# Seastar Perftune

ScyllaDB’s Seastar Perftune helps you run ScyllaDB with its maximum performance.
The [perftune.py](https://github.com/scylladb/seastar/blob/master/scripts/perftune.py) script
handles network and disk tuning, reassigning IRQs to specific CPU cores and freeing
the other cores for ScyllaDB’s exclusive use.

More specifically, running the script will:

* Ban relevant IRQs from being moved by irqbalance.
* Configure various system parameters in /proc/sys.
* Distribute the IRQs (using SMP affinity configuration) among CPUs according to
  the `irq_cpu_mask` value. Note that more than one physical core will be used
  for IRQ handling for large machines.

To ensure that the tuning persists after a host reboot, we recommend creating a systemd unit
that will run on host startup and re-tune the hosts.

## perftune.yaml

The `perfrune.yaml` file is the output file of the `perftune.py` script
(with the `--dump-options-file` option enabled). The script determines
the `irq_cpu_mask` and prints it to `perftume.yaml` on the first machine.

It’s important to use the same `irq_cpu_mask` value on other machines, even if more machines are
added to the cluster, to avoid a  mixed-cluster situation.

Note that `perftune.yaml` is generated only if the file doesn’t exist.

## Available Options

You can run `perftune.py` with the following options:

| Option                            | Description                                                                                                                                                                                                                                                                              |
|-----------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `--arfs`                          | Enables/disables aRFS.                                                                                                                                                                                                                                                                   |
| `--cpu-mask`                      | The masks of cores to use. The default is all available cores.                                                                                                                                                                                                                           |
| `--dev`                           | Specifies the device to optimize; for example, `sda1`. You can use it more than<br/>once to specify multiple devices.                                                                                                                                                                    |
| `--dir`                           | Specifies the directory to optimize. You can use it more than once to specify<br/>multiple directories.                                                                                                                                                                                  |
| `--dry-run`                       | Prints recommendations on what to do without taking any action.                                                                                                                                                                                                                          |
| `--dump-options-file`             | Prints the configuration YAML file containing the current configuration.                                                                                                                                                                                                                 |
| `--get-cpu-mask`                  | Prints the CPU mask to be used for computation.                                                                                                                                                                                                                                          |
| `--get-cpu-mask-quiet`            | Prints the CPU mask to be used for computation. Prints the zero CPU set if that’s<br/>what it is.                                                                                                                                                                                        |
| `--get-irq-cpu-mask`              | Prints the CPU mask to be used for IRQs binding.                                                                                                                                                                                                                                         |
| `--irq-core-auto-detection-ratio` | Specifies a ratio for IRQ mask auto-detection. For example, if 8 is given and<br/>auto-detection is requested, a single IRQ CPU core is going to be allocated<br/>for every 8 CPU cores out of available according to the `cpu_mask` value.<br/>The default is 16.                       |
| `--irq-cpu-mask`                  | The mask of the cores to be used for IRQs binding.                                                                                                                                                                                                                                       |
| `--nic`                           | The network interface name. The default is `eth0`. You can use it more than once<br/>to specify multiple interfaces.                                                                                                                                                                     |
| `--num-rx-queues`                 | Sets a given number of Rx queues.                                                                                                                                                                                                                                                        |
| `--options-file`                  | The configuration YAML file.                                                                                                                                                                                                                                                             |
| `--tune`                          | Specifies the components to configure. You can use it more than once to specify<br/>multiple values. The available values are:<br/><br/>> * `net` - Enables tuning the network.<br/>> * `disks` - Enables tuning the disks.<br/>> * `system` - Enables tuning system-related components. |
| `--tune-clock`                    | Forces tuning of /system/clocksource. The default is `false`. Requires the `--tune` option.                                                                                                                                                                                              |
| `--verbose`                       | Provides additional details about operations and their results.                                                                                                                                                                                                                          |
| `--write-back-cache`              | Enables/disables the *write back* write cache mode.                                                                                                                                                                                                                                      |