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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

1			NO_HZ: Reducing Scheduling-Clock Ticks
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.
10	There are three main ways of managing scheduling-clock interrupts
11	(also known as "scheduling-clock ticks" or simply "ticks"):
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.
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.
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.
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.
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.
46	This mode of operation can be selected using CONFIG_HZ_PERIODIC=y (or
47	CONFIG_NO_HZ=n for older kernels).
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.
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.
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.
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:
80	1.	It increases the number of instructions executed on the path
81		to and from the idle loop.
83	2.	On many architectures, dyntick-idle mode also increases the
84		number of expensive clock-reprogramming operations.
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.
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".
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.
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.
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.
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.
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.
141	Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
142	This is covered in the "RCU IMPLICATIONS" section below.
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.
149	Just as with dyntick-idle mode, the benefits of adaptive-tick mode do
150	not come for free:
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.
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.
161	3.	POSIX CPU timers prevent CPUs from entering adaptive-tick mode.
162		Real-time applications needing to take actions based on CPU time
163		consumption need to use other means of doing so.
165	4.	If there are more perf events pending than the hardware can
166		accommodate, they are normally round-robined so as to collect
167		all of them over time.  Adaptive-tick mode may prevent this
168		round-robining from happening.  This will likely be fixed by
169		preventing CPUs with large numbers of perf events pending from
170		entering adaptive-tick mode.
172	5.	Scheduler statistics for adaptive-tick CPUs may be computed
173		slightly differently than those for non-adaptive-tick CPUs.
174		This might in turn perturb load-balancing of real-time tasks.
176	6.	The LB_BIAS scheduler feature is disabled by adaptive ticks.
178	Although improvements are expected over time, adaptive ticks is quite
179	useful for many types of real-time and compute-intensive applications.
180	However, the drawbacks listed above mean that adaptive ticks should not
181	(yet) be enabled by default.
186	There are situations in which idle CPUs cannot be permitted to
187	enter either dyntick-idle mode or adaptive-tick mode, the most
188	common being when that CPU has RCU callbacks pending.
190	The CONFIG_RCU_FAST_NO_HZ=y Kconfig option may be used to cause such CPUs
191	to enter dyntick-idle mode or adaptive-tick mode anyway.  In this case,
192	a timer will awaken these CPUs every four jiffies in order to ensure
193	that the RCU callbacks are processed in a timely fashion.
195	Another approach is to offload RCU callback processing to "rcuo" kthreads
196	using the CONFIG_RCU_NOCB_CPU=y Kconfig option.  The specific CPUs to
197	offload may be selected using The "rcu_nocbs=" kernel boot parameter,
198	which takes a comma-separated list of CPUs and CPU ranges, for example,
199	"1,3-5" selects CPUs 1, 3, 4, and 5.
201	The offloaded CPUs will never queue RCU callbacks, and therefore RCU
202	never prevents offloaded CPUs from entering either dyntick-idle mode
203	or adaptive-tick mode.  That said, note that it is up to userspace to
204	pin the "rcuo" kthreads to specific CPUs if desired.  Otherwise, the
205	scheduler will decide where to run them, which might or might not be
206	where you want them to run.
211	So you enable all the OS-jitter features described in this document,
212	but do not see any change in your workload's behavior.  Is this because
213	your workload isn't affected that much by OS jitter, or is it because
214	something else is in the way?  This section helps answer this question
215	by providing a simple OS-jitter test suite, which is available on branch
216	master of the following git archive:
218	git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git
220	Clone this archive and follow the instructions in the README file.
221	This test procedure will produce a trace that will allow you to evaluate
222	whether or not you have succeeded in removing OS jitter from your system.
223	If this trace shows that you have removed OS jitter as much as is
224	possible, then you can conclude that your workload is not all that
225	sensitive to OS jitter.
227	Note: this test requires that your system have at least two CPUs.
228	We do not currently have a good way to remove OS jitter from single-CPU
229	systems.
234	o	Dyntick-idle slows transitions to and from idle slightly.
