[PATCH] Stop doing the (very expensive) pow() calculation pointlessly
Tomaz Canabrava
tcanabrava at kde.org
Sun Jan 19 12:34:04 UTC 2014
Testing...
On Sun, Jan 19, 2014 at 6:11 PM, Linus Torvalds <
torvalds at linux-foundation.org> wrote:
>
> From: Linus Torvalds <torvalds at linux-foundation.org>
> Date: Wed, 25 Sep 2013 20:42:19 -0700
> Subject: [PATCH] Stop doing the (very expensive) pow() calculation
> pointlessly
>
> This re-organizes the saturation calculations to be in my opinion
> clearer: we used to have the "one second" case completely separate from
> the "generic interval" case, and this undoes that.
>
> It *does* keep the special static cache for the one-second buehlmann
> factors, and expands that with a *dynamic* cache for each tissue index
> that contains the previous value of the buehlmann factor for a
> particular duration.
>
> The point is, usually we end up using some fixed duration, so the cache
> hit ratio is quite high. And doing a memory load from a cache is *much*
> faster than calculating exponentials.
>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> ---
>
> The original patch also changed the deco "time_stepsize" from 10 to 60
> seconds, which is still likely a good thing to do. But that's an
> independent issue of calling "pow()" too damn much.
>
> Tomaz, does this make things snappier for you?
>
> deco.c | 85
> ++++++++++++++++++++++++++++++++++++++++++------------------------
> 1 file changed, 54 insertions(+), 31 deletions(-)
>
> diff --git a/deco.c b/deco.c
> index 300904724ca5..6958946392e7 100644
> --- a/deco.c
> +++ b/deco.c
> @@ -129,6 +129,49 @@ static double tissue_tolerance_calc(const struct dive
> *dive)
> return ret_tolerance_limit_ambient_pressure;
> }
>
> +/*
> + * Return buelman factor for a particular period and tissue index.
> + *
> + * We cache the last factor, since we commonly call this with the
> + * same values... We have a special "fixed cache" for the one second
> + * case, although I wonder if that's even worth it considering the
> + * more general-purpose cache.
> + */
> +struct factor_cache {
> + int last_period;
> + double last_factor;
> +};
> +
> +double n2_factor(int period_in_seconds, int ci)
> +{
> + static struct factor_cache cache[16];
> +
> + if (period_in_seconds == 1)
> + return buehlmann_N2_factor_expositon_one_second[ci];
> +
> + if (period_in_seconds != cache[ci].last_period) {
> + cache[ci].last_period = period_in_seconds;
> + cache[ci].last_factor = 1 - pow(2.0, - period_in_seconds /
> (buehlmann_N2_t_halflife[ci] * 60));
> + }
> +
> + return cache[ci].last_factor;
> +}
> +
> +double he_factor(int period_in_seconds, int ci)
> +{
> + static struct factor_cache cache[16];
> +
> + if (period_in_seconds == 1)
> + return buehlmann_He_factor_expositon_one_second[ci];
> +
> + if (period_in_seconds != cache[ci].last_period) {
> + cache[ci].last_period = period_in_seconds;
> + cache[ci].last_factor = 1 - pow(2.0, - period_in_seconds /
> (buehlmann_He_t_halflife[ci] * 60));
> + }
> +
> + return cache[ci].last_factor;
> +}
> +
> /* add period_in_seconds at the given pressure and gas to the deco
> calculation */
> double add_segment(double pressure, const struct gasmix *gasmix, int
> period_in_seconds, int ccpo2, const struct dive *dive)
> {
> @@ -152,37 +195,17 @@ double add_segment(double pressure, const struct
> gasmix *gasmix, int period_in_s
> pphe *= f_dilutent;
> }
> }
> - if (period_in_seconds == 1) { /* one second interval during dive */
> - for (ci = 0; ci < 16; ci++) {
> - if (ppn2 - tissue_n2_sat[ci] > 0)
> - tissue_n2_sat[ci] +=
> buehlmann_config.satmult * (ppn2 - tissue_n2_sat[ci]) *
> -
> buehlmann_N2_factor_expositon_one_second[ci];
> - else
> - tissue_n2_sat[ci] +=
> buehlmann_config.desatmult * (ppn2 - tissue_n2_sat[ci]) *
> -
> buehlmann_N2_factor_expositon_one_second[ci];
> - if (pphe - tissue_he_sat[ci] > 0)
> - tissue_he_sat[ci] +=
> buehlmann_config.satmult * (pphe - tissue_he_sat[ci]) *
> -
> buehlmann_He_factor_expositon_one_second[ci];
> - else
> - tissue_he_sat[ci] +=
> buehlmann_config.desatmult * (pphe - tissue_he_sat[ci]) *
> -
> buehlmann_He_factor_expositon_one_second[ci];
> - }
> - } else { /* all other durations */
> - for (ci = 0; ci < 16; ci++)
> - {
> - if (ppn2 - tissue_n2_sat[ci] > 0)
> - tissue_n2_sat[ci] +=
> buehlmann_config.satmult * (ppn2 - tissue_n2_sat[ci]) *
> - (1 - pow(2.0,(- period_in_seconds
> / (buehlmann_N2_t_halflife[ci] * 60))));
> - else
> - tissue_n2_sat[ci] +=
> buehlmann_config.desatmult * (ppn2 - tissue_n2_sat[ci]) *
> - (1 - pow(2.0,(- period_in_seconds
> / (buehlmann_N2_t_halflife[ci] * 60))));
> - if (pphe - tissue_he_sat[ci] > 0)
> - tissue_he_sat[ci] +=
> buehlmann_config.satmult * (pphe - tissue_he_sat[ci]) *
> - (1 - pow(2.0,(- period_in_seconds
> / (buehlmann_He_t_halflife[ci] * 60))));
> - else
> - tissue_he_sat[ci] +=
> buehlmann_config.desatmult * (pphe - tissue_he_sat[ci]) *
> - (1 - pow(2.0,(- period_in_seconds
> / (buehlmann_He_t_halflife[ci] * 60))));
> - }
> +
> + for (ci = 0; ci < 16; ci++) {
> + double ppn2_oversat = ppn2 - tissue_n2_sat[ci];
> + double pphe_oversat = pphe - tissue_he_sat[ci];
> + double n2_f = n2_factor(period_in_seconds, ci);
> + double he_f = he_factor(period_in_seconds, ci);
> + double n2_satmult = ppn2_oversat > 0 ?
> buehlmann_config.satmult : buehlmann_config.desatmult;
> + double he_satmult = pphe_oversat > 0 ?
> buehlmann_config.satmult : buehlmann_config.desatmult;
> +
> + tissue_n2_sat[ci] += n2_satmult * ppn2_oversat * n2_f;
> + tissue_he_sat[ci] += he_satmult * pphe_oversat * he_f;
> }
> return tissue_tolerance_calc(dive);
> }
> --
> 1.8.5.1.163.gd7aced9
>
>
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