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Jacob Pand6d71ee2013-01-21 04:37:57 -08001 =======================
2 INTEL POWERCLAMP DRIVER
3 =======================
4By: Arjan van de Ven <arjan@linux.intel.com>
5 Jacob Pan <jacob.jun.pan@linux.intel.com>
6
7Contents:
8 (*) Introduction
9 - Goals and Objectives
10
11 (*) Theory of Operation
12 - Idle Injection
13 - Calibration
14
15 (*) Performance Analysis
16 - Effectiveness and Limitations
17 - Power vs Performance
18 - Scalability
19 - Calibration
20 - Comparison with Alternative Techniques
21
22 (*) Usage and Interfaces
23 - Generic Thermal Layer (sysfs)
24 - Kernel APIs (TBD)
25
26============
27INTRODUCTION
28============
29
30Consider the situation where a system’s power consumption must be
31reduced at runtime, due to power budget, thermal constraint, or noise
32level, and where active cooling is not preferred. Software managed
33passive power reduction must be performed to prevent the hardware
34actions that are designed for catastrophic scenarios.
35
36Currently, P-states, T-states (clock modulation), and CPU offlining
37are used for CPU throttling.
38
39On Intel CPUs, C-states provide effective power reduction, but so far
40they’re only used opportunistically, based on workload. With the
41development of intel_powerclamp driver, the method of synchronizing
42idle injection across all online CPU threads was introduced. The goal
43is to achieve forced and controllable C-state residency.
44
45Test/Analysis has been made in the areas of power, performance,
46scalability, and user experience. In many cases, clear advantage is
47shown over taking the CPU offline or modulating the CPU clock.
48
49
50===================
51THEORY OF OPERATION
52===================
53
54Idle Injection
55--------------
56
57On modern Intel processors (Nehalem or later), package level C-state
58residency is available in MSRs, thus also available to the kernel.
59
60These MSRs are:
61 #define MSR_PKG_C2_RESIDENCY 0x60D
62 #define MSR_PKG_C3_RESIDENCY 0x3F8
63 #define MSR_PKG_C6_RESIDENCY 0x3F9
64 #define MSR_PKG_C7_RESIDENCY 0x3FA
65
66If the kernel can also inject idle time to the system, then a
67closed-loop control system can be established that manages package
68level C-state. The intel_powerclamp driver is conceived as such a
69control system, where the target set point is a user-selected idle
70ratio (based on power reduction), and the error is the difference
71between the actual package level C-state residency ratio and the target idle
72ratio.
73
74Injection is controlled by high priority kernel threads, spawned for
75each online CPU.
76
77These kernel threads, with SCHED_FIFO class, are created to perform
78clamping actions of controlled duty ratio and duration. Each per-CPU
79thread synchronizes its idle time and duration, based on the rounding
80of jiffies, so accumulated errors can be prevented to avoid a jittery
81effect. Threads are also bound to the CPU such that they cannot be
82migrated, unless the CPU is taken offline. In this case, threads
83belong to the offlined CPUs will be terminated immediately.
84
85Running as SCHED_FIFO and relatively high priority, also allows such
86scheme to work for both preemptable and non-preemptable kernels.
87Alignment of idle time around jiffies ensures scalability for HZ
88values. This effect can be better visualized using a Perf timechart.
89The following diagram shows the behavior of kernel thread
90kidle_inject/cpu. During idle injection, it runs monitor/mwait idle
91for a given "duration", then relinquishes the CPU to other tasks,
92until the next time interval.
93
94The NOHZ schedule tick is disabled during idle time, but interrupts
95are not masked. Tests show that the extra wakeups from scheduler tick
96have a dramatic impact on the effectiveness of the powerclamp driver
97on large scale systems (Westmere system with 80 processors).
