Re: [B.A.T.M.A.N.] [RFC 05/10] batman-adv: add bat_metric_is_similar API function

Simon Wunderlich
2013-05-29T14:55:19Z

[-- Attachment #1: Type: text/plain, Size: 3049 bytes --]

On Wed, May 29, 2013 at 04:28:51PM +0200, Antonio Quartulli wrote:
> On Wed, May 29, 2013 at 04:16:57PM +0200, Simon Wunderlich wrote:
> > On Wed, May 29, 2013 at 12:20:45AM +0200, Antonio Quartulli wrote:
> > > From: Antonio Quartulli <antonio@open-mesh.com>
> > >
> > > Each routing protocol has its own metric semantic and
> > > therefore is the protocol itself the only component able to
> > > compare two metrics to check similarity similarity.
> > >
> > > This new API allows each routing protocol to implement its
> > > own logic and make the external code protocol agnostic.
> > >
> > > Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
> > > ---
> > > bat_iv_ogm.c | 7 +++++++
> > > main.c | 3 ++-
> > > main.h | 6 ++++++
> > > types.h | 3 +++
> > > 4 files changed, 18 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/bat_iv_ogm.c b/bat_iv_ogm.c
> > > index abf4cd3..18c9ae8 100644
> > > --- a/bat_iv_ogm.c
> > > +++ b/bat_iv_ogm.c
> > > @@ -1441,6 +1441,12 @@ static uint32_t batadv_iv_ogm_metric_get(struct batadv_neigh_node *neigh_node)
> > > return neigh_node->bat_iv.tq_avg;
> > > }
> > >
> > > +static bool batadv_iv_ogm_metric_is_similar(uint32_t metric,
> > > + uint32_t new_metric)
> > > +{
> > > + return (metric - new_metric < BATADV_TQ_SIMILARITY_THRESHOLD);
> >
> > You might want to use abs(metric - new_metric) here, otherwise is_similar(a, b) output
> > might differ from is_similar(b, a).
>
> Mh..imho the name of the function is bad because this has been done on purpose.

agreed, the function name is not really good. You could rename it to something like
"is_almost_or_better()" if you keep the current semantics, although this is not an
"easy name" either. Or rename it to "similar_or_greater()". Something like that

Although ...
>
> The idea is that we want to see if 'b' is at least
> as good as 'a', therefore what we want to check if is b is greater than
> 'a - threshold' only.
>
> Imagine that 'b' is greater than (a + threshold), for me the function has to
> return true, because the metric b is at least as good as a, but if I introduce
> the abs() the function would return false.
>
> Example:
>
> a=190
> b=240
> threshold=20
>
> a - b = -50 < 20 => b is at least as good as a!
>
> using abs:
>
> abs(a - b) = 50 < 20 => NO! b is worse than a....and this is not true.
>
>
> this situation can happen in the code because usually 'a' will represents some
> kind of current metric and this is not supposed to be the best ever (maybe we
> still have to switch to a new best).

... we actually compare to the biggest value (e.g. highest gateway rank, highest
bonding candidate), so I wonder if this can really happen. So adding abs() would
just make the semantics more clear without breaking the current code when we simply
replace. Although we need to check all occurences again, I'm not completely sure
that it's always compared to the greatest member.

Cheers,
Simon

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 198 bytes --]