[PATCH] Stop doing the (very expensive) pow() calculation pointlessly

Tomaz Canabrava tcanabrava at kde.org
Sun Jan 19 12:42:59 UTC 2014


The result of this patch is this:
http://wstaw.org/m/2014/01/19/plasma-desktopujz493.png
it still looks like it deserves some kind of special treatment, but it's
way faster now.


On Sun, Jan 19, 2014 at 6:37 PM, Tomaz Canabrava <tcanabrava at kde.org> wrote:

> Linus, it makes things instant. *NOW* we are talking. o/
>
>
> On Sun, Jan 19, 2014 at 6:34 PM, Tomaz Canabrava <tcanabrava at kde.org>wrote:
>
>> 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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