[PATCH v2] cpuidle: Fix the menu governor to boost IO performance

* [PATCH v2] cpuidle: Fix the menu governor to boost IO performance
@ 2009-09-15  3:42 Arjan van de Ven
  2009-09-15  3:54 ` Andrew Morton
                   ` (3 more replies)
  0 siblings, 4 replies; 14+ messages in thread
From: Arjan van de Ven @ 2009-09-15  3:42 UTC (permalink / raw)
  To: linux-kernel
  Cc: Andrew Morton, Arjan van de Ven, lenb, mingo, yanmin_zhang,
	jens.axboe, Ivan Kokshaysky

Reworked patch based on Andrew's review feedback and spelling fixes.
Rather than adding a new governor temporarily, this just puts the fixes
into the existing menu governor.

Andrew: this replaces cpuidle-a-new-variant-of-the-menu-governor-to-boost-io-performance.patch


I don't have a power meter in my hotel room so I've only been able to verify
that the patch functions as before with the timechart tool.
(no major changes were done though, only review feedback)


From: Arjan van de Ven <arjan@linux.intel.com>
Subject: cpuidle: Fix the menu governor to boost IO performance

Fix the menu idle governor which balances power savings, energy efficiency
and performance impact.

The reason for a reworked governor is that there have been serious
performance issues reported with the existing code on Nehalem server
systems.

To show this I'm sure Andrew wants to see benchmark results:
(benchmark is "fio", "no cstates" is using "idle=poll")

		no cstates	current linux	new algorithm
1 disk		107 Mb/s	85 Mb/s		105 Mb/s
2 disks		215 Mb/s	123 Mb/s	209 Mb/s
12 disks	590 Mb/s	320 Mb/s	585 Mb/s

In various power benchmark measurements, no degredation was found by our
measurement&diagnostics team.  Obviously a small percentage more power 
was used in the "fio" benchmark, due to the much higher performance.

While it would be a novel idea to describe the new algorithm in this
commit message, I cheaped out and described it in comments in the code
instead.

[changes since first post: spelling fixes from akpm, review feedback,
folded menu-tng into menu.c]

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 drivers/cpuidle/governors/menu.c |  251 ++++++++++++++++++++++++++++++++------
 include/linux/sched.h            |    4 +
 kernel/sched.c                   |   15 +++
 3 files changed, 231 insertions(+), 39 deletions(-)

diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index f1df59f..9f3d775 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -2,8 +2,12 @@
  * menu.c - the menu idle governor
  *
  * Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
+ * Copyright (C) 2009 Intel Corporation
+ * Author:
+ *        Arjan van de Ven <arjan@linux.intel.com>
  *
- * This code is licenced under the GPL.
+ * This code is licenced under the GPL version 2 as described
+ * in the COPYING file that acompanies the Linux Kernel.
  */
 
 #include <linux/kernel.h>
@@ -13,20 +17,153 @@
 #include <linux/ktime.h>
 #include <linux/hrtimer.h>
 #include <linux/tick.h>
+#include <linux/sched.h>
 
-#define BREAK_FUZZ	4	/* 4 us */
-#define PRED_HISTORY_PCT	50
+#define BUCKETS 12
+#define RESOLUTION 1024
+#define DECAY 4
+#define MAX_INTERESTING 50000
+
+/*
+ * Concepts and ideas behind the menu governor
+ *
+ * For the menu governor, there are 3 decision factors for picking a C
+ * state:
+ * 1) Energy break even point
+ * 2) Performance impact
+ * 3) Latency tolerance (from pmqos infrastructure)
+ * These these three factors are treated independently.
+ *
+ * Energy break even point
+ * -----------------------
+ * C state entry and exit have an energy cost, and a certain amount of time in
+ * the  C state is required to actually break even on this cost. CPUIDLE
+ * provides us this duration in the "target_residency" field. So all that we
+ * need is a good prediction of how long we'll be idle. Like the traditional
+ * menu governor, we start with the actual known "next timer event" time.
+ *
+ * Since there are other source of wakeups (interrupts for example) than
+ * the next timer event, this estimation is rather optimistic. To get a
+ * more realistic estimate, a correction factor is applied to the estimate,
+ * that is based on historic behavior. For example, if in the past the actual
+ * duration always was 50% of the next timer tick, the correction factor will
+ * be 0.5.
+ *
+ * menu uses a running average for this correction factor, however it uses a
+ * set of factors, not just a single factor. This stems from the realization
+ * that the ratio is dependent on the order of magnitude of the expected
+ * duration; if we expect 500 milliseconds of idle time the likelihood of
+ * getting an interrupt very early is much higher than if we expect 50 micro
+ * seconds of idle time. A second independent factor that has big impact on
+ * the actual factor is if there is (disk) IO outstanding or not.
+ * (as a special twist, we consider every sleep longer than 50 milliseconds
+ * as perfect; there are no power gains for sleeping longer than this)
+ *
+ * For these two reasons we keep an array of 12 independent factors, that gets
+ * indexed based on the magnitude of the expected duration as well as the
+ * "is IO outstanding" property.
+ *
+ * Limiting Performance Impact
+ * ---------------------------
+ * C states, especially those with large exit latencies, can have a real
+ * noticable impact on workloads, which is not acceptable for most sysadmins,
+ * and in addition, less performance has a power price of its own.
+ *
+ * As a general rule of thumb, menu assumes that the following heuristic
+ * holds:
+ *     The busier the system, the less impact of C states is acceptable
+ *
+ * This rule-of-thumb is implemented using a performance-multiplier:
+ * If the exit latency times the performance multiplier is longer than
+ * the predicted duration, the C state is not considered a candidate
+ * for selection due to a too high performance impact. So the higher
+ * this multiplier is, the longer we need to be idle to pick a deep C
+ * state, and thus the less likely a busy CPU will hit such a deep
+ * C state.
+ *
+ * Two factors are used in determing this multiplier:
+ * a value of 10 is added for each point of "per cpu load average" we have.
+ * a value of 5 points is added for each process that is waiting for
+ * IO on this CPU.
+ * (these values are experimentally determined)
+ *
+ * The load average factor gives a longer term (few seconds) input to the
+ * decision, while the iowait value gives a cpu local instantanious input.
+ * The iowait factor may look low, but realize that this is also already
+ * represented in the system load average.
+ *
+ */
 
