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> Acked-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Arjan van de Ven
Linus Torvalds
parent
45d80eea87
commit
69d25870f2
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@ -2,8 +2,12 @@
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* menu.c - the menu idle governor
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*
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* Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
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* Copyright (C) 2009 Intel Corporation
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* Author:
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* Arjan van de Ven <arjan@linux.intel.com>
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*
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* This code is licenced under the GPL.
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* This code is licenced under the GPL version 2 as described
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* in the COPYING file that acompanies the Linux Kernel.
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*/
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#include <linux/kernel.h>
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@ -13,20 +17,153 @@
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#include <linux/ktime.h>
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#include <linux/hrtimer.h>
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#include <linux/tick.h>
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#include <linux/sched.h>
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#define BREAK_FUZZ 4 /* 4 us */
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#define PRED_HISTORY_PCT 50
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#define BUCKETS 12
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#define RESOLUTION 1024
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#define DECAY 4
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#define MAX_INTERESTING 50000
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/*
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* Concepts and ideas behind the menu governor
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*
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* For the menu governor, there are 3 decision factors for picking a C
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* state:
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* 1) Energy break even point
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* 2) Performance impact
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* 3) Latency tolerance (from pmqos infrastructure)
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* These these three factors are treated independently.
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*
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* Energy break even point
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* -----------------------
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* C state entry and exit have an energy cost, and a certain amount of time in
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* the C state is required to actually break even on this cost. CPUIDLE
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* provides us this duration in the "target_residency" field. So all that we
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* need is a good prediction of how long we'll be idle. Like the traditional
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* menu governor, we start with the actual known "next timer event" time.
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*
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* Since there are other source of wakeups (interrupts for example) than
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* the next timer event, this estimation is rather optimistic. To get a
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* more realistic estimate, a correction factor is applied to the estimate,
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* that is based on historic behavior. For example, if in the past the actual
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* duration always was 50% of the next timer tick, the correction factor will
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* be 0.5.
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*
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* menu uses a running average for this correction factor, however it uses a
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* set of factors, not just a single factor. This stems from the realization
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* that the ratio is dependent on the order of magnitude of the expected
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* duration; if we expect 500 milliseconds of idle time the likelihood of
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* getting an interrupt very early is much higher than if we expect 50 micro
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* seconds of idle time. A second independent factor that has big impact on
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* the actual factor is if there is (disk) IO outstanding or not.
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* (as a special twist, we consider every sleep longer than 50 milliseconds
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* as perfect; there are no power gains for sleeping longer than this)
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*
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* For these two reasons we keep an array of 12 independent factors, that gets
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* indexed based on the magnitude of the expected duration as well as the
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* "is IO outstanding" property.
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*
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* Limiting Performance Impact
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* ---------------------------
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* C states, especially those with large exit latencies, can have a real
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* noticable impact on workloads, which is not acceptable for most sysadmins,
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* and in addition, less performance has a power price of its own.
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*
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* As a general rule of thumb, menu assumes that the following heuristic
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* holds:
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* The busier the system, the less impact of C states is acceptable
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*
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* This rule-of-thumb is implemented using a performance-multiplier:
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* If the exit latency times the performance multiplier is longer than
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* the predicted duration, the C state is not considered a candidate
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* for selection due to a too high performance impact. So the higher
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* this multiplier is, the longer we need to be idle to pick a deep C
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* state, and thus the less likely a busy CPU will hit such a deep
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* C state.
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*
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* Two factors are used in determing this multiplier:
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* a value of 10 is added for each point of "per cpu load average" we have.
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* a value of 5 points is added for each process that is waiting for
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* IO on this CPU.
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* (these values are experimentally determined)
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*
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* The load average factor gives a longer term (few seconds) input to the
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* decision, while the iowait value gives a cpu local instantanious input.
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* The iowait factor may look low, but realize that this is also already
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* represented in the system load average.
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*
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*/
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struct menu_device {
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int last_state_idx;
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unsigned int expected_us;
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unsigned int predicted_us;
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unsigned int current_predicted_us;
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unsigned int last_measured_us;
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unsigned int elapsed_us;
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u64 predicted_us;
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unsigned int measured_us;
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unsigned int exit_us;
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unsigned int bucket;
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u64 correction_factor[BUCKETS];
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};
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#define LOAD_INT(x) ((x) >> FSHIFT)
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#define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
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static int get_loadavg(void)
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{
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unsigned long this = this_cpu_load();
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return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10;
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}
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static inline int which_bucket(unsigned int duration)
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{
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int bucket = 0;
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/*
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* We keep two groups of stats; one with no
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* IO pending, one without.
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* This allows us to calculate
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* E(duration)|iowait
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*/
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if (nr_iowait_cpu())
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bucket = BUCKETS/2;
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if (duration < 10)
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return bucket;
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if (duration < 100)
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return bucket + 1;
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if (duration < 1000)
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return bucket + 2;
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if (duration < 10000)
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return bucket + 3;
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if (duration < 100000)
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return bucket + 4;
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return bucket + 5;
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}
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/*
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* Return a multiplier for the exit latency that is intended
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* to take performance requirements into account.
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* The more performance critical we estimate the system
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* to be, the higher this multiplier, and thus the higher
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* the barrier to go to an expensive C state.
