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This commit updates the description of perftune.py. It is based on the information in the reported issue (below), the contents of help for perftune.py, and the input from @vladzcloudius. Fixes https://github.com/scylladb/scylladb/issues/14233 Closes #14879
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Seastar Perftune
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================
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ScyllaDB's Seastar Perftune helps you run ScyllaDB with its maximum performance.
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The `perftune.py <https://github.com/scylladb/seastar/blob/master/scripts/perftune.py>`_ script
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handles network and disk tuning, reassigning IRQs to specific CPU cores and freeing
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the other cores for ScyllaDB’s exclusive use.
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More specifically, running the script will:
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* Ban relevant IRQs from being moved by irqbalance.
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* Configure various system parameters in /proc/sys.
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* Distribute the IRQs (using SMP affinity configuration) among CPUs according to
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the ``irq_cpu_mask`` value. Note that more than one physical core will be used
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for IRQ handling for large machines.
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To ensure that the tuning persists after a host reboot, we recommend creating a systemd unit
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that will run on host startup and re-tune the hosts.
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perftune.yaml
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------------------
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The ``perfrune.yaml`` file is the output file of the ``perftune.py`` script
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(with the ``--dump-options-file`` option enabled). The script determines
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the ``irq_cpu_mask`` and prints it to ``perftume.yaml`` on the first machine.
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It's important to use the same ``irq_cpu_mask`` value on other machines, even if more machines are
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added to the cluster, to avoid a mixed-cluster situation.
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Note that ``perftune.yaml`` is generated only if the file doesn’t exist.
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Available Options
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------------------------
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You can run ``perftune.py`` with the following options:
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.. list-table::
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:widths: 30 70
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:header-rows: 1
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* - Option
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- Description
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* - ``--arfs``
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- Enables/disables aRFS.
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* - ``--cpu-mask``
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- The masks of cores to use. The default is all available cores.
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* - ``--dev``
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- Specifies the device to optimize; for example, ``sda1``. You can use it more than
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once to specify multiple devices.
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* - ``--dir``
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- Specifies the directory to optimize. You can use it more than once to specify
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multiple directories.
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* - ``--dry-run``
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- Prints recommendations on what to do without taking any action.
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* - ``--dump-options-file``
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- Prints the configuration YAML file containing the current configuration.
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* - ``--get-cpu-mask``
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- Prints the CPU mask to be used for computation.
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* - ``--get-cpu-mask-quiet``
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- Prints the CPU mask to be used for computation. Prints the zero CPU set if that's
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what it is.
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* - ``--get-irq-cpu-mask``
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- Prints the CPU mask to be used for IRQs binding.
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* - ``--irq-core-auto-detection-ratio``
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- Specifies a ratio for IRQ mask auto-detection. For example, if 8 is given and
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auto-detection is requested, a single IRQ CPU core is going to be allocated
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for every 8 CPU cores out of available according to the ``cpu_mask`` value.
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The default is 16.
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* - ``--irq-cpu-mask``
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- The mask of the cores to be used for IRQs binding.
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* - ``--nic``
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- The network interface name. The default is ``eth0``. You can use it more than once
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to specify multiple interfaces.
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* - ``--num-rx-queues``
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- Sets a given number of Rx queues.
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* - ``--options-file``
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- The configuration YAML file.
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* - ``--tune``
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- Specifies the components to configure. You can use it more than once to specify
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multiple values. The available values are:
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* ``net`` - Enables tuning the network.
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* ``disks`` - Enables tuning the disks.
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* ``system`` - Enables tuning system-related components.
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* - ``--tune-clock``
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- Forces tuning of /system/clocksource. The default is ``false``. Requires the ``--tune`` option.
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* - ``--verbose``
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- Provides additional details about operations and their results.
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* - ``--write-back-cache``
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- Enables/disables the *write back* write cache mode.
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