askrene: mcf: trade granularity for performance

Speed in getroutes up by setting the granularity to 1000

Amount (sats) | speedup
-----------------------
          100 | 1.00
         1000 | 1.00
        10000 | 1.06
       100000 | 1.31
      1000000 | 2.64

Worst runtime of getroutes

Amount (sats) | before (ms) | after (ms)
--------------------------------------
          100 | 1507        | 761
         1000 | 2129        | 1214
        10000 | 1632        | 1043
       100000 | 2004        | 1150
      1000000 | 27170       | 3289

Changelog-None

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

Author: Lagrang3

plugins/askrene/mcf.c

@@ -969,10 +969,15 @@ 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));
+	/* -> We reduce the granularity of the flow by limiting the subdivision
+	 * of the payment amount into 1000 units of flow. 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 1000 >= amount
 	 * */
+	params->accuracy =
+	    amount_msat_max(AMOUNT_MSAT(1), amount_msat_div_ceil(amount, 1000));
 
 	// template the channel partition into linear arcs
 	params->cap_fraction[0]=0;

tests/test_askrene.py

@@ -1204,10 +1204,10 @@ def test_real_data(node_factory, bitcoind):
     # CI, it's slow.
     if SLOW_MACHINE:
         limit = 25
-        expected = (5, 25, 1567535, 142772, 91)
+        expected = (6, 25, 1568821, 143649, 91)
     else:
         limit = 100
-        expected = (9, 96, 6563767, 629671, 91)
+        expected = (9, 96, 6565467, 630668, 91)
 
     fees = {}
     for n in range(0, limit):
@@ -1324,7 +1324,7 @@ def test_real_biases(node_factory, bitcoind):
         expected = ({1: 6, 2: 6, 4: 7, 8: 12, 16: 14, 32: 19, 64: 25, 100: 25}, 0)
     else:
         limit = 100
-        expected = ({1: 22, 2: 25, 4: 36, 8: 52, 16: 69, 32: 80, 64: 96, 100: 96}, 0)
+        expected = ({1: 22, 2: 25, 4: 36, 8: 53, 16: 69, 32: 80, 64: 96, 100: 96}, 0)
 
     l1.rpc.askrene_create_layer('biases')
     num_changed = {}