From aed66f1fe06f2cbb9f4dd275d9ea050c4abf8eea Mon Sep 17 00:00:00 2001
From: Jean-loup Gailly <jloup@gailly.net>
Date: Sun, 14 Nov 2010 19:36:35 +0100
Subject: [PATCH] Distributed engine: wait for enough slave replies before
 early stop.

---
 distributed/distributed.c | 9 +++++++--
 uct/search.c              | 9 ++++++++-
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/distributed/distributed.c b/distributed/distributed.c
index 51a73c1..c19d65b 100644
--- a/distributed/distributed.c
+++ b/distributed/distributed.c
@@ -118,6 +118,9 @@ static const struct time_info default_ti = {
 /* Maximum time (seconds) to wait for answers to genmoves. */
 #define MAX_GENMOVES_WAIT 0.1 /* 100 ms */
 
+/* Minimum time (seconds) to wait before we stop early. This should
+ * ensure that most slaves have replied at least once. */
+#define MIN_EARLY_STOP_WAIT 0.3 /* 300 ms */
 
 /* Display a path as leaf<parent<grandparent...
  * Returns the path string in a static buffer; it is NOT safe for
@@ -188,6 +191,8 @@ distributed_notify(struct engine *e, struct board *b, int id, char *cmd, char *a
  * To simplify the code, we assume that master and slave have the same architecture
  * (store values identically).
  * Return the move with most playouts, and additional stats.
+ * keep_looking is set from a majority vote of the slaves seen so far for this
+ * move but should not be trusted if too few slaves have been seen.
  * Keep this code in sync with uct/slave.c:report_stats().
  * slave_lock is held on entry and on return. */
 static coord_t
@@ -314,11 +319,11 @@ distributed_genmove(struct engine *e, struct board *b, struct time_info *ti,
 		bool keep_looking;
 		best = select_best_move(b, stats, &played, &playouts, &threads, &keep_looking);
 
-		if (!keep_looking) break;
 		if (ti->dim == TD_WALLTIME) {
 			if (now - ti->len.t.timer_start >= stop.worst.time) break;
+			if (!keep_looking && now - first >= MIN_EARLY_STOP_WAIT) break;
 		} else {
-			if (played >= stop.worst.playouts) break;
+			if (!keep_looking || played >= stop.worst.playouts) break;
 		}
 		if (DEBUGVV(2)) {
 			char *coord = coord2sstr(best, b);
diff --git a/uct/search.c b/uct/search.c
index 848b252..cbd6f8b 100644
--- a/uct/search.c
+++ b/uct/search.c
@@ -271,13 +271,20 @@ uct_search_stop_early(struct uct *u, struct tree *t, struct board *b,
 	if (fullmem)
 		return true;
 
+	/* Think at least 100ms to avoid a random move. This is particularly
+	 * important in distributed mode, where this function is called frequently. */
+	double elapsed = 0.0;
+	if (ti->dim == TD_WALLTIME) {
+		elapsed = time_now() - ti->len.t.timer_start;
+		if (elapsed < TREE_BUSYWAIT_INTERVAL) return false;
+	}
+
 	/* Break early if we estimate the second-best move cannot
 	 * catch up in assigned time anymore. We use all our time
 	 * if we are in byoyomi with single stone remaining in our
 	 * period, however - it's better to pre-ponder. */
 	bool time_indulgent = (!ti->len.t.main_time && ti->len.t.byoyomi_stones == 1);
 	if (best2 && ti->dim == TD_WALLTIME && !time_indulgent) {
-		double elapsed = time_now() - ti->len.t.timer_start;
 		double remaining = stop->worst.time - elapsed;
 		double pps = ((double)played) / elapsed;
 		double estplayouts = remaining * pps + PLAYOUT_DELTA_SAFEMARGIN;
-- 
2.11.4.GIT