tree 26853fa62983f12b85badda6b9ee2197c2f10697
parent 71c696b1d0310da3ab8033d743282959bd49d28b
author Phil Carmody <ext-phil.2.carmody@nokia.com> 1300836853 -0700
committer Linus Torvalds <torvalds@linux-foundation.org> 1300841051 -0700

calibrate: home in on correct lpj value more quickly

Binary chop with a jiffy-resync on each step to find an upper bound is
slow, so just race in a tight-ish loop to find an underestimate.

If done with lots of individual steps, sometimes several hundreds of
iterations would be required, which would impose a significant overhead,
and make the initial estimate very low.  By taking slowly increasing steps
there will be less overhead.

E.g.  an x86_64 2.67GHz could have fitted in 613 individual small delays,
but in reality should have been able to fit in a single delay 644 times
longer, so underestimated by 31 steps.  To reach the equivalent of 644
small delays with the accelerating scheme now requires about 130
iterations, so has <1/4th of the overhead, and can therefore be expected
to underestimate by only 7 steps.

As now we have a better initial estimate we can binary chop over a smaller
range.  With the loop overhead in the initial estimate kept low, and the
step sizes moderate, we won't have under-estimated by much, so chose as
tight a range as we can.

Signed-off-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Tested-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>