353 lines
17 KiB
Plaintext
Executable File
353 lines
17 KiB
Plaintext
Executable File
NO_HZ: Reducing Scheduling-Clock Ticks
|
|
|
|
|
|
This document describes Kconfig options and boot parameters that can
|
|
reduce the number of scheduling-clock interrupts, thereby improving energy
|
|
efficiency and reducing OS jitter. Reducing OS jitter is important for
|
|
some types of computationally intensive high-performance computing (HPC)
|
|
applications and for real-time applications.
|
|
|
|
There are three main ways of managing scheduling-clock interrupts
|
|
(also known as "scheduling-clock ticks" or simply "ticks"):
|
|
|
|
1. Never omit scheduling-clock ticks (CONFIG_HZ_PERIODIC=y or
|
|
CONFIG_NO_HZ=n for older kernels). You normally will -not-
|
|
want to choose this option.
|
|
|
|
2. Omit scheduling-clock ticks on idle CPUs (CONFIG_NO_HZ_IDLE=y or
|
|
CONFIG_NO_HZ=y for older kernels). This is the most common
|
|
approach, and should be the default.
|
|
|
|
3. Omit scheduling-clock ticks on CPUs that are either idle or that
|
|
have only one runnable task (CONFIG_NO_HZ_FULL=y). Unless you
|
|
are running realtime applications or certain types of HPC
|
|
workloads, you will normally -not- want this option.
|
|
|
|
These three cases are described in the following three sections, followed
|
|
by a third section on RCU-specific considerations, a fourth section
|
|
discussing testing, and a fifth and final section listing known issues.
|
|
|
|
|
|
NEVER OMIT SCHEDULING-CLOCK TICKS
|
|
|
|
Very old versions of Linux from the 1990s and the very early 2000s
|
|
are incapable of omitting scheduling-clock ticks. It turns out that
|
|
there are some situations where this old-school approach is still the
|
|
right approach, for example, in heavy workloads with lots of tasks
|
|
that use short bursts of CPU, where there are very frequent idle
|
|
periods, but where these idle periods are also quite short (tens or
|
|
hundreds of microseconds). For these types of workloads, scheduling
|
|
clock interrupts will normally be delivered any way because there
|
|
will frequently be multiple runnable tasks per CPU. In these cases,
|
|
attempting to turn off the scheduling clock interrupt will have no effect
|
|
other than increasing the overhead of switching to and from idle and
|
|
transitioning between user and kernel execution.
|
|
|
|
This mode of operation can be selected using CONFIG_HZ_PERIODIC=y (or
|
|
CONFIG_NO_HZ=n for older kernels).
|
|
|
|
However, if you are instead running a light workload with long idle
|
|
periods, failing to omit scheduling-clock interrupts will result in
|
|
excessive power consumption. This is especially bad on battery-powered
|
|
devices, where it results in extremely short battery lifetimes. If you
|
|
are running light workloads, you should therefore read the following
|
|
section.
|
|
|
|
In addition, if you are running either a real-time workload or an HPC
|
|
workload with short iterations, the scheduling-clock interrupts can
|
|
degrade your applications performance. If this describes your workload,
|
|
you should read the following two sections.
|
|
|
|
|
|
OMIT SCHEDULING-CLOCK TICKS FOR IDLE CPUs
|
|
|
|
If a CPU is idle, there is little point in sending it a scheduling-clock
|
|
interrupt. After all, the primary purpose of a scheduling-clock interrupt
|
|
is to force a busy CPU to shift its attention among multiple duties,
|
|
and an idle CPU has no duties to shift its attention among.
|
|
|
|
The CONFIG_NO_HZ_IDLE=y Kconfig option causes the kernel to avoid sending
|
|
scheduling-clock interrupts to idle CPUs, which is critically important
|
|
both to battery-powered devices and to highly virtualized mainframes.
|
|
A battery-powered device running a CONFIG_HZ_PERIODIC=y kernel would
|
|
drain its battery very quickly, easily 2-3 times as fast as would the
|
|
same device running a CONFIG_NO_HZ_IDLE=y kernel. A mainframe running
|
|
1,500 OS instances might find that half of its CPU time was consumed by
|
|
unnecessary scheduling-clock interrupts. In these situations, there
|
|
is strong motivation to avoid sending scheduling-clock interrupts to
|
|
idle CPUs. That said, dyntick-idle mode is not free:
|
|
|
|
1. It increases the number of instructions executed on the path
|
|
to and from the idle loop.
|
|
|
|
2. On many architectures, dyntick-idle mode also increases the
|
|
number of expensive clock-reprogramming operations.
|
|
|
|
Therefore, systems with aggressive real-time response constraints often
|
|
run CONFIG_HZ_PERIODIC=y kernels (or CONFIG_NO_HZ=n for older kernels)
|
|
in order to avoid degrading from-idle transition latencies.