98
99CPU0
100 ____________ ____________
101kidle_inject/0 | sleep | mwait | sleep |
102 _________| |________| |_______
103 duration
104CPU1
105 ____________ ____________
106kidle_inject/1 | sleep | mwait | sleep |
107 _________| |________| |_______
108 ^
109 |
110 |
111 roundup(jiffies, interval)
112
113Only one CPU is allowed to collect statistics and update global
114control parameters. This CPU is referred to as the controlling CPU in
115this document. The controlling CPU is elected at runtime, with a
116policy that favors BSP, taking into account the possibility of a CPU
117hot-plug.
118
119In terms of dynamics of the idle control system, package level idle
120time is considered largely as a non-causal system where its behavior
121cannot be based on the past or current input. Therefore, the
122intel_powerclamp driver attempts to enforce the desired idle time
123instantly as given input (target idle ratio). After injection,
124powerclamp moniors the actual idle for a given time window and adjust
125the next injection accordingly to avoid over/under correction.
126
127When used in a causal control system, such as a temperature control,
128it is up to the user of this driver to implement algorithms where
129past samples and outputs are included in the feedback. For example, a
130PID-based thermal controller can use the powerclamp driver to
131maintain a desired target temperature, based on integral and
132derivative gains of the past samples.
133
134
135
136Calibration
137-----------
138During scalability testing, it is observed that synchronized actions
139among CPUs become challenging as the number of cores grows. This is
140also true for the ability of a system to enter package level C-states.
141
142To make sure the intel_powerclamp driver scales well, online
143calibration is implemented. The goals for doing such a calibration
144are:
145
146a) determine the effective range of idle injection ratio
147b) determine the amount of compensation needed at each target ratio
148
149Compensation to each target ratio consists of two parts:
150
151 a) steady state error compensation
152 This is to offset the error occurring when the system can
153 enter idle without extra wakeups (such as external interrupts).
154
155 b) dynamic error compensation
156 When an excessive amount of wakeups occurs during idle, an
157 additional idle ratio can be added to quiet interrupts, by
158 slowing down CPU activities.
159
160A debugfs file is provided for the user to examine compensation
161progress and results, such as on a Westmere system.
162[jacob@nex01 ~]$ cat
163/sys/kernel/debug/intel_powerclamp/powerclamp_calib
164controlling cpu: 0
165pct confidence steady dynamic (compensation)
1660 0 0 0
1671 1 0 0
1682 1 1 0
1693 3 1 0
1704 3 1 0
1715 3 1 0
1726 3 1 0
1737 3 1 0
1748 3 1 0
175...
17630 3 2 0
17731 3 2 0
17832 3 1 0
17933 3 2 0
18034 3 1 0
18135 3 2 0
18236 3 1 0
18337 3 2 0
18438 3 1 0
18539 3 2 0
18640 3 3 0
18741 3 1 0
18842 3 2 0
18943 3 1 0
19044 3 1 0
19145 3 2 0
19246 3 3 0
19347 3 0 0
19448 3 2 0
19549 3 3 0
196
197Calibration occurs during runtime. No offline method is available.
198Steady state compensation is used only when confidence levels of all
199adjacent ratios have reached satisfactory level. A confidence level
200is accumulated based on clean data collected at runtime. Data
201collected during a period without extra interrupts is considered
202clean.
203
204To compensate for excessive amounts of wakeup during idle, additional
205idle time is injected when such a condition is detected. Currently,
206we have a simple algorithm to double the injection ratio. A possible
207enhancement might be to throttle the offending IRQ, such as delaying
208EOI for level triggered interrupts. But it is a challenge to be
209non-intrusive to the scheduler or the IRQ core code.
210
211
212CPU Online/Offline
213------------------
214Per-CPU kernel threads are started/stopped upon receiving
215notifications of CPU hotplug activities. The intel_powerclamp driver
216keeps track of clamping kernel threads, even after they are migrated
217to other CPUs, after a CPU offline event.