 struct menu_device {
 	int		last_state_idx;
 
 	unsigned int	expected_us;
-	unsigned int	predicted_us;
-	unsigned int    current_predicted_us;
-	unsigned int	last_measured_us;
-	unsigned int	elapsed_us;
+	u64		predicted_us;
+	unsigned int	measured_us;
+	unsigned int	exit_us;
+	unsigned int	bucket;
+	u64		correction_factor[BUCKETS];
 };
 
+
+#define LOAD_INT(x) ((x) >> FSHIFT)
+#define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
+
+static int get_loadavg(void)
+{
+	unsigned long this = this_cpu_load();
+
+
+	return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10;
+}
+
+static inline int which_bucket(unsigned int duration)
+{
+	int bucket = 0;
+
+	/*
+	 * We keep two groups of stats; one with no
+	 * IO pending, one without.
+	 * This allows us to calculate
+	 * E(duration)|iowait
+	 */
+	if (nr_iowait_cpu())
+		bucket = BUCKETS/2;
+
+	if (duration < 10)
+		return bucket;
+	if (duration < 100)
+		return bucket + 1;
+	if (duration < 1000)
+		return bucket + 2;
+	if (duration < 10000)
+		return bucket + 3;
+	if (duration < 100000)
+		return bucket + 4;
+	return bucket + 5;
+}
+
+/*
+ * Return a multiplier for the exit latency that is intended
+ * to take performance requirements into account.
+ * The more performance critical we estimate the system
+ * to be, the higher this multiplier, and thus the higher
+ * the barrier to go to an expensive C state.
+ */
+static inline int performance_multiplier(void)
+{
+	int mult = 1;
+
+	/* for higher loadavg, we are more reluctant */
+
+	mult += 2 * get_loadavg();
+
+	/* for IO wait tasks (per cpu!) we add 5x each */
+	mult += 10 * nr_iowait_cpu();
+
+	return mult;
+}
+
 static DEFINE_PER_CPU(struct menu_device, menu_devices);
 
 /**
@@ -38,37 +175,59 @@ static int menu_select(struct cpuidle_device *dev)
 	struct menu_device *data = &__get_cpu_var(menu_devices);
 	int latency_req = pm_qos_requirement(PM_QOS_CPU_DMA_LATENCY);
 	int i;
+	int multiplier;
+
+	data->last_state_idx = 0;
+	data->exit_us = 0;
 
 	/* Special case when user has set very strict latency requirement */
-	if (unlikely(latency_req == 0)) {
-		data->last_state_idx = 0;
+	if (unlikely(latency_req == 0))
 		return 0;
-	}
 
-	/* determine the expected residency time */
+	/* determine the expected residency time, round up */
 	data->expected_us =
-		(u32) ktime_to_ns(tick_nohz_get_sleep_length()) / 1000;
+	    DIV_ROUND_UP((u32)ktime_to_ns(tick_nohz_get_sleep_length()), 1000);
+
+
+	data->bucket = which_bucket(data->expected_us);
+
+	multiplier = performance_multiplier();
+
+	/*
+	 * if the correction factor is 0 (eg first time init or cpu hotplug
+	 * etc), we actually want to start out with a unity factor.
+	 */
+	if (data->correction_factor[data->bucket] == 0)
+		data->correction_factor[data->bucket] = RESOLUTION * DECAY;
+
+	/* Make sure to round up for half microseconds */
+	data->predicted_us = DIV_ROUND_CLOSEST(
+		data->expected_us * data->correction_factor[data->bucket],
+		RESOLUTION * DECAY);
+
+	/*
+	 * We want to default to C1 (hlt), not to busy polling
+	 * unless the timer is happening really really soon.
+	 */
+	if (data->expected_us > 5)
+		data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
 
