Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 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 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. 164 165 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. 171 172 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. 175 176 6. The LB_BIAS scheduler feature is disabled by adaptive ticks. 177 178 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. 182 183 184 RCU IMPLICATIONS 185 186 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. 189 190 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. 194 195 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. 200 201 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. 207 208 209 TESTING 210 211 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: 217 218 git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git 219 220 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. 226 227 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. 230 231 232 KNOWN ISSUES 233 234 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: 241 242 a. Use PMQOS from userspace to inform the kernel of your 243 latency requirements (preferred). 244 245 b. On x86 systems, use the "idle=mwait" boot parameter. 246 247 c. On x86 systems, use the "intel_idle.max_cstate=" to limit 248 ` the maximum C-state depth. 249 250 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. 257 258 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. 263 264 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. 277 278 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. 282 283 Better handling of these sorts of situations is future work. 284 285 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. 292 293 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. 297 298 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. 308 309 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. 312 313 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. 316 317 Better handling of this situation is ongoing work. 318 319 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.