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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1			NO_HZ: Reducing Scheduling-Clock Ticks
2	
3	
4	This document describes Kconfig options and boot parameters that can
5	reduce the number of scheduling-clock interrupts, thereby improving energy
6	efficiency and reducing OS jitter.  Reducing OS jitter is important for
7	some types of computationally intensive high-performance computing (HPC)
8	applications and for real-time applications.
9	
10	There are three main ways of managing scheduling-clock interrupts
11	(also known as "scheduling-clock ticks" or simply "ticks"):
12	
13	1.	Never omit scheduling-clock ticks (CONFIG_HZ_PERIODIC=y or
14		CONFIG_NO_HZ=n for older kernels).  You normally will -not-
15		want to choose this option.
16	
17	2.	Omit scheduling-clock ticks on idle CPUs (CONFIG_NO_HZ_IDLE=y or
18		CONFIG_NO_HZ=y for older kernels).  This is the most common
19		approach, and should be the default.
20	
21	3.	Omit scheduling-clock ticks on CPUs that are either idle or that
22		have only one runnable task (CONFIG_NO_HZ_FULL=y).  Unless you
23		are running realtime applications or certain types of HPC
24		workloads, you will normally -not- want this option.
25	
26	These three cases are described in the following three sections, followed
27	by a third section on RCU-specific considerations, a fourth section
28	discussing testing, and a fifth and final section listing known issues.
29	
30	
31	NEVER OMIT SCHEDULING-CLOCK TICKS
32	
33	Very old versions of Linux from the 1990s and the very early 2000s
34	are incapable of omitting scheduling-clock ticks.  It turns out that
35	there are some situations where this old-school approach is still the
36	right approach, for example, in heavy workloads with lots of tasks
37	that use short bursts of CPU, where there are very frequent idle
38	periods, but where these idle periods are also quite short (tens or
39	hundreds of microseconds).  For these types of workloads, scheduling
40	clock interrupts will normally be delivered any way because there
41	will frequently be multiple runnable tasks per CPU.  In these cases,
42	attempting to turn off the scheduling clock interrupt will have no effect
43	other than increasing the overhead of switching to and from idle and
44	transitioning between user and kernel execution.
45	
46	This mode of operation can be selected using CONFIG_HZ_PERIODIC=y (or
47	CONFIG_NO_HZ=n for older kernels).
48	
49	However, if you are instead running a light workload with long idle
50	periods, failing to omit scheduling-clock interrupts will result in
51	excessive power consumption.  This is especially bad on battery-powered
52	devices, where it results in extremely short battery lifetimes.  If you
53	are running light workloads, you should therefore read the following
54	section.
55	
56	In addition, if you are running either a real-time workload or an HPC
57	workload with short iterations, the scheduling-clock interrupts can
58	degrade your applications performance.  If this describes your workload,
59	you should read the following two sections.
60	
61	
62	OMIT SCHEDULING-CLOCK TICKS FOR IDLE CPUs
63	
64	If a CPU is idle, there is little point in sending it a scheduling-clock
65	interrupt.  After all, the primary purpose of a scheduling-clock interrupt
66	is to force a busy CPU to shift its attention among multiple duties,
67	and an idle CPU has no duties to shift its attention among.
68	
69	The CONFIG_NO_HZ_IDLE=y Kconfig option causes the kernel to avoid sending
70	scheduling-clock interrupts to idle CPUs, which is critically important
71	both to battery-powered devices and to highly virtualized mainframes.
72	A battery-powered device running a CONFIG_HZ_PERIODIC=y kernel would
73	drain its battery very quickly, easily 2-3 times as fast as would the
74	same device running a CONFIG_NO_HZ_IDLE=y kernel.  A mainframe running
75	1,500 OS instances might find that half of its CPU time was consumed by
76	unnecessary scheduling-clock interrupts.  In these situations, there
77	is strong motivation to avoid sending scheduling-clock interrupts to
78	idle CPUs.  That said, dyntick-idle mode is not free:
79	
80	1.	It increases the number of instructions executed on the path
81		to and from the idle loop.
82	
83	2.	On many architectures, dyntick-idle mode also increases the
84		number of expensive clock-reprogramming operations.
85	
86	Therefore, systems with aggressive real-time response constraints often
87	run CONFIG_HZ_PERIODIC=y kernels (or CONFIG_NO_HZ=n for older kernels)
88	in order to avoid degrading from-idle transition latencies.
