Evan Weaver is a Ruby programmer who Twitter hired awhile back to help fix their scaling problems.

Thank you; that wasn't immediately evident by looking at his bio on his blog.

It isn't? http://blog.evanweaver.com/about/ says "I currently work at Twitter" and has a link to his resume, which says he started there in May.

Bizarre; I didn't see that there when I looked earlier.

Evan wrote bleak_house. Alongside the Nimble Method guys, he's one of the best known Rails tuners. He knows what Twitter's problems are because they hired him to fix it.