FreeNAS Power Consumption and ACPI

As I alluded to in part 4 and part 5 of my recent ‘FreeNAS 9.10 Lab Build’ series, I could achieve better power consumption figures after enabling deeper C states. I first noticed this a year or two ago when I had re-purposed a Shuttle SH67H3 cube PC for use with FreeNAS. I was familiar with the expected power draw of this system when using it with other operating systems in the past, but with FreeNAS installed, it seemed higher than it should have been.

After doing some digging on the subject, I came across a thread on the FreeNAS forums describing the default ACPI C-state used and information on how to modify it.

A Bit of Background on ACPI C-States

What are often referred to as ‘C-States’ are basically levels of CPU power savings defined by the ACPI (Advanced Configuration and Power Interface) standard. This standard defines states for other devices in the system as well – not just CPUs – but all states prefixed by a ‘C’ refer to CPU power states.

The states supported by a system will often vary depending on the age of the system and the CPU manufacturer, but most modern Intel based systems will support ACPI states C0, C1, C2 and C3. Some older systems only supported C1 and a lower power state called C1E.

C0 is essentially the state where the CPU is completely awake, operating at its full frequency and performance potential and actually executing instructions. The higher the ‘C’ value, the deeper into power savings or sleep modes the CPU can go.

All ACPI compliant systems must support another state called C1. In the C1 state, the CPU is basically at idle and isn’t executing any instructions. The key requirement for the C1 state is that the CPU must be able to return to the C0 state to execute instructions immediately without any latency or delay. Because of this requirement, there are only so many power saving tweaks that the CPU can implement.

This is where the C2 state comes in, also known as ‘Stop-Clock’. In this ACPI idle state, additional power savings features can be used. This is where modern Intel processors shine. They can do all sorts of power saving wizardry, like turning off unused portions of the CPU or even entire idle CPU cores. Because there can be a very slight delay in returning to the C0 state when using these these features, they cannot be implemented when limited to ACPI C1.

Enabling Deeper ACPI C-States

By default, FreeNAS has ACPI configured to use only the C1 power state. Presumably, this is to guarantee maximum performance and prevent any quirks with older CPUs switching between power states.

If maximum performance and performance consistency is desired over power savings – as is often the case in critical production environments – leaving the default at C1 is probably a wise choice. This choice also becomes much less important if your system is heavily utilized and spends very little time at idle anyway. But if you are like me, running a home lab, idle power consumption is an important consideration.

From an SSH or console prompt, you can determine the detected and supported C-States by querying the relevant ACPI sysctls:

[root@freenas] ~# sysctl -a | grep cx_
hw.acpi.cpu.cx_lowest: C1
dev.cpu.3.cx_usage: 100.00% 0.00% last 19us
dev.cpu.3.cx_lowest: C1
dev.cpu.3.cx_supported: C1/1/1 C2/3/96
dev.cpu.2.cx_usage: 100.00% 0.00% last 136us
dev.cpu.2.cx_lowest: C1
dev.cpu.2.cx_supported: C1/1/1 C2/3/96
dev.cpu.1.cx_usage: 100.00% 0.00% last 2006us
dev.cpu.1.cx_lowest: C1
dev.cpu.1.cx_supported: C1/1/1 C2/3/96
dev.cpu.0.cx_usage: 100.00% 0.00% last 2101us
dev.cpu.0.cx_lowest: C1
dev.cpu.0.cx_supported: C1/1/1 C2/3/96

As you can see above, my Xeon X3430 supports C1, C2 and C3 states. The cx_usage value appears to report how much time the processor spends in that particular state and the cx_lowest reports the deepest allowed state.

In the C1 state, my system is idling at about 95W total power consumption.

