After upgrading to Mac OS X 10.10.3 the discoveryd process would ramp up to use 100% of my MacBook Pro's CPU. I noticed this when I felt like my machine was working far too hard for what I was doing at the time, and the bottom felt like it was going to melt my lap.
This quick shell script temporarily fixes it by unloading and reloading the plist file. I don't know how effective this is in the long run, but it does work.
Paste the following two lines into a file, and name it whatever you like, with a '.sh' extension. I named mine "ddfix.sh."
sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.discoveryd.plist sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.discoveryd.plist
Save this as a shell file somewhere. I'm hoping for a quick fix from Apple, so I saved mine to the desktop for now. When it feels like my MacBook Pro could cook an egg, or it sounds like a jet plane ready for take-off, I check Activity Monitor to see if discoveryd is the culprit. If it is, I just need to open a Terminal window and do the following:
~: cd Desktop/ ~/Desktop: sh ddfix.sh
The discoveryd process goes back down to 0.0% and the issue is solved (at least temporarily). Credit goes to this support thread on the Apple forums.