|
|
|
|
An idle CPU that is not receiving scheduling-clock interrupts is said to
|
|
be "dyntick-idle", "in dyntick-idle mode", "in nohz mode", or "running
|
|
tickless". The remainder of this document will use "dyntick-idle mode".
|
|
|
|
There is also a boot parameter "nohz=" that can be used to disable
|
|
dyntick-idle mode in CONFIG_NO_HZ_IDLE=y kernels by specifying "nohz=off".
|
|
By default, CONFIG_NO_HZ_IDLE=y kernels boot with "nohz=on", enabling
|
|
dyntick-idle mode.
|
|
|
|
|
|
OMIT SCHEDULING-CLOCK TICKS FOR CPUs WITH ONLY ONE RUNNABLE TASK
|
|
|
|
If a CPU has only one runnable task, there is little point in sending it
|
|
a scheduling-clock interrupt because there is no other task to switch to.
|
|
Note that omitting scheduling-clock ticks for CPUs with only one runnable
|
|
task implies also omitting them for idle CPUs.
|
|
|
|
The CONFIG_NO_HZ_FULL=y Kconfig option causes the kernel to avoid
|
|
sending scheduling-clock interrupts to CPUs with a single runnable task,
|
|
and such CPUs are said to be "adaptive-ticks CPUs". This is important
|
|
for applications with aggressive real-time response constraints because
|
|
it allows them to improve their worst-case response times by the maximum
|
|
duration of a scheduling-clock interrupt. It is also important for
|
|
computationally intensive short-iteration workloads: If any CPU is
|
|
delayed during a given iteration, all the other CPUs will be forced to
|
|
wait idle while the delayed CPU finishes. Thus, the delay is multiplied
|
|
by one less than the number of CPUs. In these situations, there is
|
|
again strong motivation to avoid sending scheduling-clock interrupts.
|
|
|
|
By default, no CPU will be an adaptive-ticks CPU. The "nohz_full="
|
|
boot parameter specifies the adaptive-ticks CPUs. For example,
|
|
"nohz_full=1,6-8" says that CPUs 1, 6, 7, and 8 are to be adaptive-ticks
|
|
CPUs. Note that you are prohibited from marking all of the CPUs as
|
|
adaptive-tick CPUs: At least one non-adaptive-tick CPU must remain
|
|
online to handle timekeeping tasks in order to ensure that system
|
|
calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
|
|
(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
|
|
user processes to observe slight drifts in clock rate.) Therefore, the
|
|
boot CPU is prohibited from entering adaptive-ticks mode. Specifying a
|
|
"nohz_full=" mask that includes the boot CPU will result in a boot-time
|
|
error message, and the boot CPU will be removed from the mask. Note that
|
|
this means that your system must have at least two CPUs in order for
|
|
CONFIG_NO_HZ_FULL=y to do anything for you.
|
|
|
|
Alternatively, the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter specifies
|
|
that all CPUs other than the boot CPU are adaptive-ticks CPUs. This
|
|
Kconfig parameter will be overridden by the "nohz_full=" boot parameter,
|
|
so that if both the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter and
|
|
the "nohz_full=1" boot parameter is specified, the boot parameter will
|
|
prevail so that only CPU 1 will be an adaptive-ticks CPU.
|
|
|
|
Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
|
|
This is covered in the "RCU IMPLICATIONS" section below.
|
|
|
|
Normally, a CPU remains in adaptive-ticks mode as long as possible.
|
|
In particular, transitioning to kernel mode does not automatically change
|
|
the mode. Instead, the CPU will exit adaptive-ticks mode only if needed,
|
|
for example, if that CPU enqueues an RCU callback.
|
|
|
|
Just as with dyntick-idle mode, the benefits of adaptive-tick mode do
|
|
not come for free:
|
|
|
|
1. CONFIG_NO_HZ_FULL selects CONFIG_NO_HZ_COMMON, so you cannot run
|
|
adaptive ticks without also running dyntick idle. This dependency
|
|
extends down into the implementation, so that all of the costs
|
|
of CONFIG_NO_HZ_IDLE are also incurred by CONFIG_NO_HZ_FULL.
|
|
|
|
2. The user/kernel transitions are slightly more expensive due
|
|
to the need to inform kernel subsystems (such as RCU) about
|
|
the change in mode.
|
|
|
|
3. POSIX CPU timers on adaptive-tick CPUs may miss their deadlines
|
|
(perhaps indefinitely) because they currently rely on
|
|
scheduling-tick interrupts. This will likely be fixed in
|
|
one of two ways: (1) Prevent CPUs with POSIX CPU timers from
|
|
entering adaptive-tick mode, or (2) Use hrtimers or other
|
|
adaptive-ticks-immune mechanism to cause the POSIX CPU timer to
|
|
fire properly.
