askrene: don't *completely* ignore fees to start.

I noticed that increasing mu a little bit sometimes made a big difference,
because by completely ignoring fees we were choosing the worst of two channels
in some cases.

Start at 1% fees; this saves a lot on initial fees in this test!

Here's the new stats on mu levels:

     96  mu=1
     90  mu=10
     41  mu=20
     30  mu=30
     24  mu=40
     19  mu=50
     22  mu=60
      8  mu=70
     95  mu=80
     19  mu=90

Signed-off-by: Rusty Russell <[email protected]>
Changelog-EXPERIMENTAL: `askrene` is now better at finding low-fee paths.

Author: rustyrussell

plugins/askrene/askrene.c

@@ -399,8 +399,8 @@ static const char *get_routes(const tal_t *ctx,
 
 	delay_feefactor = 1.0/1000000;
 
-	/* First up, don't care about fees.   */
-	mu = 0;
+	/* First up, don't care about fees (well, just enough to tiebreak!) */
+	mu = 1;
 	flows = minflow(rq, rq, srcnode, dstnode, amount,
 			mu, delay_feefactor);
 	if (!flows) {
@@ -435,7 +435,10 @@ static const char *get_routes(const tal_t *ctx,
 	while (amount_msat_greater(flowset_fee(rq->plugin, flows), maxfee)) {
 		struct flow **new_flows;
 
-		mu += 10;
+		if (mu == 1)
+			mu = 10;
+		else
+			mu += 10;
 		rq_log(tmpctx, rq, LOG_UNUSUAL,
 		       "The flows had a fee of %s, greater than max of %s, retrying with mu of %u%%...",
 		       fmt_amount_msat(tmpctx, flowset_fee(rq->plugin, flows)),

tests/test_askrene.py

@@ -992,4 +992,4 @@ def test_real_data(node_factory, bitcoind):
         if len(fees[n]) > len(fees[best]):
             best = n
 
-    assert (len(fees[best]), len(improved), total_first_fee, total_final_fee, percent_fee_reduction) == (9, 96, 19961361, 650099, 97)
+    assert (len(fees[best]), len(improved), total_first_fee, total_final_fee, percent_fee_reduction) == (9, 96, 9451780, 650099, 94)