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# scheduler

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Module

scheduler

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Module Summary

Measure scheduler utilization

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Since

Module scheduler was introduced in OTP 21.0.

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Description

This module contains utility functions for easy measurement and calculation of scheduler utilization. It act as a wrapper around the more primitive API erlang:statistics(scheduler_wall_time).

The simplest usage is to call the blocking scheduler:utilization(Seconds).

For non blocking and/or continuous calculation of scheduler utilization, the recommended usage is:

- First call erlang:system_flag(scheduler_wall_time,true) to enable scheduler wall time measurements.

- Call get_sample/0 to collect samples with some time in between.

- Call utilization/2 to calculate the scheduler utilization in the interval between two samples.

- When done call erlang:system_flag(scheduler_wall_time,false) to disable scheduler wall time measurements and avoid unecessary cpu overhead.

To get correct values from utilization/2, it is important that scheduler_wall_time is kept enabled during the entire interval between the two samples. To ensure this, the process that called erlang:system_flag(scheduler_wall_time,true) must be kept alive, as scheduler_wall_time will automatically be disabled if it terminates.

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Data Types

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sched_sample()

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sched_type() = normal | cpu | io

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sched_id() = integer()

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sched_util_result() =
[{sched_type(), sched_id(), float(), string()} |
{total, float(), string()} |
{weighted, float(), string()}]

A list of tuples containing results for individual schedulers as well as aggregated averages. Util is the scheduler utilization as a floating point value between 0.0 and 1.0. Percent is the same utilization as a more human readable string expressed in percent.

{normal, SchedulerId, Util, Percent}
  Scheduler utilization of a normal scheduler with number SchedulerId. Schedulers that are not online will also be included. Online schedulers have the lowest SchedulerId.
{cpu, SchedulerId, Util, Percent}
  Scheduler utilization of a dirty-cpu scheduler with number SchedulerId.
{io, SchedulerId, Util, Percent}
  Scheduler utilization of a dirty-io scheduler with number SchedulerId. This tuple will only exist if both samples were taken with sample_all/0.
{total, Util, Percent}
  Total utilization of all normal and dirty-cpu schedulers.
{weighted, Util, Percent}
  Total utilization of all normal and dirty-cpu schedulers, weighted against maximum amount of available CPU time.

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Exports

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get_sample() -> sched_sample() | undefined
OTP 24.3

Returns a scheduler utilization sample for normal and dirty-cpu schedulers. Returns undefined if system flag scheduler_wall_time has not been enabled.

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get_sample_all() -> sched_sample() | undefined
OTP 24.3

Return a scheduler utilization sample for all schedulers, including dirty-io schedulers. Returns undefined if system flag scheduler_wall_time has not been enabled.

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sample() -> sched_sample()
OTP 21.0

Return a scheduler utilization sample for normal and dirty-cpu schedulers. Will call erlang:system_flag(scheduler_wall_time,true) first if not already already enabled.

Note

This function is not recommended as there is no way to detect if scheduler_wall_time already was enabled or not. If scheduler_wall_time has been disabled between two samples, passing them to utilization/2 will yield invalid results.

Instead use get_sample/0 together with erlang:system_flag(scheduler_wall_time,_).

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sample_all() -> sched_sample()
OTP 21.0

Return a scheduler utilization sample for all schedulers, including dirty-io schedulers. Will call erlang:system_flag(scheduler_wall_time,true) first if not already already enabled.

Note

This function is not recommended for same reason as sample/0. Instead use get_sample_all/0 together with erlang:system_flag(scheduler_wall_time,_).

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utilization(Seconds) -> sched_util_result()
OTP 21.0

### Types

Seconds = integer() >= 1

Measure utilization for normal and dirty-cpu schedulers during Seconds seconds, and then return the result.

Will automatically first enable and then disable scheduler_wall_time.

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utilization(Sample) -> sched_util_result()
OTP 21.0

### Types

Sample = sched_sample()

Calculate scheduler utilizations for the time interval from when Sample was taken and "now". The same as calling scheduler:utilization(Sample, scheduler:sample_all()).

Note

This function is not recommended as it's so easy to get invalid results without noticing. In particular do not do this:

    scheduler:utilization(scheduler:sample()). % DO NOT DO THIS!

The above example takes two samples in rapid succession and calculates the scheduler utilization between them. The resulting values will probably be more misleading than informative.

Instead use scheduler:utilization/2 and call get_sample/0 to get samples with some time in between.

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utilization(Sample1, Sample2) -> sched_util_result()
OTP 21.0

### Types

Sample1 = Sample2 = sched_sample()

Calculates scheduler utilizations for the time interval between the two samples obtained from calling get_sample/0 or get_sample_all/0.

This function itself, does not need scheduler_wall_time to be enabled. However, for a correct result, scheduler_wall_time must have been enabled during the entire interval between the two samples.