|
|
|
|
4. If there are more perf events pending than the hardware can
|
|
accommodate, they are normally round-robined so as to collect
|
|
all of them over time. Adaptive-tick mode may prevent this
|
|
round-robining from happening. This will likely be fixed by
|
|
preventing CPUs with large numbers of perf events pending from
|
|
entering adaptive-tick mode.
|
|
|
|
5. Scheduler statistics for adaptive-tick CPUs may be computed
|
|
slightly differently than those for non-adaptive-tick CPUs.
|
|
This might in turn perturb load-balancing of real-time tasks.
|
|
|
|
6. The LB_BIAS scheduler feature is disabled by adaptive ticks.
|
|
|
|
Although improvements are expected over time, adaptive ticks is quite
|
|
useful for many types of real-time and compute-intensive applications.
|
|
However, the drawbacks listed above mean that adaptive ticks should not
|
|
(yet) be enabled by default.
|
|
|
|
|
|
RCU IMPLICATIONS
|
|
|
|
There are situations in which idle CPUs cannot be permitted to
|
|
enter either dyntick-idle mode or adaptive-tick mode, the most
|
|
common being when that CPU has RCU callbacks pending.
|
|
|
|
The CONFIG_RCU_FAST_NO_HZ=y Kconfig option may be used to cause such CPUs
|
|
to enter dyntick-idle mode or adaptive-tick mode anyway. In this case,
|
|
a timer will awaken these CPUs every four jiffies in order to ensure
|
|
that the RCU callbacks are processed in a timely fashion.
|
|
|
|
Another approach is to offload RCU callback processing to "rcuo" kthreads
|
|
using the CONFIG_RCU_NOCB_CPU=y Kconfig option. The specific CPUs to
|
|
offload may be selected via several methods:
|
|
|
|
1. One of three mutually exclusive Kconfig options specify a
|
|
build-time default for the CPUs to offload:
|
|
|
|
a. The CONFIG_RCU_NOCB_CPU_NONE=y Kconfig option results in
|
|
no CPUs being offloaded.
|
|
|
|
b. The CONFIG_RCU_NOCB_CPU_ZERO=y Kconfig option causes
|
|
CPU 0 to be offloaded.
|
|
|
|
c. The CONFIG_RCU_NOCB_CPU_ALL=y Kconfig option causes all
|
|
CPUs to be offloaded. Note that the callbacks will be
|
|
offloaded to "rcuo" kthreads, and that those kthreads
|
|
will in fact run on some CPU. However, this approach
|
|
gives fine-grained control on exactly which CPUs the
|
|
callbacks run on, along with their scheduling priority
|
|
(including the default of SCHED_OTHER), and it further
|
|
allows this control to be varied dynamically at runtime.
|
|
|
|
2. The "rcu_nocbs=" kernel boot parameter, which takes a comma-separated
|
|
list of CPUs and CPU ranges, for example, "1,3-5" selects CPUs 1,
|
|
3, 4, and 5. The specified CPUs will be offloaded in addition to
|
|
any CPUs specified as offloaded by CONFIG_RCU_NOCB_CPU_ZERO=y or
|
|
CONFIG_RCU_NOCB_CPU_ALL=y. This means that the "rcu_nocbs=" boot
|
|
parameter has no effect for kernels built with RCU_NOCB_CPU_ALL=y.
|
|
|
|
The offloaded CPUs will never queue RCU callbacks, and therefore RCU
|
|
never prevents offloaded CPUs from entering either dyntick-idle mode
|
|
or adaptive-tick mode. That said, note that it is up to userspace to
|
|
pin the "rcuo" kthreads to specific CPUs if desired. Otherwise, the
|
|
scheduler will decide where to run them, which might or might not be
|
|
where you want them to run.
|
|
|
|
|
|
TESTING
|
|
|
|
So you enable all the OS-jitter features described in this document,
|
|
but do not see any change in your workload's behavior. Is this because
|
|
your workload isn't affected that much by OS jitter, or is it because
|
|
something else is in the way? This section helps answer this question
|
|
by providing a simple OS-jitter test suite, which is available on branch
|
|
master of the following git archive:
|
|
|
|
git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git
|
|
|
|
Clone this archive and follow the instructions in the README file.
|
|
This test procedure will produce a trace that will allow you to evaluate
|
|
whether or not you have succeeded in removing OS jitter from your system.
|
|
If this trace shows that you have removed OS jitter as much as is
|
|
possible, then you can conclude that your workload is not all that
|
|
sensitive to OS jitter.
|
|
|
|
Note: this test requires that your system have at least two CPUs.
|
|
We do not currently have a good way to remove OS jitter from single-CPU
|
|
systems.
