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Intel Powerclamp Driver
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By:
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- Arjan van de Ven <arjan@linux.intel.com>
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- Jacob Pan <jacob.jun.pan@linux.intel.com>
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.. Contents:
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(*) Introduction
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- Goals and Objectives
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(*) Theory of Operation
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- Idle Injection
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- Calibration
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(*) Performance Analysis
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- Effectiveness and Limitations
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- Power vs Performance
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- Scalability
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- Calibration
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- Comparison with Alternative Techniques
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(*) Usage and Interfaces
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- Generic Thermal Layer (sysfs)
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- Kernel APIs (TBD)
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INTRODUCTION
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============
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Consider the situation where a system’s power consumption must be
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reduced at runtime, due to power budget, thermal constraint, or noise
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level, and where active cooling is not preferred. Software managed
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passive power reduction must be performed to prevent the hardware
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actions that are designed for catastrophic scenarios.
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Currently, P-states, T-states (clock modulation), and CPU offlining
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are used for CPU throttling.
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On Intel CPUs, C-states provide effective power reduction, but so far
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they’re only used opportunistically, based on workload. With the
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development of intel_powerclamp driver, the method of synchronizing
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idle injection across all online CPU threads was introduced. The goal
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is to achieve forced and controllable C-state residency.
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Test/Analysis has been made in the areas of power, performance,
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scalability, and user experience. In many cases, clear advantage is
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shown over taking the CPU offline or modulating the CPU clock.
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THEORY OF OPERATION
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===================
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Idle Injection
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--------------
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On modern Intel processors (Nehalem or later), package level C-state
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residency is available in MSRs, thus also available to the kernel.
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These MSRs are::
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#define MSR_PKG_C2_RESIDENCY 0x60D
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#define MSR_PKG_C3_RESIDENCY 0x3F8
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#define MSR_PKG_C6_RESIDENCY 0x3F9
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#define MSR_PKG_C7_RESIDENCY 0x3FA
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If the kernel can also inject idle time to the system, then a
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closed-loop control system can be established that manages package
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level C-state. The intel_powerclamp driver is conceived as such a
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control system, where the target set point is a user-selected idle
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ratio (based on power reduction), and the error is the difference
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between the actual package level C-state residency ratio and the target idle
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ratio.
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Injection is controlled by high priority kernel threads, spawned for
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each online CPU.
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These kernel threads, with SCHED_FIFO class, are created to perform
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clamping actions of controlled duty ratio and duration. Each per-CPU
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thread synchronizes its idle time and duration, based on the rounding
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of jiffies, so accumulated errors can be prevented to avoid a jittery
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effect. Threads are also bound to the CPU such that they cannot be
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migrated, unless the CPU is taken offline. In this case, threads
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belong to the offlined CPUs will be terminated immediately.
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Running as SCHED_FIFO and relatively high priority, also allows such
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scheme to work for both preemptable and non-preemptable kernels.
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Alignment of idle time around jiffies ensures scalability for HZ
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values. This effect can be better visualized using a Perf timechart.
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The following diagram shows the behavior of kernel thread
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kidle_inject/cpu. During idle injection, it runs monitor/mwait idle
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for a given "duration", then relinquishes the CPU to other tasks,
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until the next time interval.
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The NOHZ schedule tick is disabled during idle time, but interrupts
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are not masked. Tests show that the extra wakeups from scheduler tick
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have a dramatic impact on the effectiveness of the powerclamp driver
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on large scale systems (Westmere system with 80 processors).
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::
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CPU0
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____________ ____________
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kidle_inject/0 | sleep | mwait | sleep |
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_________| |________| |_______
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duration
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CPU1
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____________ ____________
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kidle_inject/1 | sleep | mwait | sleep |
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_________| |________| |_______
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^
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roundup(jiffies, interval)
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Only one CPU is allowed to collect statistics and update global
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control parameters. This CPU is referred to as the controlling CPU in
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this document. The controlling CPU is elected at runtime, with a
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policy that favors BSP, taking into account the possibility of a CPU
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hot-plug.
