318 lines
15 KiB
ReStructuredText
318 lines
15 KiB
ReStructuredText
======================================
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NO_HZ: Reducing Scheduling-Clock Ticks
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======================================
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This document describes Kconfig options and boot parameters that can
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reduce the number of scheduling-clock interrupts, thereby improving energy
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efficiency and reducing OS jitter. Reducing OS jitter is important for
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some types of computationally intensive high-performance computing (HPC)
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applications and for real-time applications.
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There are three main ways of managing scheduling-clock interrupts
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(also known as "scheduling-clock ticks" or simply "ticks"):
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1. Never omit scheduling-clock ticks (CONFIG_HZ_PERIODIC=y or
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CONFIG_NO_HZ=n for older kernels). You normally will -not-
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want to choose this option.
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2. Omit scheduling-clock ticks on idle CPUs (CONFIG_NO_HZ_IDLE=y or
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CONFIG_NO_HZ=y for older kernels). This is the most common
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approach, and should be the default.
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3. Omit scheduling-clock ticks on CPUs that are either idle or that
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have only one runnable task (CONFIG_NO_HZ_FULL=y). Unless you
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are running realtime applications or certain types of HPC
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workloads, you will normally -not- want this option.
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These three cases are described in the following three sections, followed
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by a third section on RCU-specific considerations, a fourth section
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discussing testing, and a fifth and final section listing known issues.
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Never Omit Scheduling-Clock Ticks
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=================================
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Very old versions of Linux from the 1990s and the very early 2000s
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are incapable of omitting scheduling-clock ticks. It turns out that
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there are some situations where this old-school approach is still the
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right approach, for example, in heavy workloads with lots of tasks
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that use short bursts of CPU, where there are very frequent idle
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periods, but where these idle periods are also quite short (tens or
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hundreds of microseconds). For these types of workloads, scheduling
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clock interrupts will normally be delivered any way because there
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will frequently be multiple runnable tasks per CPU. In these cases,
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attempting to turn off the scheduling clock interrupt will have no effect
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other than increasing the overhead of switching to and from idle and
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transitioning between user and kernel execution.
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This mode of operation can be selected using CONFIG_HZ_PERIODIC=y (or
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CONFIG_NO_HZ=n for older kernels).
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However, if you are instead running a light workload with long idle
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periods, failing to omit scheduling-clock interrupts will result in
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excessive power consumption. This is especially bad on battery-powered
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devices, where it results in extremely short battery lifetimes. If you
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are running light workloads, you should therefore read the following
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section.
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In addition, if you are running either a real-time workload or an HPC
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workload with short iterations, the scheduling-clock interrupts can
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degrade your applications performance. If this describes your workload,
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you should read the following two sections.
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Omit Scheduling-Clock Ticks For Idle CPUs
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=========================================
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If a CPU is idle, there is little point in sending it a scheduling-clock
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interrupt. After all, the primary purpose of a scheduling-clock interrupt
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is to force a busy CPU to shift its attention among multiple duties,
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and an idle CPU has no duties to shift its attention among.
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An idle CPU that is not receiving scheduling-clock interrupts is said to
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be "dyntick-idle", "in dyntick-idle mode", "in nohz mode", or "running
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tickless". The remainder of this document will use "dyntick-idle mode".
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The CONFIG_NO_HZ_IDLE=y Kconfig option causes the kernel to avoid sending
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scheduling-clock interrupts to idle CPUs, which is critically important
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both to battery-powered devices and to highly virtualized mainframes.
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A battery-powered device running a CONFIG_HZ_PERIODIC=y kernel would
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drain its battery very quickly, easily 2-3 times as fast as would the
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same device running a CONFIG_NO_HZ_IDLE=y kernel. A mainframe running
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1,500 OS instances might find that half of its CPU time was consumed by
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unnecessary scheduling-clock interrupts. In these situations, there
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is strong motivation to avoid sending scheduling-clock interrupts to
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idle CPUs. That said, dyntick-idle mode is not free:
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1. It increases the number of instructions executed on the path
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to and from the idle loop.
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2. On many architectures, dyntick-idle mode also increases the
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number of expensive clock-reprogramming operations.
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Therefore, systems with aggressive real-time response constraints often
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run CONFIG_HZ_PERIODIC=y kernels (or CONFIG_NO_HZ=n for older kernels)
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in order to avoid degrading from-idle transition latencies.
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There is also a boot parameter "nohz=" that can be used to disable
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dyntick-idle mode in CONFIG_NO_HZ_IDLE=y kernels by specifying "nohz=off".
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By default, CONFIG_NO_HZ_IDLE=y kernels boot with "nohz=on", enabling
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dyntick-idle mode.
