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In the context of high-performance computing (HPC), the Operating System
Noise (*osnoise*) refers to the interference experienced by an application
due to activities inside the operating system. In the context of Linux,
NMIs, IRQs, SoftIRQs, and any other system thread can cause noise to the
system. Moreover, hardware-related jobs can also cause noise, for example,
via SMIs.
hwlat_detector is one of the tools used to identify the most complex
source of noise: *hardware noise*.
In a nutshell, the hwlat_detector creates a thread that runs
periodically for a given period. At the beginning of a period, the thread
disables interrupt and starts sampling. While running, the hwlatd
thread reads the time in a loop. As interrupts are disabled, threads,
IRQs, and SoftIRQs cannot interfere with the hwlatd thread. Hence, the
cause of any gap between two different reads of the time roots either on
NMI or in the hardware itself. At the end of the period, hwlatd enables
interrupts and reports the max observed gap between the reads. It also
prints a NMI occurrence counter. If the output does not report NMI
executions, the user can conclude that the hardware is the culprit for
the latency. The hwlat detects the NMI execution by observing
the entry and exit of a NMI.
The osnoise tracer leverages the hwlat_detector by running a
similar loop with preemption, SoftIRQs and IRQs enabled, thus allowing
all the sources of *osnoise* during its execution. Using the same approach
of hwlat, osnoise takes note of the entry and exit point of any
source of interferences, increasing a per-cpu interference counter. The
osnoise tracer also saves an interference counter for each source of
interference. The interference counter for NMI, IRQs, SoftIRQs, and
threads is increased anytime the tool observes these interferences' entry
events. When a noise happens without any interference from the operating
system level, the hardware noise counter increases, pointing to a
hardware-related noise. In this way, osnoise can account for any
source of interference. At the end of the period, the osnoise tracer
prints the sum of all noise, the max single noise, the percentage of CPU
available for the thread, and the counters for the noise sources.
Write the ASCII text "osnoise" into the current_tracer file of the
tracing system (generally mounted at /sys/kernel/tracing).
For example::
[root@f32 ~]# cd /sys/kernel/tracing/
[root@f32 tracing]# echo osnoise > current_tracer
It is possible to follow the trace by reading the trace trace file::
[root@f32 tracing]# cat trace
# tracer: osnoise
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth MAX
# || / SINGLE Interference counters:
# |||| RUNTIME NOISE % OF CPU NOISE +-----------------------------+
# | | | |||| | | | | | | | | | |
<...>-859 [000] .... 81.637220: 1000000 190 99.98100 9 18 0 1007 18 1
<...>-860 [001] .... 81.638154: 1000000 656 99.93440 74 23 0 1006 16 3
<...>-861 [002] .... 81.638193: 1000000 5675 99.43250 202 6 0 1013 25 21
<...>-862 [003] .... 81.638242: 1000000 125 99.98750 45 1 0 1011 23 0
<...>-863 [004] .... 81.638260: 1000000 1721 99.82790 168 7 0 1002 49 41
<...>-864 [005] .... 81.638286: 1000000 263 99.97370 57 6 0 1006 26 2
<...>-865 [006] .... 81.638302: 1000000 109 99.98910 21 3 0 1006 18 1
<...>-866 [007] .... 81.638326: 1000000 7816 99.21840 107 8 0 1016 39 19
In addition to the regular trace fields (from TASK-PID to TIMESTAMP), the
tracer prints a message at the end of each period for each CPU that is
running an osnoise/ thread. The osnoise specific fields report:
- The RUNTIME IN US reports the amount of time in microseconds that
the osnoise thread kept looping reading the time.
- The NOISE IN US reports the sum of noise in microseconds observed
by the osnoise tracer during the associated runtime.
- The % OF CPU AVAILABLE reports the percentage of CPU available for
the osnoise thread during the runtime window.
- The MAX SINGLE NOISE IN US reports the maximum single noise observed
during the runtime window.
- The Interference counters display how many each of the respective
interference happened during the runtime window.
Note that the example above shows a high number of HW noise samples.
The reason being is that this sample was taken on a virtual machine,
and the host interference is detected as a hardware interference.
Tracer options
The tracer has a set of options inside the osnoise directory, they are:
- osnoise/cpus: CPUs at which a osnoise thread will execute.
- osnoise/period_us: the period of the osnoise thread.
- osnoise/runtime_us: how long an osnoise thread will look for noise.
- osnoise/stop_tracing_us: stop the system tracing if a single noise
higher than the configured value happens. Writing 0 disables this
- osnoise/stop_tracing_total_us: stop the system tracing if total noise
higher than the configured value happens. Writing 0 disables this
- tracing_threshold: the minimum delta between two time() reads to be
considered as noise, in us. When set to 0, the default value will
will be used, which is currently 5 us.
Additional Tracing
In addition to the tracer, a set of tracepoints were added to
facilitate the identification of the osnoise source.
- osnoise:sample_threshold: printed anytime a noise is higher than
the configurable tolerance_ns.
- osnoise:nmi_noise: noise from NMI, including the duration.
- osnoise:irq_noise: noise from an IRQ, including the duration.
- osnoise:softirq_noise: noise from a SoftIRQ, including the
- osnoise:thread_noise: noise from a thread, including the duration.
Note that all the values are *net values*. For example, if while osnoise
is running, another thread preempts the osnoise thread, it will start a
thread_noise duration at the start. Then, an IRQ takes place, preempting
the thread_noise, starting a irq_noise. When the IRQ ends its execution,
it will compute its duration, and this duration will be subtracted from
the thread_noise, in such a way as to avoid the double accounting of the
IRQ execution. This logic is valid for all sources of noise.
Here is one example of the usage of these tracepoints::
osnoise/8-961 [008] d.h. 5789.857532: irq_noise: local_timer:236 start 5789.857529929 duration 1845 ns
osnoise/8-961 [008] dNh. 5789.858408: irq_noise: local_timer:236 start 5789.858404871 duration 2848 ns
migration/8-54 [008] d... 5789.858413: thread_noise: migration/8:54 start 5789.858409300 duration 3068 ns
osnoise/8-961 [008] .... 5789.858413: sample_threshold: start 5789.858404555 duration 8812 ns interferences 2
In this example, a noise sample of 8 microseconds was reported in the last
line, pointing to two interferences. Looking backward in the trace, the
two previous entries were about the migration thread running after a
timer IRQ execution. The first event is not part of the noise because
it took place one millisecond before.
It is worth noticing that the sum of the duration reported in the
tracepoints is smaller than eight us reported in the sample_threshold.
The reason roots in the overhead of the entry and exit code that happens
before and after any interference execution. This justifies the dual
approach: measuring thread and tracing.