Messages in this thread | | | Date | Thu, 14 Aug 2008 15:49:54 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with preemptible kernel and CPU hotplug |
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* H. Peter Anvin (hpa@zytor.com) wrote: > Mathieu Desnoyers wrote: >> Sure, here are the updated tables. Basically, they show no significant >> difference between the NOP and the DS segment selector prefix >> approaches. > > Actually, unless I have blown my T-test completely, they show with a 80% > and 74% confidence (respective for the two benchmarks) that the DS case is > slightly *better* (0.26% and 0.20% better, respective), which makes it a > no-brainer. Doing around 10 runs of each is likely to confirm this > conclusion by pushing it into the 90+% interval. >
I did more runs (20 runs of each) to compare the nop case to the DS prefix case. Results in seconds. They actually does not seems to show a significant difference.
NOP
34.155 33.955 34.012 35.299 35.679 34.141 33.995 35.016 34.254 33.957 33.957 34.008 35.013 34.494 33.893 34.295 34.314 34.854 33.991 34.132
DS
34.080 34.304 34.374 35.095 34.291 34.135 33.940 34.208 35.276 34.288 33.861 33.898 34.610 34.709 33.851 34.256 35.161 34.283 33.865 35.078
Used http://www.graphpad.com/quickcalcs/ttest1.cfm?Format=C to do the T-test (yeah, I'm lazy) :
Group Group One (DS prefix) Group Two (nops) Mean 34.37815 34.37070 SD 0.46108 0.51905 SEM 0.10310 0.11606 N 20 20
P value and statistical significance: The two-tailed P value equals 0.9620 By conventional criteria, this difference is considered to be not statistically significant.
Confidence interval: The mean of Group One minus Group Two equals 0.00745 95% confidence interval of this difference: From -0.30682 to 0.32172
Intermediate values used in calculations: t = 0.0480 df = 38 standard error of difference = 0.155
So, unless these calculus are completely bogus, the difference between the nop and the DS case seems not to be statistically significant.
Mathieu
> Note that since the difference is so small, and so can also be due to some > kind of systematic error (lower ambient temperature during the DS run > making the disk drive slightly faster, what have you.) > > -hpa
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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