Q: discoveryd process uses 100% CPU - Safari Can't find the server

OS X Yosemite: Reset your computer’s PRAM

Here is permanent fix I found on another site osxdaily.com

1: Remove Network Configuration & Preference Files

Manually trashing the network plist files should be your first line of troubleshooting. This is one of those tricks that consistently resolves even the most stubborn wireless problems on Macs of nearly any OS X version. This is particularly effective for Macs who updated to Yosemite that may have a corrupt or dysfunctional preference file mucking things up:

1. Turn Off Wi-Fi from the Wireless menu item
2. From the OS X Finder, hit Command+Shift+G and enter the following path:

/Library/Preferences/SystemConfiguration/

1. Within this folder locate and select the following files:

com.apple.airport.preferences.plist

com.apple.network.identification.plist
com.apple.wifi.message-tracer.plist

NetworkInterfaces.plist

preferences.plist

1. Move all of these files into a folder on your Desktop called ‘wifi backups’ or something similar – we’re backing these up just in case you break something but if you regularly backup your Mac you can just delete the files instead since you could restore from Time Machine if need be
2. Reboot the Mac
3. Turn ON WI-Fi from the wireless network menu again

This forces OS X to recreate all network configuration files. This alone may resolve your problems, but if you’re continuing to have trouble we recommend following through with the second step which means using some custom network settings.

If you don't already have a current backup, back up all data, then reinstall the OS.* You don't need to erase the startup volume, and you won't need the backup unless something goes wrong. If the system was upgraded from an older version of OS X, you may need the Apple ID and password you used.

If you use FileVault 2, then before running the Installer you must launch Disk Utility and select the icon of the FileVault startup volume ("Macintosh HD," unless you gave it a different name.) It will be nested below another icon with the same name. Click the Unlock button in the toolbar and enter your login password when prompted. Then quit Disk Utility to be returned to the main Recovery screen.

There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode. All other data will be preserved.

*The linked support article refers to OS X 10.10 ("Yosemite"), but the procedure is the same for OS X 10.7 ("Lion") and later.

reset / delete system network preferences settings, restart computer  for a permanent fix as explained in details at http://osxdaily.com/2014/10/25/fix-wi-fi-problems-os-x-yosemite/

I would not go through the trouble of reinstalling the OS.

I have the same issue you describe across two completely different iMacs, one of which is a brand new 5k Retina.  There is definitely something broken in the discoveryd process, perhaps a memory leak.

After running some amount of time, it increases in both CPU demand and memory.  Once it grows to a certain point, it starts blowing all sorts of errors in console and cannot resolve IP addresses.

Rebooting causes the process to reset so provides temporary relief, as does the terminal command referenced above.  However, I am unsure whether the terminal command reestablishes the bonjour connections necessary for networked Macs to see each other.  I tried just quitting the process within Activity manager and going that route definitely disconnects Mac to Mac networking until reboot. :-(

Any other suggestions from the community are welcome as I've run out of ideas.  I have an open case with Apple Engineering, but that's a long road with limited interaction.

*Update*

By reducing the number of apps that I had running in background making DNS calls, this last reboot lasted about 48 hours before discoveryd started blowing lots of console errors.  For the most part, this occurred by repeated use of Safari although I'm sure some other aspects of the system were making DNS calls as well.

Memory and CPU usage never went above 50MB and 9% respectively.  CPU usage settled down after I stopped actively surfing pages so, strictly speaking, a reboot wasn't required this last time since had I not been checking logs, i wouldn't have been aware of the errors.

I'm going to let the system work and see what happens this time...if it's the same as above, then I'll reboot once the errors start and reintroduce those Apps and see if things go casters up faster and with more effect.

I'd encourage everyone to get an active case going with a Senior Apple Tech and Engineering...if they start seeing those console logs coming from multiple places, it might get more focus than would otherwise.

This is also happening to me (and this guy). I haven't found a permanent fix. Quitting discoveryd in Activity Monitor clears it up instantly, but only temporarily. I don't use Adium or Wi-Fi, and I've installed the 10.10.1 update.

Do you have any 3rd party apps that continually poll web sites...for example...and RSS reader like Newsbar?

As for quitting the discoveryd process from Activity Monitor, like you, I found it temporarily fixed the problem, however, it also disconnects the computer from the local network for fillesharing, i.e., it is not longer seen in the sidebar of other macs.

I have not found a way to restart whatever aspect of discoveryd that enables peer-to-peer networking after quitting it via activity monitor.  You might not have other macs on your network so didn't notice.  However, if you've found a way to restart local networking, please share :-)

Meantime...I continue to have an open case with Apple Engineering and will post here with anything I find out.

Do you have any 3rd party apps that continually poll web sites...for example...and RSS reader like Newsbar?

Not that I know of… I do have Dropbox, Backblaze, FortiClient, Creative Cloud, Fantastical, and 1Password in my menu bar (and Mail checking for new messages every 5 minutes).

Those definitely do request DNS responses so, overtime, could contribute to discoveryd blowing up...not that it should.

My guess is that some Apps like Safari, Backblaze, Dropbox, etc are either trying to use a deprecated mdnsresolver process which was replaced by discoveryd in Yosemite, or...there's a memory leak in discoveryd and it can only handle so many requests before going to pot.

When your discoveryd next goes casters up, check how much memory it's using.  Anything over 50MB seems to indicate a problem where the Sockets Delegate error starts appearing in Console.  When it grows beyond 100MB is where I've found that the process needs to be restarted either manually via Activity Monitor (which breaks local networking) or via reboot.