Update the default scoring parameters to use historical model only

Utilizing the results of probes sent once a minute to a random node
in the network for a random amount (within a reasonable range), we
were able to analyze the accuracy of our resulting success
probability estimation with various PDFs across the historical and
live-bounds models.

For each candidate PDF (as well as other parameters, to be tuned in
the coming commits), we used the `min_zero_implies_no_successes`
fudge factor in `success_probability` as well as a total
probability multiple fudge factor to get both the historical
success model and the a priori model to be neither too optimistic
nor too pessimistic (as measured by the relative log-loss between
succeeding and failing hops in our sample data).

We then compared the resulting log-loss for the historical success
model and selected the candidate PDF with the lowest log-loss,
skipping a few candidates with similar resulting log-loss but with
more extreme constants (such as a power of 11 with a higher
`min_zero_implies_no_successes` penalty).

In every case, the historical model performed substantially better
than the live-bounds model, so here we simply disable the
live-bounds model by default and use only the historical model.
Further, we use the calculated total probability multiple fudge
factor (0.7886892844179266) to choose the ratio between the
historical model and the per-hop penalty (as multiplying each hop's
probability by 78% is equivalent to adding a per-hop penalty of
log10(0.78) of our probabilistic penalty).

We take this opportunity to bump the penalties up a bit as well, as
anecdotally LDK users are willing to pay more than they do today to
get more successful paths.

Fixes lightningdevkit#3040

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -488,7 +488,13 @@ where L::Target: Logger {
 pub struct ProbabilisticScoringFeeParameters {
 	/// A fixed penalty in msats to apply to each channel.
 	///
-	/// Default value: 500 msat
+	/// In testing, a value of roughly 1/10th of [`historical_liquidity_penalty_multiplier_msat`]
+	/// (implying scaling all estimated probabilities down by a factor of ~79%) resulted in the
+	/// most accurate total success probabilities.
+	///
+	/// Default value: 1,024 msat (i.e. we're willing to pay 1 sat to avoid each additional hop).
+	///
+	/// [`historical_liquidity_penalty_multiplier_msat`]: Self::historical_liquidity_penalty_multiplier_msat
 	pub base_penalty_msat: u64,
 
 	/// A multiplier used with the payment amount to calculate a fixed penalty applied to each
@@ -500,9 +506,16 @@ pub struct ProbabilisticScoringFeeParameters {
 	///
 	/// ie `base_penalty_amount_multiplier_msat * amount_msat / 2^30`
 	///
-	/// Default value: 8,192 msat
+	/// In testing, a value of roughly ~100x (1/10th * 2^10) of
+	/// [`historical_liquidity_penalty_amount_multiplier_msat`] (implying scaling all estimated
+	/// probabilities down by a factor of ~79%) resulted in the most accurate total success
+	/// probabilities.
+	///
+	/// Default value: 131,072 msat (i.e. we're willing to pay 0.125bps to avoid each additional
+	///                              hop).
 	///
 	/// [`base_penalty_msat`]: Self::base_penalty_msat
+	/// [`historical_liquidity_penalty_amount_multiplier_msat`]: Self::historical_liquidity_penalty_amount_multiplier_msat
 	pub base_penalty_amount_multiplier_msat: u64,
 
 	/// A multiplier used in conjunction with the negative `log10` of the channel's success
@@ -518,9 +531,14 @@ pub struct ProbabilisticScoringFeeParameters {
 	///
 	/// `-log10(success_probability) * liquidity_penalty_multiplier_msat`
 	///
-	/// Default value: 30,000 msat
+	/// In testing, this scoring model performs much worse than the historical scoring model
+	/// configured with the [`historical_liquidity_penalty_multiplier_msat`] and thus is disabled
+	/// by default.
+	///
+	/// Default value: 0 msat
 	///
 	/// [`liquidity_offset_half_life`]: ProbabilisticScoringDecayParameters::liquidity_offset_half_life
+	/// [`historical_liquidity_penalty_multiplier_msat`]: Self::historical_liquidity_penalty_multiplier_msat
 	pub liquidity_penalty_multiplier_msat: u64,
 