89	
90	An idle CPU that is not receiving scheduling-clock interrupts is said to
91	be "dyntick-idle", "in dyntick-idle mode", "in nohz mode", or "running
92	tickless".  The remainder of this document will use "dyntick-idle mode".
93	
94	There is also a boot parameter "nohz=" that can be used to disable
95	dyntick-idle mode in CONFIG_NO_HZ_IDLE=y kernels by specifying "nohz=off".
96	By default, CONFIG_NO_HZ_IDLE=y kernels boot with "nohz=on", enabling
97	dyntick-idle mode.
98	
99	
100	OMIT SCHEDULING-CLOCK TICKS FOR CPUs WITH ONLY ONE RUNNABLE TASK
101	
102	If a CPU has only one runnable task, there is little point in sending it
103	a scheduling-clock interrupt because there is no other task to switch to.
104	Note that omitting scheduling-clock ticks for CPUs with only one runnable
105	task implies also omitting them for idle CPUs.
106	
107	The CONFIG_NO_HZ_FULL=y Kconfig option causes the kernel to avoid
108	sending scheduling-clock interrupts to CPUs with a single runnable task,
109	and such CPUs are said to be "adaptive-ticks CPUs".  This is important
110	for applications with aggressive real-time response constraints because
111	it allows them to improve their worst-case response times by the maximum
112	duration of a scheduling-clock interrupt.  It is also important for
113	computationally intensive short-iteration workloads:  If any CPU is
114	delayed during a given iteration, all the other CPUs will be forced to
115	wait idle while the delayed CPU finishes.  Thus, the delay is multiplied
116	by one less than the number of CPUs.  In these situations, there is
117	again strong motivation to avoid sending scheduling-clock interrupts.
118	
119	By default, no CPU will be an adaptive-ticks CPU.  The "nohz_full="
120	boot parameter specifies the adaptive-ticks CPUs.  For example,
121	"nohz_full=1,6-8" says that CPUs 1, 6, 7, and 8 are to be adaptive-ticks
122	CPUs.  Note that you are prohibited from marking all of the CPUs as
123	adaptive-tick CPUs:  At least one non-adaptive-tick CPU must remain
124	online to handle timekeeping tasks in order to ensure that system
125	calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
126	(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
127	user processes to observe slight drifts in clock rate.)  Therefore, the
128	boot CPU is prohibited from entering adaptive-ticks mode.  Specifying a
129	"nohz_full=" mask that includes the boot CPU will result in a boot-time
130	error message, and the boot CPU will be removed from the mask.  Note that
131	this means that your system must have at least two CPUs in order for
132	CONFIG_NO_HZ_FULL=y to do anything for you.
133	
134	Alternatively, the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter specifies
135	that all CPUs other than the boot CPU are adaptive-ticks CPUs.  This
136	Kconfig parameter will be overridden by the "nohz_full=" boot parameter,
137	so that if both the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter and
138	the "nohz_full=1" boot parameter is specified, the boot parameter will
139	prevail so that only CPU 1 will be an adaptive-ticks CPU.
140	
141	Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
142	This is covered in the "RCU IMPLICATIONS" section below.
143	
144	Normally, a CPU remains in adaptive-ticks mode as long as possible.
145	In particular, transitioning to kernel mode does not automatically change
146	the mode.  Instead, the CPU will exit adaptive-ticks mode only if needed,
147	for example, if that CPU enqueues an RCU callback.
148	
149	Just as with dyntick-idle mode, the benefits of adaptive-tick mode do
150	not come for free:
151	
152	1.	CONFIG_NO_HZ_FULL selects CONFIG_NO_HZ_COMMON, so you cannot run
153		adaptive ticks without also running dyntick idle.  This dependency
154		extends down into the implementation, so that all of the costs
155		of CONFIG_NO_HZ_IDLE are also incurred by CONFIG_NO_HZ_FULL.
156	
157	2.	The user/kernel transitions are slightly more expensive due
158		to the need to inform kernel subsystems (such as RCU) about
159		the change in mode.
160	
161	3.	POSIX CPU timers on adaptive-tick CPUs may miss their deadlines
162		(perhaps indefinitely) because they currently rely on
163		scheduling-tick interrupts.  This will likely be fixed in
164		one of two ways: (1) Prevent CPUs with POSIX CPU timers from
165		entering adaptive-tick mode, or (2) Use hrtimers or other
166		adaptive-ticks-immune mechanism to cause the POSIX CPU timer to
167		fire properly.