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In terms of dynamics of the idle control system, package level idle
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time is considered largely as a non-causal system where its behavior
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cannot be based on the past or current input. Therefore, the
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intel_powerclamp driver attempts to enforce the desired idle time
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instantly as given input (target idle ratio). After injection,
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powerclamp monitors the actual idle for a given time window and adjust
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the next injection accordingly to avoid over/under correction.
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When used in a causal control system, such as a temperature control,
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it is up to the user of this driver to implement algorithms where
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past samples and outputs are included in the feedback. For example, a
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PID-based thermal controller can use the powerclamp driver to
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maintain a desired target temperature, based on integral and
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derivative gains of the past samples.
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Calibration
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-----------
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During scalability testing, it is observed that synchronized actions
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among CPUs become challenging as the number of cores grows. This is
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also true for the ability of a system to enter package level C-states.
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To make sure the intel_powerclamp driver scales well, online
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calibration is implemented. The goals for doing such a calibration
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are:
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a) determine the effective range of idle injection ratio
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b) determine the amount of compensation needed at each target ratio
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Compensation to each target ratio consists of two parts:
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a) steady state error compensation
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This is to offset the error occurring when the system can
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enter idle without extra wakeups (such as external interrupts).
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b) dynamic error compensation
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When an excessive amount of wakeups occurs during idle, an
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additional idle ratio can be added to quiet interrupts, by
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slowing down CPU activities.
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A debugfs file is provided for the user to examine compensation
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progress and results, such as on a Westmere system::
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[jacob@nex01 ~]$ cat
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/sys/kernel/debug/intel_powerclamp/powerclamp_calib
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controlling cpu: 0
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pct confidence steady dynamic (compensation)
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0 0 0 0
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1 1 0 0
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2 1 1 0
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3 3 1 0
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4 3 1 0
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5 3 1 0
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6 3 1 0
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7 3 1 0
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8 3 1 0
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...
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30 3 2 0
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31 3 2 0
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32 3 1 0
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33 3 2 0
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34 3 1 0
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35 3 2 0
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36 3 1 0
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37 3 2 0
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38 3 1 0
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39 3 2 0
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40 3 3 0
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41 3 1 0
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42 3 2 0
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43 3 1 0
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44 3 1 0
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45 3 2 0
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46 3 3 0
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47 3 0 0
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48 3 2 0
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49 3 3 0
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Calibration occurs during runtime. No offline method is available.
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Steady state compensation is used only when confidence levels of all
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adjacent ratios have reached satisfactory level. A confidence level
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is accumulated based on clean data collected at runtime. Data
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collected during a period without extra interrupts is considered
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clean.
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To compensate for excessive amounts of wakeup during idle, additional
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idle time is injected when such a condition is detected. Currently,
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we have a simple algorithm to double the injection ratio. A possible
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enhancement might be to throttle the offending IRQ, such as delaying
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EOI for level triggered interrupts. But it is a challenge to be
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non-intrusive to the scheduler or the IRQ core code.
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CPU Online/Offline
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------------------
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Per-CPU kernel threads are started/stopped upon receiving
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notifications of CPU hotplug activities. The intel_powerclamp driver
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keeps track of clamping kernel threads, even after they are migrated
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to other CPUs, after a CPU offline event.
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Performance Analysis
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====================
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This section describes the general performance data collected on
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multiple systems, including Westmere (80P) and Ivy Bridge (4P, 8P).
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Effectiveness and Limitations
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-----------------------------
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The maximum range that idle injection is allowed is capped at 50
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percent. As mentioned earlier, since interrupts are allowed during
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forced idle time, excessive interrupts could result in less
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effectiveness. The extreme case would be doing a ping -f to generated
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flooded network interrupts without much CPU acknowledgement. In this
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case, little can be done from the idle injection threads. In most
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normal cases, such as scp a large file, applications can be throttled
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by the powerclamp driver, since slowing down the CPU also slows down
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network protocol processing, which in turn reduces interrupts.