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*/
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static inline int performance_multiplier(void)
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{
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int mult = 1;
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/* for higher loadavg, we are more reluctant */
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mult += 2 * get_loadavg();
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/* for IO wait tasks (per cpu!) we add 5x each */
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mult += 10 * nr_iowait_cpu();
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return mult;
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}
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static DEFINE_PER_CPU(struct menu_device, menu_devices);
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/**
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@ -38,37 +175,59 @@ static int menu_select(struct cpuidle_device *dev)
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struct menu_device *data = &__get_cpu_var(menu_devices);
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int latency_req = pm_qos_requirement(PM_QOS_CPU_DMA_LATENCY);
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int i;
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int multiplier;
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data->last_state_idx = 0;
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data->exit_us = 0;
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/* Special case when user has set very strict latency requirement */
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if (unlikely(latency_req == 0)) {
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data->last_state_idx = 0;
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if (unlikely(latency_req == 0))
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return 0;
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}
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/* determine the expected residency time */
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/* determine the expected residency time, round up */
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data->expected_us =
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(u32) ktime_to_ns(tick_nohz_get_sleep_length()) / 1000;
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DIV_ROUND_UP((u32)ktime_to_ns(tick_nohz_get_sleep_length()), 1000);
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data->bucket = which_bucket(data->expected_us);
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multiplier = performance_multiplier();
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/*
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* if the correction factor is 0 (eg first time init or cpu hotplug
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* etc), we actually want to start out with a unity factor.
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*/
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if (data->correction_factor[data->bucket] == 0)
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data->correction_factor[data->bucket] = RESOLUTION * DECAY;
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/* Make sure to round up for half microseconds */
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data->predicted_us = DIV_ROUND_CLOSEST(
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data->expected_us * data->correction_factor[data->bucket],
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RESOLUTION * DECAY);
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/*
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* We want to default to C1 (hlt), not to busy polling
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* unless the timer is happening really really soon.
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*/
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if (data->expected_us > 5)
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data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
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/* Recalculate predicted_us based on prediction_history_pct */
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data->predicted_us *= PRED_HISTORY_PCT;
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data->predicted_us += (100 - PRED_HISTORY_PCT) *
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data->current_predicted_us;
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data->predicted_us /= 100;
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/* find the deepest idle state that satisfies our constraints */
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for (i = CPUIDLE_DRIVER_STATE_START + 1; i < dev->state_count; i++) {
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for (i = CPUIDLE_DRIVER_STATE_START; i < dev->state_count; i++) {
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struct cpuidle_state *s = &dev->states[i];
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if (s->target_residency > data->expected_us)
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break;
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if (s->target_residency > data->predicted_us)
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break;
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if (s->exit_latency > latency_req)
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break;
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if (s->exit_latency * multiplier > data->predicted_us)
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break;
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data->exit_us = s->exit_latency;
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data->last_state_idx = i;
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}
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data->last_state_idx = i - 1;
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return i - 1;
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return data->last_state_idx;
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}
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/**
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@ -85,35 +244,49 @@ static void menu_reflect(struct cpuidle_device *dev)
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unsigned int last_idle_us = cpuidle_get_last_residency(dev);
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struct cpuidle_state *target = &dev->states[last_idx];
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unsigned int measured_us;
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u64 new_factor;
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/*
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* Ugh, this idle state doesn't support residency measurements, so we
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* are basically lost in the dark. As a compromise, assume we slept
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* for one full standard timer tick. However, be aware that this
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* could potentially result in a suboptimal state transition.
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* for the whole expected time.
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*/
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if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID)))
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last_idle_us = USEC_PER_SEC / HZ;
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last_idle_us = data->expected_us;
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measured_us = last_idle_us;
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/*
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* measured_us and elapsed_us are the cumulative idle time, since the
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* last time we were woken out of idle by an interrupt.
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* We correct for the exit latency; we are assuming here that the
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* exit latency happens after the event that we're interested in.
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*/
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if (data->elapsed_us <= data->elapsed_us + last_idle_us)
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measured_us = data->elapsed_us + last_idle_us;
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if (measured_us > data->exit_us)
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measured_us -= data->exit_us;
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/* update our correction ratio */
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new_factor = data->correction_factor[data->bucket]
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* (DECAY - 1) / DECAY;
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if (data->expected_us > 0 && data->measured_us < MAX_INTERESTING)
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new_factor += RESOLUTION * measured_us / data->expected_us;
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else
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measured_us = -1;
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/*
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* we were idle so long that we count it as a perfect
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* prediction
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*/
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new_factor += RESOLUTION;
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/* Predict time until next break event */
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data->current_predicted_us = max(measured_us, data->last_measured_us);
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/*
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* We don't want 0 as factor; we always want at least
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* a tiny bit of estimated time.
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*/
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if (new_factor == 0)
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new_factor = 1;
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if (last_idle_us + BREAK_FUZZ <
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data->expected_us - target->exit_latency) {
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data->last_measured_us = measured_us;
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data->elapsed_us = 0;
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} else {
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data->elapsed_us = measured_us;
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}
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data->correction_factor[data->bucket] = new_factor;
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}
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/**
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@ -140,6 +140,10 @@ extern int nr_processes(void);
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extern unsigned long nr_running(void);
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extern unsigned long nr_uninterruptible(void);
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extern unsigned long nr_iowait(void);
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extern unsigned long nr_iowait_cpu(void);
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extern unsigned long this_cpu_load(void);
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extern void calc_global_load(void);
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extern u64 cpu_nr_migrations(int cpu);
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@ -2904,6 +2904,19 @@ unsigned long nr_iowait(void)
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return sum;
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}
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unsigned long nr_iowait_cpu(void)
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{
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struct rq *this = this_rq();
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return atomic_read(&this->nr_iowait);
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}
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unsigned long this_cpu_load(void)
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{
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struct rq *this = this_rq();
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return this->cpu_load[0];
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}
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/* Variables and functions for calc_load */
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static atomic_long_t calc_load_tasks;
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static unsigned long calc_load_update;
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