168	
169	4.	If there are more perf events pending than the hardware can
170		accommodate, they are normally round-robined so as to collect
171		all of them over time.  Adaptive-tick mode may prevent this
172		round-robining from happening.  This will likely be fixed by
173		preventing CPUs with large numbers of perf events pending from
174		entering adaptive-tick mode.
175	
176	5.	Scheduler statistics for adaptive-tick CPUs may be computed
177		slightly differently than those for non-adaptive-tick CPUs.
178		This might in turn perturb load-balancing of real-time tasks.
179	
180	6.	The LB_BIAS scheduler feature is disabled by adaptive ticks.
181	
182	Although improvements are expected over time, adaptive ticks is quite
183	useful for many types of real-time and compute-intensive applications.
184	However, the drawbacks listed above mean that adaptive ticks should not
185	(yet) be enabled by default.
186	
187	
188	RCU IMPLICATIONS
189	
190	There are situations in which idle CPUs cannot be permitted to
191	enter either dyntick-idle mode or adaptive-tick mode, the most
192	common being when that CPU has RCU callbacks pending.
193	
194	The CONFIG_RCU_FAST_NO_HZ=y Kconfig option may be used to cause such CPUs
195	to enter dyntick-idle mode or adaptive-tick mode anyway.  In this case,
196	a timer will awaken these CPUs every four jiffies in order to ensure
197	that the RCU callbacks are processed in a timely fashion.
198	
199	Another approach is to offload RCU callback processing to "rcuo" kthreads
200	using the CONFIG_RCU_NOCB_CPU=y Kconfig option.  The specific CPUs to
201	offload may be selected via several methods:
202	
203	1.	One of three mutually exclusive Kconfig options specify a
204		build-time default for the CPUs to offload:
205	
206		a.	The CONFIG_RCU_NOCB_CPU_NONE=y Kconfig option results in
207			no CPUs being offloaded.
208	
209		b.	The CONFIG_RCU_NOCB_CPU_ZERO=y Kconfig option causes
210			CPU 0 to be offloaded.
211	
212		c.	The CONFIG_RCU_NOCB_CPU_ALL=y Kconfig option causes all
213			CPUs to be offloaded.  Note that the callbacks will be
214			offloaded to "rcuo" kthreads, and that those kthreads
215			will in fact run on some CPU.  However, this approach
216			gives fine-grained control on exactly which CPUs the
217			callbacks run on, along with their scheduling priority
218			(including the default of SCHED_OTHER), and it further
219			allows this control to be varied dynamically at runtime.
220	
221	2.	The "rcu_nocbs=" kernel boot parameter, which takes a comma-separated
222		list of CPUs and CPU ranges, for example, "1,3-5" selects CPUs 1,
223		3, 4, and 5.  The specified CPUs will be offloaded in addition to
224		any CPUs specified as offloaded by CONFIG_RCU_NOCB_CPU_ZERO=y or
225		CONFIG_RCU_NOCB_CPU_ALL=y.  This means that the "rcu_nocbs=" boot
226		parameter has no effect for kernels built with RCU_NOCB_CPU_ALL=y.
227	
228	The offloaded CPUs will never queue RCU callbacks, and therefore RCU
229	never prevents offloaded CPUs from entering either dyntick-idle mode
230	or adaptive-tick mode.  That said, note that it is up to userspace to
231	pin the "rcuo" kthreads to specific CPUs if desired.  Otherwise, the
232	scheduler will decide where to run them, which might or might not be
233	where you want them to run.
234	
235	
236	TESTING
237	
238	So you enable all the OS-jitter features described in this document,
239	but do not see any change in your workload's behavior.  Is this because
240	your workload isn't affected that much by OS jitter, or is it because
241	something else is in the way?  This section helps answer this question
242	by providing a simple OS-jitter test suite, which is available on branch
243	master of the following git archive:
244	
245	git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git
246	
247	Clone this archive and follow the instructions in the README file.
248	This test procedure will produce a trace that will allow you to evaluate
249	whether or not you have succeeded in removing OS jitter from your system.
250	If this trace shows that you have removed OS jitter as much as is
251	possible, then you can conclude that your workload is not all that
252	sensitive to OS jitter.
253	
254	Note: this test requires that your system have at least two CPUs.
255	We do not currently have a good way to remove OS jitter from single-CPU
256	systems.
