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SubjectRe: [PATCH] tcp_cubic: use 32 bit math
Hi Stephen,

On Wed, Mar 21, 2007 at 11:54:19AM -0700, Stephen Hemminger wrote:
> On Tue, 13 Mar 2007 21:50:20 +0100
> Willy Tarreau <w@1wt.eu> wrote:

[...] ( cut my boring part )

> > Here are the results classed by speed :
> >
> > /* Sample output on a Pentium-M 600 MHz :
> >
> > Function clocks mean(us) max(us) std(us) Avg err size
> > ncubic_tab0 79 0.66 7.20 1.04 0.613% 160
> > ncubic_0div 84 0.70 7.64 1.57 4.521% 192
> > ncubic_1div 178 1.48 16.27 1.81 0.443% 336
> > ncubic_tab1 179 1.49 16.34 1.85 0.195% 320
> > ncubic_ndiv3 263 2.18 24.04 3.59 0.250% 512
> > ncubic_2div 270 2.24 24.70 2.77 0.187% 512
> > ncubic32_1 359 2.98 32.81 3.59 0.238% 544
> > ncubic_3div 361 2.99 33.08 3.79 0.170% 656
> > ncubic32 364 3.02 33.29 3.51 0.247% 544
> > ncubic 529 4.39 48.39 4.92 0.247% 720
> > hcbrt 539 4.47 49.25 5.98 1.580% 96
> > ocubic 732 4.93 61.83 7.22 0.274% 320
> > acbrt 842 6.98 76.73 8.55 0.275% 192
> > bictcp 1032 6.95 86.30 9.04 0.172% 768
> >

[...]

> The following version of div64_64 is faster because do_div already
> optimized for the 32 bit case..

Cool, this is interesting because I first wanted to optimize it but did
not find how to start with this. You seem to get very good results. BTW,
you did not append your changes.

However, one thing I do not understand is why your avg error is about 1/3
below the original one. Was there a precision bug in the original div_64_64
or did you extend the values used in the test ?

Or perhaps you used -fast-math to build and the original cbrt() is less
precise in this case ?

> I get the following results on ULV Core Solo (ie slow current processor)
> and the following on 64bit Core Duo. ncubic_tab1 seems like
> the best (no additional error and about as fast)

OK. It was the one I preferred too unless tab0's avg error was acceptable.

> ULV Core Solo
>
> Function clocks mean(us) max(us) std(us) Avg err size
> ncubic_tab0 192 11.24 45.10 15.28 0.450% -2262
> ncubic_0div 201 11.77 47.23 27.40 3.357% -2404
> ncubic_1div 324 19.02 76.32 25.82 0.189% -2567
> ncubic_tab1 326 19.13 76.73 23.71 0.043% -2059
> ncubic_2div 456 26.72 108.92 493.16 0.028% -2790
> ncubic_ndiv3 463 27.15 133.37 1889.39 0.104% -3344
> ncubic32 549 32.18 130.59 508.97 0.041% -3794
> ncubic32_1 574 33.66 138.32 548.48 0.029% -3604
> ncubic_3div 581 34.04 140.24 608.55 0.018% -3050
> ncubic 733 42.92 173.35 523.19 0.041% 299
> ocubic 1046 61.25 283.68 3305.65 0.027% -2232
> acbrt 1149 67.32 284.91 1941.55 0.029% 168
> bictcp 1663 97.41 394.29 604.86 0.017% 628
>
> Core 2 Duo
>
> Function clocks mean(us) max(us) std(us) Avg err size
> ncubic_0div 74 0.03 1.60 0.07 3.357% -2101
> ncubic_tab0 74 0.03 1.60 0.04 0.450% -2029
> ncubic_1div 142 0.07 3.11 1.05 0.189% -2195
> ncubic_tab1 144 0.07 3.18 1.02 0.043% -1638
> ncubic_2div 216 0.10 4.74 1.07 0.028% -2326
> ncubic_ndiv3 219 0.10 4.76 1.04 0.104% -2709
> ncubic32 269 0.13 5.87 1.13 0.041% -1500
> ncubic32_1 272 0.13 5.92 1.10 0.029% -2881
> ncubic 273 0.13 5.96 1.13 0.041% -1763
> ncubic_3div 290 0.14 6.32 1.01 0.018% -2499
> acbrt 430 0.20 9.42 1.18 0.029% 77
> ocubic 444 0.21 9.82 1.82 0.027% -1924
> bictcp 549 0.26 12.06 1.68 0.017% 236

Thanks,
Willy

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