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
2018-08-04T09:31:27Z

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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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