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plugins/pay: fix capacity bias.

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We attempted to introduce a "capacity bias" so we would penalize
channels where we were using a large amount of their total capacity.

Unfortunately, this was both naive, and applied wrongly:

	-log((capmsat + 1 - amtmsat) / (capmsat + 1));

As an integer gives 0 up to about 65% capacity.  We then applied this
by multiplying:

	(cmsat * rmsat * bias) / (cmsat + rmsat + bias + 1);

Giving a total score of 0 in these cases!  If we're using the
arithmetic mean we really need to add 1 to bias.  We might as well
use a double the whole way through, for a slightly more fine-grained
approach.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell 2024-03-07 13:26:36 +10:30 • committed by Christian Decker
parent fdfffdc232
commit e6cf8c0a24
1 changed files with 9 additions and 8 deletions
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11
plugins/libplugin-pay.c
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@ -747,7 +747,7 @@ static bool payment_route_can_carry_even_disabled(const struct gossmap *map,
* \mu*fee(amt) which is the linearized version which for small amounts and
* suitable value of \mu should be good enough)
*/
static u64 capacity_bias(const struct gossmap *map,
static double capacity_bias(const struct gossmap *map,
const struct gossmap_chan *c,
int dir,
struct amount_msat amount)
@ -773,14 +773,15 @@ static u64 route_score(u32 distance,
{
u64 cmsat = cost.millisatoshis; /* Raw: lengthy math */
u64 rmsat = risk.millisatoshis; /* Raw: lengthy math */
u64 bias = capacity_bias(global_gossmap, c, dir, cost);
/* We multiply this, so 1 is neutral, higher is a penalty. */
double bias = capacity_bias(global_gossmap, c, dir, cost) + 1;
/* Smoothed harmonic mean to avoid division by 0 */
u64 costs = (cmsat * rmsat * bias) / (cmsat + rmsat + bias + 1);
double costs = (cmsat * rmsat * bias) / (cmsat + rmsat + bias + 1);
if (costs > 0xFFFFFFFF)
costs = 0xFFFFFFFF;
return costs;
return 0xFFFFFFFF;
return (u64)costs;
}
static struct route_hop *route(const tal_t *ctx,