askrene: prune some arcs for the MCF computation

Constraint the number of flow units to 1M and prune arcs that are
provably not used in the MCF computation.

Changelog-None.

Signed-off-by: Lagrang3 <[email protected]>

Author: Lagrang3

plugins/askrene/mcf.c

@@ -365,9 +365,13 @@ static void linearize_channel(const struct pay_parameters *params,
 	    b = 1 + amount_msat_ratio_floor(maxcap, params->accuracy);
 
 	/* An extra bound on capacity, here we use it to reduce the flow such
-	 * that it does not exceed htlcmax. */
+	 * that it does not exceed htlcmax.
+	 * Also there is no need to keep track of more capacity than the payment
+	 * amount, this can help us prune some arcs. */
 	u64 cap_on_capacity =
-	    amount_msat_ratio_floor(gossmap_chan_htlc_max(c, dir), params->accuracy);
+	    MIN(amount_msat_ratio_floor(gossmap_chan_htlc_max(c, dir),
+					params->accuracy),
+		amount_msat_ratio_ceil(params->amount, params->accuracy));
 
 	set_capacity(&capacity[0], a, &cap_on_capacity);
 	cost[0]=0;
@@ -603,8 +607,9 @@ static void init_linear_network(const tal_t *ctx,
 			// when the `i` hits the `next` node.
 			for(size_t k=0;k<CHANNEL_PARTS;++k)
 			{
-				/* FIXME: Can we prune arcs with 0 capacity?
-				 * if(capacity[k]==0)continue; */
+				/* prune arcs with 0 capacity */
+				if (capacity[k] == 0)
+					continue;
 
 				struct arc arc = arc_from_parts(chan_id, half, k, false);
 
@@ -964,10 +969,14 @@ struct flow **minflow(const tal_t *ctx,
 	params->source = source;
 	params->target = target;
 	params->amount = amount;
-	params->accuracy = AMOUNT_MSAT(1000);
-	/* FIXME: params->accuracy = amount_msat_max(amount_msat_div(amount,
-	 * 1000), AMOUNT_MSAT(1));
+	/* -> At most 1M units of flow are allowed, that reduces the
+	 * computational burden for algorithms that depend on it, eg. "capacity
+	 * scaling" and "successive shortest path".
+	 * -> Using Ceil operation instead of Floor so that
+	 *      accuracy x 1M >= amount
 	 * */
+	params->accuracy = amount_msat_max(
+	    AMOUNT_MSAT(1), amount_msat_div_ceil(amount, 1000000));
 
 	// template the channel partition into linear arcs
 	params->cap_fraction[0]=0;

tests/test_askrene.py

@@ -1204,7 +1204,7 @@ def test_real_data(node_factory, bitcoind):
     # CI, it's slow.
     if SLOW_MACHINE:
         limit = 25
-        expected = (6, 25, 1544756, 142986, 91)
+        expected = (5, 25, 1567536, 142772, 91)
     else:
         limit = 100
         expected = (9, 95, 6347877, 566288, 92)
@@ -1321,7 +1321,7 @@ def test_real_biases(node_factory, bitcoind):
     # CI, it's slow.
     if SLOW_MACHINE:
         limit = 25
-        expected = ({1: 5, 2: 7, 4: 7, 8: 11, 16: 14, 32: 19, 64: 25, 100: 25}, 0)
+        expected = ({1: 6, 2: 6, 4: 7, 8: 12, 16: 14, 32: 19, 64: 25, 100: 25}, 0)
     else:
         limit = 100
         expected = ({1: 23, 2: 31, 4: 40, 8: 53, 16: 70, 32: 82, 64: 96, 100: 96}, 0)