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authorPhil Carmody <ext-phil.2.carmody@nokia.com>2011-03-22 16:34:13 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2011-03-22 17:44:11 -0700
commit191e56880a6a638ce931859317f37deb084b6433 (patch)
tree26853fa62983f12b85badda6b9ee2197c2f10697 /init/do_mounts.c
parent71c696b1d0310da3ab8033d743282959bd49d28b (diff)
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>
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