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When control parameters change at runtime by the controlling CPU, it
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may take an additional period for the rest of the CPUs to catch up
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with the changes. During this time, idle injection is out of sync,
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thus not able to enter package C- states at the expected ratio. But
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this effect is minor, in that in most cases change to the target
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ratio is updated much less frequently than the idle injection
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frequency.
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Scalability
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-----------
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Tests also show a minor, but measurable, difference between the 4P/8P
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Ivy Bridge system and the 80P Westmere server under 50% idle ratio.
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More compensation is needed on Westmere for the same amount of
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target idle ratio. The compensation also increases as the idle ratio
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gets larger. The above reason constitutes the need for the
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calibration code.
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On the IVB 8P system, compared to an offline CPU, powerclamp can
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achieve up to 40% better performance per watt. (measured by a spin
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counter summed over per CPU counting threads spawned for all running
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CPUs).
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Usage and Interfaces
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====================
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The powerclamp driver is registered to the generic thermal layer as a
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cooling device. Currently, it’s not bound to any thermal zones::
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jacob@chromoly:/sys/class/thermal/cooling_device14$ grep . *
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cur_state:0
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max_state:50
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type:intel_powerclamp
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cur_state allows user to set the desired idle percentage. Writing 0 to
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cur_state will stop idle injection. Writing a value between 1 and
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max_state will start the idle injection. Reading cur_state returns the
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actual and current idle percentage. This may not be the same value
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set by the user in that current idle percentage depends on workload
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and includes natural idle. When idle injection is disabled, reading
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cur_state returns value -1 instead of 0 which is to avoid confusing
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100% busy state with the disabled state.
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Example usage:
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- To inject 25% idle time::
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$ sudo sh -c "echo 25 > /sys/class/thermal/cooling_device80/cur_state
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If the system is not busy and has more than 25% idle time already,
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then the powerclamp driver will not start idle injection. Using Top
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will not show idle injection kernel threads.
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If the system is busy (spin test below) and has less than 25% natural
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idle time, powerclamp kernel threads will do idle injection. Forced
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idle time is accounted as normal idle in that common code path is
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taken as the idle task.
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In this example, 24.1% idle is shown. This helps the system admin or
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user determine the cause of slowdown, when a powerclamp driver is in action::
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Tasks: 197 total, 1 running, 196 sleeping, 0 stopped, 0 zombie
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Cpu(s): 71.2%us, 4.7%sy, 0.0%ni, 24.1%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
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Mem: 3943228k total, 1689632k used, 2253596k free, 74960k buffers
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Swap: 4087804k total, 0k used, 4087804k free, 945336k cached
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PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
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3352 jacob 20 0 262m 644 428 S 286 0.0 0:17.16 spin
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3341 root -51 0 0 0 0 D 25 0.0 0:01.62 kidle_inject/0
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3344 root -51 0 0 0 0 D 25 0.0 0:01.60 kidle_inject/3
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3342 root -51 0 0 0 0 D 25 0.0 0:01.61 kidle_inject/1
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3343 root -51 0 0 0 0 D 25 0.0 0:01.60 kidle_inject/2
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2935 jacob 20 0 696m 125m 35m S 5 3.3 0:31.11 firefox
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1546 root 20 0 158m 20m 6640 S 3 0.5 0:26.97 Xorg
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2100 jacob 20 0 1223m 88m 30m S 3 2.3 0:23.68 compiz
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Tests have shown that by using the powerclamp driver as a cooling
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device, a PID based userspace thermal controller can manage to
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control CPU temperature effectively, when no other thermal influence
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is added. For example, a UltraBook user can compile the kernel under
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certain temperature (below most active trip points).
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