On Sunday 17 May 2015 00:11:52 Sven Eckelmann wrote: > And please also include the commit which caused the regression. This makes > it easier to find out which kernel versions must be patched in case the > problem is critical. Here is an example how this information should be > included:
You are most likely want to fix the regression introduced in 0853ec7 ("batman-
adv: tvlv - gateway download/upload bandwidth container"). The gateway client
code for fast node selection was previously using batadv_gw_bandwidth_to_kbit
to convert the input parameters to Kibit/s. After the TLV restructuring it
should have multiplied the TLV value by 100 to get the exact same result
because the batadv_tvlv_gateway_data bandwidth_down/bandwidth_up stores the
throughput with the base unit of 100 Kibit/s.
The problem here would be to decide if it changes the result of the algorithm.
The code goes through all gateways. For each gateway it calculates the formula
(gw_down is in 100 Kibit/s; gw_divisor is constant ):
gw_factor = |_ (tq_avg ** 2 * gw_down * 10 ** 4) / gw_divisor _|
You want to change it to
gw_factor = |_ (tq_avg ** 2 * gw_down * 10 ** 6) / gw_divisor _|
The rest of the algorithm is just comparing the gw_factor of each gateway. The
question which should have been answered by the commit message would be:
Why is this constant of 100 affecting your results? Is it rounding related?
How much does it affect the algorithm?
tq_avg is a value <= 255. A realistic value for bandwidth_down is maybe
something like 20. gw_divisor is 2 ** 18.
So each difference in "tq_avg ** 2 * gw_down * 10 ** 4" for each gateway must
be larger than 2 ** 18 to always be distinguishable by the algorithm. For a
fixed tq_avg value of 255 the algorithm could currently compare two gateways
with a gw_down difference of (1. / 2480) and still distinguish them. It would
still detect a difference when the tq_avg is 6 and the gw_down difference is
1.
For a fixed gw_down value of 1, the current algorithm would have problems to
distinguish the tq_avg values < 14 when the tq_avg difference is small.
Your change would make it 100x more precise for gw_down. But I am currently
not seeing the actual impact. Maybe you could explain it in more detail in the
commit message.
Kind regards,
Sven
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