Ok, granted it's _very_ conservative. I really don't want to risk its presence
being a burden on anything, and the iowait it induces probably makes it turn
itself off for another PREFETCH_DELAY (5s). I really don't want to cross the
line to where it is detrimental in any way. Not dropping out on a
cond_resched and perhaps making the delay tunable should be enough to make it
a little less "sleepy".
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-ck
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