I may be wrong, but I don't think the CPU cost of this code matters a
lot.  People will rarely turn it on and disk IO is a lot slower than
CPU actions and it's waaaaaaay more important to get high-quality info
about readahead than it is to squeeze out a few CPU cycles.

In its current form it would cache line bounce, which tends to be 
extremly slow. But the solution is probably to make it per CPU.

-Andi

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