
-#include <sys/kauth.h>
#include <sys/lockdebug.h>
-#include <sys/kmem.h>
#include <sys/intr.h>
static void updatepri(struct lwp *);
@@ -95,7 +92,7 @@ static void resetpriority(struct lwp *);
extern unsigned int sched_pstats_ticks; /* defined in kern_synch.c */
/* Number of hardclock ticks per sched_tick() */
-static int rrticks;
+static int rrticks __read_mostly;
/*
* Force switch among equal priority processes every 100ms.
@@ -133,7 +130,7 @@ sched_tick(struct cpu_info *ci)
if (spc->spc_flags & SPCF_SHOULDYIELD) {
/*
* Process is stuck in kernel somewhere, probably
- * due to buggy or inefficient code. Force a
+ * due to buggy or inefficient code. Force a
* kernel preemption.
*/
cpu_need_resched(ci, RESCHED_KPREEMPT);
@@ -170,71 +167,90 @@ sched_tick(struct cpu_info *ci)
#define ESTCPULIM(e) min((e), ESTCPU_MAX)
/*
- * Constants for digital decay and forget:
- * 90% of (l_estcpu) usage in 5 * loadav time
- * 95% of (l_pctcpu) usage in 60 seconds (load insensitive)
- * Note that, as ps(1) mentions, this can let percentages
- * total over 100% (I've seen 137.9% for 3 processes).
+ * The main parameter used by this algorithm is 'l_estcpu'. It is an estimate
+ * of the recent CPU utilization of the thread.
+ *
+ * l_estcpu is:
+ * - increased each time the hardclock ticks and the thread is found to
+ * be executing, in sched_schedclock() called from hardclock()
+ * - decreased (filtered) on each sched tick, in sched_pstats_hook()
+ * If the lwp is sleeping for more than a second, we don't touch l_estcpu: it
+ * will be updated in sched_setrunnable() when the lwp wakes up, in burst mode
+ * (ie, we decrease it n times).
*
* Note that hardclock updates l_estcpu and l_cpticks independently.
*
- * We wish to decay away 90% of l_estcpu in (5 * loadavg) seconds.
- * That is, the system wants to compute a value of decay such
- * that the following for loop:
- * for (i = 0; i < (5 * loadavg); i++)
- * l_estcpu *= decay;
- * will compute
- * l_estcpu *= 0.1;
- * for all values of loadavg:
+ * -----------------------------------------------------------------------------
+ *
+ * Here we describe how l_estcpu is decreased.
+ *
+ * Constants for digital decay (filter):
+ * 90% of l_estcpu usage in (5 * loadavg) seconds
+ *
+ * We wish to decay away 90% of l_estcpu in (5 * loadavg) seconds. That is, we
+ * want to compute a value of decay such that the following loop:
+ * for (i = 0; i < (5 * loadavg); i++)
+ * l_estcpu *= decay;
+ * will result in
+ * l_estcpu *= 0.1;
+ * for all values of loadavg.
*
* Mathematically this loop can be expressed by saying:
- * decay ** (5 * loadavg) ~= .1
+ * decay ** (5 * loadavg) ~= .1
+ *
+ * And finally, the corresponding value of decay we're using is:
+ * decay = (2 * loadavg) / (2 * loadavg + 1)
*
- * The system computes decay as:
- * decay = (2 * loadavg) / (2 * loadavg + 1)
+ * -----------------------------------------------------------------------------
*
- * We wish to prove that the system's computation of decay
- * will always fulfill the equation:
- * decay ** (5 * loadavg) ~= .1
+ * Now, let's prove that the value of decay stated above will always fulfill
+ * the equation:
+ * decay ** (5 * loadavg) ~= .1
*
* If we compute b as:
- * b = 2 * loadavg
+ * b = 2 * loadavg
* then
- * decay = b / (b + 1)
+ * decay = b / (b + 1)
*
* We now need to prove two things:
- * 1) Given factor ** (5 * loadavg) ~= .1, prove factor == b/(b+1)
- * 2) Given b/(b+1) ** power ~= .1, prove power == (5 * loadavg)
+ * 1) Given [factor ** (5 * loadavg) =~ .1], prove [factor == b/(b+1)].
+ * 2) Given [b/(b+1) ** power =~ .1], prove [power == (5 * loadavg)].
*
* Facts:
- * For x close to zero, exp(x) =~ 1 + x, since
- * exp(x) = 0! + x**1/1! + x**2/2! + ... .
- * therefore exp(-1/b) =~ 1 - (1/b) = (b-1)/b.
- * For x close to zero, ln(1+x) =~ x, since
- * ln(1+x) = x - x**2/2 + x**3/3 - ... -1 < x < 1
- * therefore ln(b/(b+1)) = ln(1 - 1/(b+1)) =~ -1/(b+1).
- * ln(.1) =~ -2.30
+ * * For x real: exp(x) = 0! + x**1/1! + x**2/2! + ...
+ * Therefore, for x close to zero, exp(x) =~ 1 + x.
