This article shows how ACPI can effectively be disabled. This method is "semi-permanent" in that it will probably need to be disabled again when the firewall is upgraded.
There is typically no need to disable ACPI on current versions of pfSense with reasonably recent hardware. There may still be some edge cases that require it to be disabled, however.
There have been 2 main reports floating around the forums, mailing list, and IRC as to why someone might want to disable ACPI.
Calcru messages are typically harmless, though they can get quite annoying because they can fill up logs very fast.
The messages look like this:
calcru: negative runtime of -350418 usec for bufdaemon
To alleviate the problem, add the following line to /boot/device.hints at the very bottom:
Reboot after making the change.
This issue came by way of the mailing list. The admin reported that on an IBM X335 with Intel Xeon 2.8ghz and 2.5 GB of RAM), it would take the system upwards of 6 minutes to get fully booted with an older version of pfSense. The majority of the time was spent at the following message:
smp: AP CPU #1 Launched!
Booting the same system with ACPI disabled (via the boot menu), it came up in under a minute. Use the same procedure as above to disable ACPI.
The hardware may not power itself off when halting with ACPI disabled.