218
219
220=====================
221Performance Analysis
222=====================
223This section describes the general performance data collected on
224multiple systems, including Westmere (80P) and Ivy Bridge (4P, 8P).
225
226Effectiveness and Limitations
227-----------------------------
228The maximum range that idle injection is allowed is capped at 50
229percent. As mentioned earlier, since interrupts are allowed during
230forced idle time, excessive interrupts could result in less
231effectiveness. The extreme case would be doing a ping -f to generated
232flooded network interrupts without much CPU acknowledgement. In this
233case, little can be done from the idle injection threads. In most
234normal cases, such as scp a large file, applications can be throttled
235by the powerclamp driver, since slowing down the CPU also slows down
236network protocol processing, which in turn reduces interrupts.
237
238When control parameters change at runtime by the controlling CPU, it
239may take an additional period for the rest of the CPUs to catch up
240with the changes. During this time, idle injection is out of sync,
241thus not able to enter package C- states at the expected ratio. But
242this effect is minor, in that in most cases change to the target
243ratio is updated much less frequently than the idle injection
244frequency.
245
246Scalability
247-----------
248Tests also show a minor, but measurable, difference between the 4P/8P
249Ivy Bridge system and the 80P Westmere server under 50% idle ratio.
250More compensation is needed on Westmere for the same amount of
251target idle ratio. The compensation also increases as the idle ratio
252gets larger. The above reason constitutes the need for the
253calibration code.
254
255On the IVB 8P system, compared to an offline CPU, powerclamp can
256achieve up to 40% better performance per watt. (measured by a spin
257counter summed over per CPU counting threads spawned for all running
258CPUs).
259
260====================
261Usage and Interfaces
262====================
263The powerclamp driver is registered to the generic thermal layer as a
264cooling device. Currently, it’s not bound to any thermal zones.
265
266jacob@chromoly:/sys/class/thermal/cooling_device14$ grep . *
267cur_state:0
268max_state:50
269type:intel_powerclamp
270
271Example usage:
272- To inject 25% idle time
273$ sudo sh -c "echo 25 > /sys/class/thermal/cooling_device80/cur_state
274"
275
276If the system is not busy and has more than 25% idle time already,
277then the powerclamp driver will not start idle injection. Using Top
278will not show idle injection kernel threads.
279
280If the system is busy (spin test below) and has less than 25% natural
281idle time, powerclamp kernel threads will do idle injection, which
282appear running to the scheduler. But the overall system idle is still
283reflected. In this example, 24.1% idle is shown. This helps the
284system admin or user determine the cause of slowdown, when a
285powerclamp driver is in action.
286
287
288Tasks: 197 total, 1 running, 196 sleeping, 0 stopped, 0 zombie
289Cpu(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
290Mem: 3943228k total, 1689632k used, 2253596k free, 74960k buffers
291Swap: 4087804k total, 0k used, 4087804k free, 945336k cached
292
293 PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
294 3352 jacob 20 0 262m 644 428 S 286 0.0 0:17.16 spin
295 3341 root -51 0 0 0 0 D 25 0.0 0:01.62 kidle_inject/0
296 3344 root -51 0 0 0 0 D 25 0.0 0:01.60 kidle_inject/3
297 3342 root -51 0 0 0 0 D 25 0.0 0:01.61 kidle_inject/1
298 3343 root -51 0 0 0 0 D 25 0.0 0:01.60 kidle_inject/2
299 2935 jacob 20 0 696m 125m 35m S 5 3.3 0:31.11 firefox
300 1546 root 20 0 158m 20m 6640 S 3 0.5 0:26.97 Xorg
301 2100 jacob 20 0 1223m 88m 30m S 3 2.3 0:23.68 compiz
302
303Tests have shown that by using the powerclamp driver as a cooling
304device, a PID based userspace thermal controller can manage to
305control CPU temperature effectively, when no other thermal influence
306is added. For example, a UltraBook user can compile the kernel under
307certain temperature (below most active trip points).