Commit dac220b made pages-in-use accounting continuous, but this accounting has a bug where the count can go negative and wrap around to ~2^64. This causes the sweep ratio computed thereafter to be effectively infinite, so the first allocation on each P in the sweep phase will sweep until the whole heap is swept.

The fix is trivial and should go in to Go 1.6.x. I'll send it out once I can reproduce it in a test.