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Omit Scheduling-Clock Ticks For CPUs With Only One Runnable Task
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================================================================
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If a CPU has only one runnable task, there is little point in sending it
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a scheduling-clock interrupt because there is no other task to switch to.
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Note that omitting scheduling-clock ticks for CPUs with only one runnable
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task implies also omitting them for idle CPUs.
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The CONFIG_NO_HZ_FULL=y Kconfig option causes the kernel to avoid
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sending scheduling-clock interrupts to CPUs with a single runnable task,
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and such CPUs are said to be "adaptive-ticks CPUs". This is important
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for applications with aggressive real-time response constraints because
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it allows them to improve their worst-case response times by the maximum
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duration of a scheduling-clock interrupt. It is also important for
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computationally intensive short-iteration workloads: If any CPU is
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delayed during a given iteration, all the other CPUs will be forced to
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wait idle while the delayed CPU finishes. Thus, the delay is multiplied
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by one less than the number of CPUs. In these situations, there is
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again strong motivation to avoid sending scheduling-clock interrupts.
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By default, no CPU will be an adaptive-ticks CPU. The "nohz_full="
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boot parameter specifies the adaptive-ticks CPUs. For example,
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"nohz_full=1,6-8" says that CPUs 1, 6, 7, and 8 are to be adaptive-ticks
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CPUs. Note that you are prohibited from marking all of the CPUs as
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adaptive-tick CPUs: At least one non-adaptive-tick CPU must remain
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online to handle timekeeping tasks in order to ensure that system
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calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
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(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
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user processes to observe slight drifts in clock rate.) Note that this
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means that your system must have at least two CPUs in order for
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CONFIG_NO_HZ_FULL=y to do anything for you.
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Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
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This is covered in the "RCU IMPLICATIONS" section below.
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Normally, a CPU remains in adaptive-ticks mode as long as possible.
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In particular, transitioning to kernel mode does not automatically change
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the mode. Instead, the CPU will exit adaptive-ticks mode only if needed,
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for example, if that CPU enqueues an RCU callback.
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Just as with dyntick-idle mode, the benefits of adaptive-tick mode do
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not come for free:
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1. CONFIG_NO_HZ_FULL selects CONFIG_NO_HZ_COMMON, so you cannot run
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adaptive ticks without also running dyntick idle. This dependency
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extends down into the implementation, so that all of the costs
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of CONFIG_NO_HZ_IDLE are also incurred by CONFIG_NO_HZ_FULL.
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2. The user/kernel transitions are slightly more expensive due
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to the need to inform kernel subsystems (such as RCU) about
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the change in mode.
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3. POSIX CPU timers prevent CPUs from entering adaptive-tick mode.
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Real-time applications needing to take actions based on CPU time
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consumption need to use other means of doing so.
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4. If there are more perf events pending than the hardware can
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accommodate, they are normally round-robined so as to collect
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all of them over time. Adaptive-tick mode may prevent this
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round-robining from happening. This will likely be fixed by
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preventing CPUs with large numbers of perf events pending from
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entering adaptive-tick mode.
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5. Scheduler statistics for adaptive-tick CPUs may be computed
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slightly differently than those for non-adaptive-tick CPUs.
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This might in turn perturb load-balancing of real-time tasks.
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Although improvements are expected over time, adaptive ticks is quite
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useful for many types of real-time and compute-intensive applications.
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However, the drawbacks listed above mean that adaptive ticks should not
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(yet) be enabled by default.
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RCU Implications
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================
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There are situations in which idle CPUs cannot be permitted to
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enter either dyntick-idle mode or adaptive-tick mode, the most
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common being when that CPU has RCU callbacks pending.
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Avoid this by offloading RCU callback processing to "rcuo" kthreads
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using the CONFIG_RCU_NOCB_CPU=y Kconfig option. The specific CPUs to
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offload may be selected using The "rcu_nocbs=" kernel boot parameter,
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which takes a comma-separated list of CPUs and CPU ranges, for example,
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"1,3-5" selects CPUs 1, 3, 4, and 5. Note that CPUs specified by
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the "nohz_full" kernel boot parameter are also offloaded.
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The offloaded CPUs will never queue RCU callbacks, and therefore RCU
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never prevents offloaded CPUs from entering either dyntick-idle mode
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or adaptive-tick mode. That said, note that it is up to userspace to
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pin the "rcuo" kthreads to specific CPUs if desired. Otherwise, the
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scheduler will decide where to run them, which might or might not be
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where you want them to run.
