Device manager

sys-fs/eudev

To switch from udev to eudev, simply do:

$ cat /etc/portage/package.mask/systemd 
sys-apps/systemd
sys-fs/udev

In /etc/portage/make.conf:

# Do NOT add -udev simply because you want to use eudev!
USE=" ... -systemd ..."

Then switch:

emerge -C sys-fs/udev && emerge -1 sys-fs/eudev

Do not reboot the computer until you are sure that the process has successfully completed!

If you wish to use eudev instead of udev, you may mask the package sys-fs/udev, but do NOT make the mistake of adding the -udev US flag in make.conf.

You may have:

$ cat /etc/portage/package.mask/systemd 
sys-apps/systemd
sys-fs/udev

But in /etc/portage/make.conf:

# Do NOT add -udev simply because you want to use eudev!
USE=" ... -systemd ..."