Re: Reducing power consumption on idle servers

Hi,

On 2022-11-17 13:06:23 +0530, Bharath Rupireddy wrote:
> I understand. I know it's a bit hard to measure the power savings, I'm
> wondering if there's any info, maybe not necessarily related to
> postgres, but in general how much power gets saved if a certain number
> of waits/polls/system calls are reduced.

It's heavily hardware and hardware settings dependent.

On some systems you can get an *approximate* idea of the power usage even
while plugged in. On others you can only see it while on battery power.

On systems with RAPL support (most Intel and I think newer AMD CPUs) you can
query power usage with:
powerstat -R 1 (adding -D provides a bit more detail)

But note that RAPL typically severely undercounts power usage because it
doesn't cover a lot of sources of power usage (display, sometimes memory,
all other peripherals).

Sometimes powerstat -R -D can split power usage up more granularly,
e.g. between the different CPU sockets and memory.

On laptops you can often measure the discharge rate when not plugged in, with
powerstat -d 0 1. But IME the latency till the values update makes it harder
to interpret.

On some workstations / servers you can read the power usage via ACPI. E.g. on
my workstation 'sensors' shows a power_meter-acpi-0

As an example of the difference it can make, here's the output of
powerstat -D 1
on my laptop.

Time User Nice Sys Idle IO Run Ctxt/s IRQ/s Fork Exec Exit Watts
16:41:55 0.1 0.0 0.1 99.8 0.0 1 403 246 1 0 1 2.62
16:41:56 0.1 0.0 0.1 99.8 0.0 1 357 196 1 0 1 2.72
16:41:57 0.1 0.0 0.1 99.6 0.2 1 510 231 4 0 4 2.64
16:41:58 0.1 0.0 0.1 99.9 0.0 2 1350 758 64 62 63 4.06
16:41:59 0.3 0.0 1.0 98.7 0.0 2 4166 2406 244 243 244 7.20
16:42:00 0.2 0.0 0.7 99.1 0.0 2 4203 2353 247 246 247 7.21
16:42:01 0.5 0.0 1.6 98.0 0.0 2 4079 2395 240 239 240 7.08
16:42:02 0.5 0.0 0.9 98.7 0.0 2 4097 2405 245 243 245 7.20
16:42:03 0.4 0.0 1.3 98.3 0.0 2 4117 2311 243 242 243 7.14
16:42:04 0.1 0.0 0.4 99.4 0.1 1 1721 1152 70 70 71 4.48
16:42:05 0.1 0.0 0.2 99.8 0.0 1 433 250 1 0 1 2.92
16:42:06 0.0 0.0 0.3 99.7 0.0 1 400 231 1 0 1 2.66

In the period of higher power etc usage I ran
while true;do sleep 0.001;done
and then interupted that after a bit with ctrl-c.

Same thing on my workstation (:

Time User Nice Sys Idle IO Run Ctxt/s IRQ/s Fork Exec Exit Watts
16:43:48 1.0 0.0 0.2 98.7 0.1 1 8218 2354 0 0 0 46.43
16:43:49 1.1 0.0 0.3 98.7 0.0 1 7866 2477 0 0 0 45.99
16:43:50 1.1 0.0 0.4 98.5 0.0 2 7753 2996 0 0 0 48.93
16:43:51 0.8 0.0 1.7 97.5 0.0 1 9395 5285 0 0 0 75.48
16:43:52 0.5 0.0 1.7 97.8 0.0 1 9141 4806 0 0 0 75.30
16:43:53 1.1 0.0 1.8 97.1 0.0 2 10065 5504 0 0 0 76.27
16:43:54 1.3 0.0 1.5 97.2 0.0 1 10962 5165 0 0 0 76.33
16:43:55 0.9 0.0 0.8 98.3 0.0 1 8452 3939 0 0 0 61.99
16:43:56 0.6 0.0 0.1 99.3 0.0 2 6541 1999 0 0 0 40.92
16:43:57 0.9 0.0 0.2 98.9 0.0 2 8199 2477 0 0 0 42.91

And if I query the power supply via ACPI instead:

while true;do sensors power_meter-acpi-0|grep power1|awk '{print $2, $3}';sleep 1;done
...
163.00 W
173.00 W
173.00 W
172.00 W
203.00 W
206.00 W
206.00 W
206.00 W
209.00 W
205.00 W
211.00 W
213.00 W
203.00 W
175.00 W
166.00 W
...

As you can see the difference is quite substantial. This is solely due to a
1ms sleep loop (albeit one where we fork a process after each sleep, which
likely is a good chunk of the CPU usage).

However, the difference in power usage will be far smaller if the system
already busy, because the CPU will already run at a high frequency.

Greetings,

Andres Freund