Re: [B.A.T.M.A.N.] [PATCH v2 6/7] batman-adv: ELP - use tp meter to estimate the throughput if otherwise not available

[Antonio Quartulli <a@unstable.cc>]

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Hi,

On 22/05/18 03:06, Sven Eckelmann wrote:
> On Montag, 21. Mai 2018 15:17:11 CEST Linus Lüssing wrote:
>>> +               throughput = total_bytes * 8 >> ilog2(test_time) / 10;
> [...]
>> -----
> [...]
>>
>>         throughput = total_bytes * 8 >> log_test_time / 10;
>>
>>         // Straightforward approach?
>>         throughput2 = total_bytes * 8 / test_time * 1000 / 1024 / 100;
> [...]
>> -----
>> $ ./test
>> Result: 80000000 (log_test_time: 13)
>> Result2: 156
>> $ file ./test
>> ./test: ELF 32-bit LSB pie executable ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 3.2.0, BuildID[sha1]=d18f32829cdd2bc42cf744cdcafde7cdbd315cb0, not stripped
>> -----
> 
> Thanks for this small example program. Yes, there are parenthesis missing in 
> the calculation. Right now, following is calculated:
> 
>     (total_bytes * 8) >> (ilog2(test_time) / 10);
> 
> But the author most likely wanted following precedence:
> 
>     ((total_bytes * 8) >> ilog2(test_time)) / 10;
> 
> And together with the fixed unit, you would get:
> 
>     (total_bytes * 8 >> ilog2(test_time) / 100;
> 
> Your example program would then show following result because the shifting 
> stuff is still the wrong approach:
> 
>     Result: 195 (log_test_time: 13)
>     Result2: 156
> 
> The calculation still has to be changed to something like this to get 
> 
>     // when 0.1 Mbit/s == 100 kbit/s
>     throughput = total_bytes * 5;
> 	 do_div(throughput, test_time * 64);
> 
>     // when 0.1 Mbit/s == 102.4 kbit/s
>     throughput = total_bytes * 625;
> 	 do_div(throughput, test_time * 8192);
> 
>     // when 0.1 Mbit/s == 100 kbit/s, and 1kbit/s == 1000 bit (instead of 1024 bit):
>     throughput = total_bytes;
>     do_div(throughput, test_time * 125);
> 
> Please keep in mind that we must do a check of the divisor (for 0) before 
> doing this do_div.

Thanks for the code and the correction :-)

Yes, we wanted to avoid 64 bits explicit divisions and we assumed
test_time would have been a power of 2 (otherwise we'd accept a small
error).

Anyway, the last case brought by Sven depicts what we want to implement.
The patch will be changed accordingly.

Cheers,

-- 
Antonio Quartulli


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