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# PID Control Tuning & Logging
The openBMC PID control daemon, swampd (phosphor-pid-control) requires the user
to specify the sensors and PID coefficients. Determining good coefficients is
beyond the scope of this document.
**NOTE** The steps below may not be applicable if you are using D-Bus based configuration.
## Tuning Fan PID Using a Fixed RPM Setpoint
Flag `"-t"` can be specified to enabled the daemon to read the fan RPM setpoint
from a file `/etc/thermal.d/setpoint` instead from D-Bus.
The value in the setpoint file is expected to be a normal decimal integer, such
as `4000` or `5000`, and is in RPM.
## Tuning Fan PID Control Parameters
`phosphor-pid-control` reads PID control values from
`/usr/share/swampd/config.json`, one can modify the PID parameters in the config
file and restart the daemon to make the new values effective.
```
systemctl restart swampd.service
```
## Logging
Flag `"-l \<path\>"` can be specified to enable the daemon to log fan control
data into `path`. The log output is in CSV format with the following header:
```
epoch_ms,setpt,fan1,fan2,...fanN,fleeting,failsafe
```
`phosphor-pid-control` will create a log for each PID control zone.
## Fan RPM Tuning Helper script
`https://github.com/openbmc/phosphor-pid-control/blob/master/tools/fan_rpm_loop_test.sh`
is an example script on how to sweep through available RPM setpoints and log the
fan responses.
## Thermal Tuning Example
1. Create initial `/usr/share/swampd/config.json` used for PID control
2. (Option 1) If using a fixed setpoint, write the value to
`/etc/thermal.d/setpoint`, run swampd manually with `swampd -l ${LOG_PATH}&`, and kill the
process after desired duration.
3. (Option 2) If sweeping fan setpoint, using the tuning helper script `fan_rpm_loop_test.sh` to
configure fan setpoint in steps and collect logs
4. Parse logs from `${LOG_PATH}/zone_*.log` and analyze response data
6. Modify `/usr/share/swampd/config.json` as needed
7. Repeat from step 2 or step 3
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