257	
258	
259	KNOWN ISSUES
260	
261	o	Dyntick-idle slows transitions to and from idle slightly.
262		In practice, this has not been a problem except for the most
263		aggressive real-time workloads, which have the option of disabling
264		dyntick-idle mode, an option that most of them take.  However,
265		some workloads will no doubt want to use adaptive ticks to
266		eliminate scheduling-clock interrupt latencies.  Here are some
267		options for these workloads:
268	
269		a.	Use PMQOS from userspace to inform the kernel of your
270			latency requirements (preferred).
271	
272		b.	On x86 systems, use the "idle=mwait" boot parameter.
273	
274		c.	On x86 systems, use the "intel_idle.max_cstate=" to limit
275		`	the maximum C-state depth.
276	
277		d.	On x86 systems, use the "idle=poll" boot parameter.
278			However, please note that use of this parameter can cause
279			your CPU to overheat, which may cause thermal throttling
280			to degrade your latencies -- and that this degradation can
281			be even worse than that of dyntick-idle.  Furthermore,
282			this parameter effectively disables Turbo Mode on Intel
283			CPUs, which can significantly reduce maximum performance.
284	
285	o	Adaptive-ticks slows user/kernel transitions slightly.
286		This is not expected to be a problem for computationally intensive
287		workloads, which have few such transitions.  Careful benchmarking
288		will be required to determine whether or not other workloads
289		are significantly affected by this effect.
290	
291	o	Adaptive-ticks does not do anything unless there is only one
292		runnable task for a given CPU, even though there are a number
293		of other situations where the scheduling-clock tick is not
294		needed.  To give but one example, consider a CPU that has one
295		runnable high-priority SCHED_FIFO task and an arbitrary number
296		of low-priority SCHED_OTHER tasks.  In this case, the CPU is
297		required to run the SCHED_FIFO task until it either blocks or
298		some other higher-priority task awakens on (or is assigned to)
299		this CPU, so there is no point in sending a scheduling-clock
300		interrupt to this CPU.	However, the current implementation
301		nevertheless sends scheduling-clock interrupts to CPUs having a
302		single runnable SCHED_FIFO task and multiple runnable SCHED_OTHER
303		tasks, even though these interrupts are unnecessary.
304	
305		And even when there are multiple runnable tasks on a given CPU,
306		there is little point in interrupting that CPU until the current
307		running task's timeslice expires, which is almost always way
308		longer than the time of the next scheduling-clock interrupt.
309	
310		Better handling of these sorts of situations is future work.
311	
312	o	A reboot is required to reconfigure both adaptive idle and RCU
313		callback offloading.  Runtime reconfiguration could be provided
314		if needed, however, due to the complexity of reconfiguring RCU at
315		runtime, there would need to be an earthshakingly good reason.
316		Especially given that you have the straightforward option of
317		simply offloading RCU callbacks from all CPUs and pinning them
318		where you want them whenever you want them pinned.
319	
320	o	Additional configuration is required to deal with other sources
321		of OS jitter, including interrupts and system-utility tasks
322		and processes.  This configuration normally involves binding
323		interrupts and tasks to particular CPUs.
324	
325	o	Some sources of OS jitter can currently be eliminated only by
326		constraining the workload.  For example, the only way to eliminate
327		OS jitter due to global TLB shootdowns is to avoid the unmapping
328		operations (such as kernel module unload operations) that
329		result in these shootdowns.  For another example, page faults
330		and TLB misses can be reduced (and in some cases eliminated) by
331		using huge pages and by constraining the amount of memory used
332		by the application.  Pre-faulting the working set can also be
333		helpful, especially when combined with the mlock() and mlockall()
334		system calls.
335	
336	o	Unless all CPUs are idle, at least one CPU must keep the
337		scheduling-clock interrupt going in order to support accurate
338		timekeeping.
339	
340	o	If there might potentially be some adaptive-ticks CPUs, there
341		will be at least one CPU keeping the scheduling-clock interrupt
342		going, even if all CPUs are otherwise idle.
343	
344		Better handling of this situation is ongoing work.
345	
346	o	Some process-handling operations still require the occasional
347		scheduling-clock tick.	These operations include calculating CPU
348		load, maintaining sched average, computing CFS entity vruntime,
349		computing avenrun, and carrying out load balancing.  They are
350		currently accommodated by scheduling-clock tick every second
351		or so.	On-going work will eliminate the need even for these
352		infrequent scheduling-clock ticks.
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