235		In practice, this has not been a problem except for the most
236		aggressive real-time workloads, which have the option of disabling
237		dyntick-idle mode, an option that most of them take.  However,
238		some workloads will no doubt want to use adaptive ticks to
239		eliminate scheduling-clock interrupt latencies.  Here are some
240		options for these workloads:
242		a.	Use PMQOS from userspace to inform the kernel of your
243			latency requirements (preferred).
245		b.	On x86 systems, use the "idle=mwait" boot parameter.
247		c.	On x86 systems, use the "intel_idle.max_cstate=" to limit
248		`	the maximum C-state depth.
250		d.	On x86 systems, use the "idle=poll" boot parameter.
251			However, please note that use of this parameter can cause
252			your CPU to overheat, which may cause thermal throttling
253			to degrade your latencies -- and that this degradation can
254			be even worse than that of dyntick-idle.  Furthermore,
255			this parameter effectively disables Turbo Mode on Intel
256			CPUs, which can significantly reduce maximum performance.
258	o	Adaptive-ticks slows user/kernel transitions slightly.
259		This is not expected to be a problem for computationally intensive
260		workloads, which have few such transitions.  Careful benchmarking
261		will be required to determine whether or not other workloads
262		are significantly affected by this effect.
264	o	Adaptive-ticks does not do anything unless there is only one
265		runnable task for a given CPU, even though there are a number
266		of other situations where the scheduling-clock tick is not
267		needed.  To give but one example, consider a CPU that has one
268		runnable high-priority SCHED_FIFO task and an arbitrary number
269		of low-priority SCHED_OTHER tasks.  In this case, the CPU is
270		required to run the SCHED_FIFO task until it either blocks or
271		some other higher-priority task awakens on (or is assigned to)
272		this CPU, so there is no point in sending a scheduling-clock
273		interrupt to this CPU.	However, the current implementation
274		nevertheless sends scheduling-clock interrupts to CPUs having a
275		single runnable SCHED_FIFO task and multiple runnable SCHED_OTHER
276		tasks, even though these interrupts are unnecessary.
278		And even when there are multiple runnable tasks on a given CPU,
279		there is little point in interrupting that CPU until the current
280		running task's timeslice expires, which is almost always way
281		longer than the time of the next scheduling-clock interrupt.
283		Better handling of these sorts of situations is future work.
285	o	A reboot is required to reconfigure both adaptive idle and RCU
286		callback offloading.  Runtime reconfiguration could be provided
287		if needed, however, due to the complexity of reconfiguring RCU at
288		runtime, there would need to be an earthshakingly good reason.
289		Especially given that you have the straightforward option of
290		simply offloading RCU callbacks from all CPUs and pinning them
291		where you want them whenever you want them pinned.
293	o	Additional configuration is required to deal with other sources
294		of OS jitter, including interrupts and system-utility tasks
295		and processes.  This configuration normally involves binding
296		interrupts and tasks to particular CPUs.
298	o	Some sources of OS jitter can currently be eliminated only by
299		constraining the workload.  For example, the only way to eliminate
300		OS jitter due to global TLB shootdowns is to avoid the unmapping
301		operations (such as kernel module unload operations) that
302		result in these shootdowns.  For another example, page faults
303		and TLB misses can be reduced (and in some cases eliminated) by
304		using huge pages and by constraining the amount of memory used
305		by the application.  Pre-faulting the working set can also be
306		helpful, especially when combined with the mlock() and mlockall()
307		system calls.
309	o	Unless all CPUs are idle, at least one CPU must keep the
310		scheduling-clock interrupt going in order to support accurate
311		timekeeping.
313	o	If there might potentially be some adaptive-ticks CPUs, there
314		will be at least one CPU keeping the scheduling-clock interrupt
315		going, even if all CPUs are otherwise idle.
317		Better handling of this situation is ongoing work.
319	o	Some process-handling operations still require the occasional
320		scheduling-clock tick.	These operations include calculating CPU
321		load, maintaining sched average, computing CFS entity vruntime,
322		computing avenrun, and carrying out load balancing.  They are
323		currently accommodated by scheduling-clock tick every second
324		or so.	On-going work will eliminate the need even for these
325		infrequent scheduling-clock ticks.
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