Re: [gpsd-dev] [PATCH] Speed up gpsfake.

[Beat Bolli]

Hi Terje

On 17.05.12 20:35, Terje Mathisen wrote:
> Beat Bolli wrote:
>> By advancing the twirly baton only each 11th line, the whole
>> regress-driver run is sped up by 7% when run over SSH.
> 
> The idea is good, but I cringe when I see the implementation! :-)
> 
>> +            elif self.count % 11 == 0:
>>                   self.stream.write("-/|\\"[self.count % 4])
>>                   self.stream.write("\010")
>> -            self.stream.flush()
>> +                self.stream.flush()
>>           self.count = self.count + 1
> 
> The "count % 11" operation is _very_ expensive you know, best case it
> will be implemented as a reciprocal multiplication plus
> back-multiplication (via shift&add) and subtraction, but still taking 10
> cycles or so.
> 
> If implemented as a regular DIV operation we're talking 20-40 cycles.
> 
> The effect is of course to run the baton _backwards_, at a rate which is
> 1:11 of the original one. If you had used % 13 it would still be going
> forward, but I agree this doesn't really matter. :-)
> 
> OTOH, what's wrong with something like this:
> 
>   elif self.count & 15 == 0:
>     self.stream.write("-/|\\"[(self.count >> 4) & 3])
> 
> I.e. run at 1:16, with zero mul or div operations?

Wow, I didn't think my patch would get such scrutiny ;-)

Your version would of course be even faster, but as I explained in the
commit message, the main problem with the original implementation
(before my patch) was the relatively frequent flush() that causes
sending a new SSH packet. This is an I/O-bound problem, not a CPU-bound
one ;-)

At least I thought of using a reduction factor that has no common
divisors with the number of baton states...

Beat
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