+ * In turn, for b large enough, exp(-1/b) =~ 1 - (1/b) = (b-1)/b.
+ *
+ * * For b large enough, (b-1)/b =~ b/(b+1).
+ *
+ * * For x belonging to [-1;1[, ln(1-x) = - x - x**2/2 - x**3/3 - ...
+ * Therefore ln(b/(b+1)) = ln(1 - 1/(b+1)) =~ -1/(b+1).
+ *
+ * * ln(0.1) =~ -2.30
*
* Proof of (1):
- * Solve (factor)**(power) =~ .1 given power (5*loadav):
- * solving for factor,
- * ln(factor) =~ (-2.30/5*loadav), or
- * factor =~ exp(-1/((5/2.30)*loadav)) =~ exp(-1/(2*loadav)) =
- * exp(-1/b) =~ (b-1)/b =~ b/(b+1). QED
+ * factor ** (5 * loadavg) =~ 0.1
+ * => ln(factor) =~ -2.30 / (5 * loadavg)
+ * => factor =~ exp(-1 / ((5 / 2.30) * loadavg))
+ * =~ exp(-1 / (2 * loadavg))
+ * =~ exp(-1 / b)
+ * =~ (b - 1) / b
+ * =~ b / (b + 1)
+ * =~ (2 * loadavg) / ((2 * loadavg) + 1)
*
* Proof of (2):
- * Solve (factor)**(power) =~ .1 given factor == (b/(b+1)):
- * solving for power,
- * power*ln(b/(b+1)) =~ -2.30, or
- * power =~ 2.3 * (b + 1) = 4.6*loadav + 2.3 =~ 5*loadav. QED
- *
- * Actual power values for the implemented algorithm are as follows:
- * loadav: 1 2 3 4
- * power: 5.68 10.32 14.94 19.55
+ * (b / (b + 1)) ** power =~ .1
+ * => power * ln(b / (b + 1)) =~ -2.30
+ * => power * (-1 / (b + 1)) =~ -2.30
+ * => power =~ 2.30 * (b + 1)
+ * => power =~ 4.60 * loadavg + 2.30
+ * => power =~ 5 * loadavg
+ *
+ * Conclusion: decay = (2 * loadavg) / (2 * loadavg + 1)
*/
-/* calculations for digital decay to forget 90% of usage in 5*loadav sec */
-#define loadfactor(loadav) (2 * (loadav) / ncpu)
+/* See calculations above */
+#define loadfactor(loadavg) (2 * (loadavg) / ncpu)
static fixpt_t
decay_cpu(fixpt_t loadfac, fixpt_t estcpu)
@@ -250,22 +266,24 @@ decay_cpu(fixpt_t loadfac, fixpt_t estcp
if (__predict_true(loadfac <= FIXPT_MAX / ESTCPU_MAX)) {
return estcpu * loadfac / (loadfac + FSCALE);
}
-#endif /* !defined(_LP64) */
+#endif
return (uint64_t)estcpu * loadfac / (loadfac + FSCALE);
}
-/*
- * For all load averages >= 1 and max l_estcpu of (255 << ESTCPU_SHIFT),
- * sleeping for at least seven times the loadfactor will decay l_estcpu to
- * less than (1 << ESTCPU_SHIFT).
- *
- * note that our ESTCPU_MAX is actually much smaller than (255 << ESTCPU_SHIFT).
- */
static fixpt_t
decay_cpu_batch(fixpt_t loadfac, fixpt_t estcpu, unsigned int n)
{
+ /*
+ * For all load averages >= 1 and max l_estcpu of (255 << ESTCPU_SHIFT),
+ * if we slept for at least seven times the loadfactor, we will decay
+ * l_estcpu to less than (1 << ESTCPU_SHIFT), and therefore we can
+ * return zero directly.
+ *
+ * Note that our ESTCPU_MAX is actually much smaller than
+ * (255 << ESTCPU_SHIFT).
+ */
if ((n << FSHIFT) >= 7 * loadfac) {
return 0;
}
@@ -299,13 +317,13 @@ sched_pstats_hook(struct lwp *l, int bat
return;
}
}
- loadfac = 2 * (averunnable.ldavg[0]);
+ loadfac = 2 * (averunnable.ldavg[0]); /* XXX: should be loadfactor? */
l->l_estcpu = decay_cpu(loadfac, l->l_estcpu);
resetpriority(l);
}
/*
- * Recalculate the priority of a process after it has slept for a while.
+ * Recalculate the priority of an LWP after it has slept for a while.
*/
static void
updatepri(struct lwp *l)
@@ -383,10 +401,9 @@ resetpriority(struct lwp *l)
* is running (linearly), and decays away exponentially, at a rate which is
* proportionally slower when the system is busy. The basic principle is
* that the system will 90% forget that the process used a lot of CPU time
- * in 5 * loadav seconds. This causes the system to favor processes which
+ * in (5 * loadavg) seconds. This causes the system to favor processes which
* haven't run much recently, and to round-robin among other processes.
*/
-
void
sched_schedclock(struct lwp *l)
{