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Testing
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=======
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So you enable all the OS-jitter features described in this document,
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but do not see any change in your workload's behavior. Is this because
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your workload isn't affected that much by OS jitter, or is it because
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something else is in the way? This section helps answer this question
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by providing a simple OS-jitter test suite, which is available on branch
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master of the following git archive:
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git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git
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Clone this archive and follow the instructions in the README file.
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This test procedure will produce a trace that will allow you to evaluate
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whether or not you have succeeded in removing OS jitter from your system.
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If this trace shows that you have removed OS jitter as much as is
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possible, then you can conclude that your workload is not all that
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sensitive to OS jitter.
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Note: this test requires that your system have at least two CPUs.
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We do not currently have a good way to remove OS jitter from single-CPU
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systems.
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Known Issues
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============
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* Dyntick-idle slows transitions to and from idle slightly.
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In practice, this has not been a problem except for the most
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aggressive real-time workloads, which have the option of disabling
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dyntick-idle mode, an option that most of them take. However,
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some workloads will no doubt want to use adaptive ticks to
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eliminate scheduling-clock interrupt latencies. Here are some
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options for these workloads:
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a. Use PMQOS from userspace to inform the kernel of your
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latency requirements (preferred).
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b. On x86 systems, use the "idle=mwait" boot parameter.
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c. On x86 systems, use the "intel_idle.max_cstate=" to limit
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` the maximum C-state depth.
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d. On x86 systems, use the "idle=poll" boot parameter.
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However, please note that use of this parameter can cause
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your CPU to overheat, which may cause thermal throttling
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to degrade your latencies -- and that this degradation can
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be even worse than that of dyntick-idle. Furthermore,
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this parameter effectively disables Turbo Mode on Intel
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CPUs, which can significantly reduce maximum performance.
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* Adaptive-ticks slows user/kernel transitions slightly.
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This is not expected to be a problem for computationally intensive
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workloads, which have few such transitions. Careful benchmarking
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will be required to determine whether or not other workloads
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are significantly affected by this effect.
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* Adaptive-ticks does not do anything unless there is only one
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runnable task for a given CPU, even though there are a number
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of other situations where the scheduling-clock tick is not
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needed. To give but one example, consider a CPU that has one
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runnable high-priority SCHED_FIFO task and an arbitrary number
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of low-priority SCHED_OTHER tasks. In this case, the CPU is
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required to run the SCHED_FIFO task until it either blocks or
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some other higher-priority task awakens on (or is assigned to)
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this CPU, so there is no point in sending a scheduling-clock
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interrupt to this CPU. However, the current implementation
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nevertheless sends scheduling-clock interrupts to CPUs having a
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single runnable SCHED_FIFO task and multiple runnable SCHED_OTHER
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tasks, even though these interrupts are unnecessary.
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And even when there are multiple runnable tasks on a given CPU,
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there is little point in interrupting that CPU until the current
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running task's timeslice expires, which is almost always way
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longer than the time of the next scheduling-clock interrupt.
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Better handling of these sorts of situations is future work.
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* A reboot is required to reconfigure both adaptive idle and RCU
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callback offloading. Runtime reconfiguration could be provided
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if needed, however, due to the complexity of reconfiguring RCU at
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runtime, there would need to be an earthshakingly good reason.
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Especially given that you have the straightforward option of
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simply offloading RCU callbacks from all CPUs and pinning them
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where you want them whenever you want them pinned.
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* Additional configuration is required to deal with other sources
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of OS jitter, including interrupts and system-utility tasks
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and processes. This configuration normally involves binding
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interrupts and tasks to particular CPUs.
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* Some sources of OS jitter can currently be eliminated only by
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constraining the workload. For example, the only way to eliminate
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OS jitter due to global TLB shootdowns is to avoid the unmapping
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operations (such as kernel module unload operations) that
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result in these shootdowns. For another example, page faults
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and TLB misses can be reduced (and in some cases eliminated) by
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using huge pages and by constraining the amount of memory used
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by the application. Pre-faulting the working set can also be
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helpful, especially when combined with the mlock() and mlockall()
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system calls.
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* Unless all CPUs are idle, at least one CPU must keep the
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scheduling-clock interrupt going in order to support accurate
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timekeeping.
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* If there might potentially be some adaptive-ticks CPUs, there
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will be at least one CPU keeping the scheduling-clock interrupt
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going, even if all CPUs are otherwise idle.
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Better handling of this situation is ongoing work.
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* Some process-handling operations still require the occasional
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scheduling-clock tick. These operations include calculating CPU
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load, maintaining sched average, computing CFS entity vruntime,
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computing avenrun, and carrying out load balancing. They are
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currently accommodated by scheduling-clock tick every second
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or so. On-going work will eliminate the need even for these
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infrequent scheduling-clock ticks.
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