|
|
|
|
|
|
KNOWN ISSUES
|
|
|
|
o Dyntick-idle slows transitions to and from idle slightly.
|
|
In practice, this has not been a problem except for the most
|
|
aggressive real-time workloads, which have the option of disabling
|
|
dyntick-idle mode, an option that most of them take. However,
|
|
some workloads will no doubt want to use adaptive ticks to
|
|
eliminate scheduling-clock interrupt latencies. Here are some
|
|
options for these workloads:
|
|
|
|
a. Use PMQOS from userspace to inform the kernel of your
|
|
latency requirements (preferred).
|
|
|
|
b. On x86 systems, use the "idle=mwait" boot parameter.
|
|
|
|
c. On x86 systems, use the "intel_idle.max_cstate=" to limit
|
|
` the maximum C-state depth.
|
|
|
|
d. On x86 systems, use the "idle=poll" boot parameter.
|
|
However, please note that use of this parameter can cause
|
|
your CPU to overheat, which may cause thermal throttling
|
|
to degrade your latencies -- and that this degradation can
|
|
be even worse than that of dyntick-idle. Furthermore,
|
|
this parameter effectively disables Turbo Mode on Intel
|
|
CPUs, which can significantly reduce maximum performance.
|
|
|
|
o Adaptive-ticks slows user/kernel transitions slightly.
|
|
This is not expected to be a problem for computationally intensive
|
|
workloads, which have few such transitions. Careful benchmarking
|
|
will be required to determine whether or not other workloads
|
|
are significantly affected by this effect.
|
|
|
|
o Adaptive-ticks does not do anything unless there is only one
|
|
runnable task for a given CPU, even though there are a number
|
|
of other situations where the scheduling-clock tick is not
|
|
needed. To give but one example, consider a CPU that has one
|
|
runnable high-priority SCHED_FIFO task and an arbitrary number
|
|
of low-priority SCHED_OTHER tasks. In this case, the CPU is
|
|
required to run the SCHED_FIFO task until it either blocks or
|
|
some other higher-priority task awakens on (or is assigned to)
|
|
this CPU, so there is no point in sending a scheduling-clock
|
|
interrupt to this CPU. However, the current implementation
|
|
nevertheless sends scheduling-clock interrupts to CPUs having a
|
|
single runnable SCHED_FIFO task and multiple runnable SCHED_OTHER
|
|
tasks, even though these interrupts are unnecessary.
|
|
|
|
And even when there are multiple runnable tasks on a given CPU,
|
|
there is little point in interrupting that CPU until the current
|
|
running task's timeslice expires, which is almost always way
|
|
longer than the time of the next scheduling-clock interrupt.
|
|
|
|
Better handling of these sorts of situations is future work.
|
|
|
|
o A reboot is required to reconfigure both adaptive idle and RCU
|
|
callback offloading. Runtime reconfiguration could be provided
|
|
if needed, however, due to the complexity of reconfiguring RCU at
|
|
runtime, there would need to be an earthshakingly good reason.
|
|
Especially given that you have the straightforward option of
|
|
simply offloading RCU callbacks from all CPUs and pinning them
|
|
where you want them whenever you want them pinned.
|
|
|
|
o Additional configuration is required to deal with other sources
|
|
of OS jitter, including interrupts and system-utility tasks
|
|
and processes. This configuration normally involves binding
|
|
interrupts and tasks to particular CPUs.
|
|
|
|
o Some sources of OS jitter can currently be eliminated only by
|
|
constraining the workload. For example, the only way to eliminate
|
|
OS jitter due to global TLB shootdowns is to avoid the unmapping
|
|
operations (such as kernel module unload operations) that
|
|
result in these shootdowns. For another example, page faults
|
|
and TLB misses can be reduced (and in some cases eliminated) by
|
|
using huge pages and by constraining the amount of memory used
|
|
by the application. Pre-faulting the working set can also be
|
|
helpful, especially when combined with the mlock() and mlockall()
|
|
system calls.
|
|
|
|
o Unless all CPUs are idle, at least one CPU must keep the
|
|
scheduling-clock interrupt going in order to support accurate
|
|
timekeeping.
|
|
|
|
o If there might potentially be some adaptive-ticks CPUs, there
|
|
will be at least one CPU keeping the scheduling-clock interrupt
|
|
going, even if all CPUs are otherwise idle.
|
|
|
|
Better handling of this situation is ongoing work.
|
|
|
|
o Some process-handling operations still require the occasional
|
|
scheduling-clock tick. These operations include calculating CPU
|
|
load, maintaining sched average, computing CFS entity vruntime,
|
|
computing avenrun, and carrying out load balancing. They are
|
|
currently accommodated by scheduling-clock tick every second
|
|
or so. On-going work will eliminate the need even for these
|
|
infrequent scheduling-clock ticks.
|