 	/// A multiplier used in conjunction with the payment amount and the negative `log10` of the
@@ -540,7 +558,13 @@ pub struct ProbabilisticScoringFeeParameters {
 	/// probabilities, the multiplier will have a decreasing effect as the negative `log10` will
 	/// fall below `1`.
 	///
-	/// Default value: 192 msat
+	/// In testing, this scoring model performs much worse than the historical scoring model
+	/// configured with the [`historical_liquidity_penalty_amount_multiplier_msat`] and thus is
+	/// disabled by default.
+	///
+	/// Default value: 0 msat
+	///
+	/// [`historical_liquidity_penalty_amount_multiplier_msat`]: Self::historical_liquidity_penalty_amount_multiplier_msat
 	pub liquidity_penalty_amount_multiplier_msat: u64,
 
 	/// A multiplier used in conjunction with the negative `log10` of the channel's success
@@ -554,7 +578,8 @@ pub struct ProbabilisticScoringFeeParameters {
 	/// track which of several buckets those bounds fall into, exponentially decaying the
 	/// probability of each bucket as new samples are added.
 	///
-	/// Default value: 10,000 msat
+	/// Default value: 10,000 msat (i.e. willing to pay 1 sat to avoid an 80% probability channel,
+	///                            or 6 sats to avoid a 25% probability channel).
 	///
 	/// [`liquidity_penalty_multiplier_msat`]: Self::liquidity_penalty_multiplier_msat
 	pub historical_liquidity_penalty_multiplier_msat: u64,
@@ -575,7 +600,9 @@ pub struct ProbabilisticScoringFeeParameters {
 	/// channel, we track which of several buckets those bounds fall into, exponentially decaying
 	/// the probability of each bucket as new samples are added.
 	///
-	/// Default value: 64 msat
+	/// Default value: 1,250 msat (i.e. willing to pay about 0.125 bps per hop to avoid 78%
+	///                            probability channels, or 0.5bps to avoid a 38% probability
+	///                            channel).
 	///
 	/// [`liquidity_penalty_amount_multiplier_msat`]: Self::liquidity_penalty_amount_multiplier_msat
 	pub historical_liquidity_penalty_amount_multiplier_msat: u64,
@@ -642,15 +669,15 @@ pub struct ProbabilisticScoringFeeParameters {
 impl Default for ProbabilisticScoringFeeParameters {
 	fn default() -> Self {
 		Self {
-			base_penalty_msat: 500,
-			base_penalty_amount_multiplier_msat: 8192,
-			liquidity_penalty_multiplier_msat: 30_000,
-			liquidity_penalty_amount_multiplier_msat: 192,
+			base_penalty_msat: 1024,
+			base_penalty_amount_multiplier_msat: 131_072,
+			liquidity_penalty_multiplier_msat: 0,
+			liquidity_penalty_amount_multiplier_msat: 0,
 			manual_node_penalties: new_hash_map(),
 			anti_probing_penalty_msat: 250,
 			considered_impossible_penalty_msat: 1_0000_0000_000,
 			historical_liquidity_penalty_multiplier_msat: 10_000,
-			historical_liquidity_penalty_amount_multiplier_msat: 64,
+			historical_liquidity_penalty_amount_multiplier_msat: 1_250,
 			linear_success_probability: false,
 		}
 	}
@@ -3017,47 +3044,47 @@ mod tests {
 			info,
 			short_channel_id: 42,
 		});
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 11577);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 42_252);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 1_950_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 8462);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 36_005);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 2_950_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 6889);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 32_851);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 3_950_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 5883);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 30_832);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 4_950_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 5412);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 29_886);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 5_950_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 4940);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 28_939);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 6_950_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 4689);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 28_435);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 7_450_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 4468);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 27_993);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 7_950_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 4468);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 27_993);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 8_950_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 4217);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 27_488);
 		let usage = ChannelUsage {
 			effective_capacity: EffectiveCapacity::Total { capacity_msat: 9_950_000_000, htlc_maximum_msat: 1_000 }, ..usage
 		};
-		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 3996);
+		assert_eq!(scorer.channel_penalty_msat(&candidate, usage, &params), 27_047);
 	}
 
 	#[test]