-	/* Recalculate predicted_us based on prediction_history_pct */
-	data->predicted_us *= PRED_HISTORY_PCT;
-	data->predicted_us += (100 - PRED_HISTORY_PCT) *
-				data->current_predicted_us;
-	data->predicted_us /= 100;
 
 	/* find the deepest idle state that satisfies our constraints */
-	for (i = CPUIDLE_DRIVER_STATE_START + 1; i < dev->state_count; i++) {
+	for (i = CPUIDLE_DRIVER_STATE_START; i < dev->state_count; i++) {
 		struct cpuidle_state *s = &dev->states[i];
 
-		if (s->target_residency > data->expected_us)
-			break;
 		if (s->target_residency > data->predicted_us)
 			break;
 		if (s->exit_latency > latency_req)
 			break;
+		if (s->exit_latency * multiplier > data->predicted_us)
+			break;
+		data->exit_us = s->exit_latency;
+		data->last_state_idx = i;
 	}
 
-	data->last_state_idx = i - 1;
-	return i - 1;
+	return data->last_state_idx;
 }
 
 /**
@@ -85,35 +244,49 @@ static void menu_reflect(struct cpuidle_device *dev)
 	unsigned int last_idle_us = cpuidle_get_last_residency(dev);
 	struct cpuidle_state *target = &dev->states[last_idx];
 	unsigned int measured_us;
+	u64 new_factor;
 
 	/*
 	 * Ugh, this idle state doesn't support residency measurements, so we
 	 * are basically lost in the dark.  As a compromise, assume we slept
-	 * for one full standard timer tick.  However, be aware that this
-	 * could potentially result in a suboptimal state transition.
+	 * for the whole expected time.
 	 */
 	if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID)))
-		last_idle_us = USEC_PER_SEC / HZ;
+		last_idle_us = data->expected_us;
+
+
+	measured_us = last_idle_us;
 
 	/*
-	 * measured_us and elapsed_us are the cumulative idle time, since the
-	 * last time we were woken out of idle by an interrupt.
+	 * We correct for the exit latency; we are assuming here that the
+	 * exit latency happens after the event that we're interested in.
 	 */
-	if (data->elapsed_us <= data->elapsed_us + last_idle_us)
-		measured_us = data->elapsed_us + last_idle_us;
+	if (measured_us > data->exit_us)
+		measured_us -= data->exit_us;
+
+
+	/* update our correction ratio */
+
+	new_factor = data->correction_factor[data->bucket]
+			* (DECAY - 1) / DECAY;
+
+	if (data->expected_us > 0 && data->measured_us < MAX_INTERESTING)
+		new_factor += RESOLUTION * measured_us / data->expected_us;
 	else
-		measured_us = -1;
+		/*
+		 * we were idle so long that we count it as a perfect
+		 * prediction
+		 */
+		new_factor += RESOLUTION;
 
-	/* Predict time until next break event */
-	data->current_predicted_us = max(measured_us, data->last_measured_us);
+	/*
+	 * We don't want 0 as factor; we always want at least
+	 * a tiny bit of estimated time.
+	 */
+	if (new_factor == 0)
+		new_factor = 1;
 
-	if (last_idle_us + BREAK_FUZZ <
-	    data->expected_us - target->exit_latency) {
-		data->last_measured_us = measured_us;
-		data->elapsed_us = 0;
-	} else {
-		data->elapsed_us = measured_us;
-	}
+	data->correction_factor[data->bucket] = new_factor;
 }
 
 /**
diff --git a/include/linux/sched.h b/include/linux/sched.h
index c0d9944..6b29155 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -140,6 +140,10 @@ extern int nr_processes(void);
 extern unsigned long nr_running(void);
 extern unsigned long nr_uninterruptible(void);
 extern unsigned long nr_iowait(void);
+extern unsigned long nr_iowait_cpu(void);
+extern unsigned long this_cpu_load(void);
+
+
 extern void calc_global_load(void);
 extern u64 cpu_nr_migrations(int cpu);
 
diff --git a/kernel/sched.c b/kernel/sched.c
index c512a02..1e2f1d0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3064,6 +3064,21 @@ unsigned long nr_iowait(void)
 	return sum;
 }
 
+unsigned long nr_iowait_cpu(void)
+{
+	int this_cpu = smp_processor_id();
+	struct rq *this_rq = cpu_rq(this_cpu);
+	return atomic_read(&this_rq->nr_iowait);
+}
+
+unsigned long this_cpu_load(void)
+{
+	int this_cpu = smp_processor_id();
+	struct rq *this_rq = cpu_rq(this_cpu);
+	return this_rq->cpu_load[0];
+}
+
+
 /* Variables and functions for calc_load */
 static atomic_long_t calc_load_tasks;
 static unsigned long calc_load_update;
-- 
1.6.0.6


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org

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