I had originally suspected that Intel SpeedStep technology (a feature that provides dynamic frequency and voltage scaling at idle) was not functioning in ACPI C1, but that doesn’t seem to be the case. My CPU’s normal (non turbo-boost) frequency is 2.4GHz. If SpeedStep is functional, I’d expect it to use one of the following lower power states as defined in the following sysctl:

[root@freenas] ~# sysctl -a | grep dev.cpu.0.freq_levels
dev.cpu.0.freq_levels: 2395/95000 2394/95000 2261/78000 2128/63000 1995/57000 1862/46000 1729/36000 1596/32000 1463/25000 1330/19000 1197/17000

As seen above, the processor should be able to scale down to 1197MHz at idle. Even with the powerd daemon stopped, you can still use the powerd command line tool to see what the current CPU frequency is as well as any changes as load increases. Using powerd with the -v verbose option, we can see that the processor frequency does indeed jump up and down in the ACPI C1 state and stabilizes at 1197MHz when idle:

[root@freenas] ~# powerd -v
changing clock speed from 1463 MHz to 1330 MHz
load   4%, current freq 1330 MHz ( 9), wanted freq 1263 MHz
load   0%, current freq 1330 MHz ( 9), wanted freq 1223 MHz
load   0%, current freq 1330 MHz ( 9), wanted freq 1197 MHz
changing clock speed from 1330 MHz to 1197 MHz
load  26%, current freq 1197 MHz (10), wanted freq 1197 MHz
load   3%, current freq 1197 MHz (10), wanted freq 1197 MHz
load   0%, current freq 1197 MHz (10), wanted freq 1197 MHz
load   4%, current freq 1197 MHz (10), wanted freq 1197 MHz

You can change the lowest allowed ACPI state by using the following command. In this example, I will allow the system to use more advanced power saving features by setting cx_lowest to C2:

[root@freenas] ~# sysctl hw.acpi.cpu.cx_lowest=C2
hw.acpi.cpu.cx_lowest: C1 -> C2

After making this change, the system power consumption immediately dropped down to about 75W. That’s more than 25% – not bad!

Now if we repeat the previous command, we can see some different reporting:

[root@freenas] ~# sysctl -a | grep cx_
hw.acpi.cpu.cx_lowest: C2
dev.cpu.3.cx_usage: 0.00% 100.00% last 19695us
dev.cpu.3.cx_lowest: C2
dev.cpu.3.cx_supported: C1/1/1 C2/3/96
dev.cpu.2.cx_usage: 5.12% 94.87% last 23us
dev.cpu.2.cx_lowest: C2
dev.cpu.2.cx_supported: C1/1/1 C2/3/96
dev.cpu.1.cx_usage: 1.87% 98.12% last 255us
dev.cpu.1.cx_lowest: C2
dev.cpu.1.cx_supported: C1/1/1 C2/3/96
dev.cpu.0.cx_usage: 1.28% 98.71% last 1200us
dev.cpu.0.cx_lowest: C2
dev.cpu.0.cx_supported: C1/1/1 C2/3/96

Part of the reason the power savings was so significant is because the system is spending over 95% of it’s time at idle in the C2 state.

Making it Stick

One thing you’ll notice is that after rebooting, this change will revert back to the default again. Since this is a sysctl, you’d think this could just be added as a system tunable parameter in the UI. I tried this to no avail.

After doing some digging, I found a bug reported on this. It appears that this is a known problem due to other rc.d scripts interfering with the cx_lowest sysctl.

I took a look in the /etc/rc.d directory and found a script called power_profile. It does indeed appear to be overwriting the ACPI lowest state at bootup:

[root@freenas] ~# cat /etc/rc.d/power_profile
# Set the various sysctls based on the profile's values.
eval value=\$${profile}_cx_lowest

I could probably get by this issue by modifying the startup scripts, but the better solution is to simply add the required command to the list of post-init scripts. This can be done from the UI in the following location:


They key thing is to ensure is that the command is run ‘post init’. When that is done, the setting sticks and is applied after the power_profile script.

Hopefully this could save you a few dollars on your power bill as well!

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