Today I was using my laptop on battery power and I remembered that I hadn't disabled wifi power management since I reinstalled Ubuntu in late 2016. With power management enabled for wifi, I find I get laggy internet on battery power, and pings are noticeably higher.

If you suspect this is affecting you, it's easy to check:

$ iwconfig 2> /dev/null | grep "Power Management"
Power Management:on

Unfortunately, the method I wrote about previously to disable wifi power management no longer works. My guess is that this is due to Ubuntu switching to systemd in 2015.

Instead, you need to use an alternative approach as suggested in this post.

First, look at your existing configuration:

$ cd /etc/NetworkManager/conf.d/
$ ls
$ sudo cp default-wifi-powersave-on.conf wifi-powersave-off.conf

The new .conf filename must follow any other files alphabetically, in this case the filename must be one that's alphabetically later than default-wifi-powersave-on, so that it gets applied last, and your modified setting overrides the default. If we named this file, e.g. a-wifi-powersave-off-config.conf, our new settings wouldn't apply since the filename would then be ahead of default-wifi-powersave-on.conf alphabetically.

Now, edit the new file:

$ sudoedit wifi-powersave-off.conf

Change what is likely a 3 in the wifi.powersave line to 2. This disables wireless power management.

It likely will look like this when you've edited it:

wifi.powersave = 2

Now, restart NetworkManager and you should see that this change is now in effect. It should also persist across reboots. You can confirm this again with iwconfig:

$ sudo systemctl restart NetworkManager
$ iwconfig 2> /dev/null | grep "Power Management"
Power Management:off