--- old/src/hotspot/share/gc/g1/g1Analytics.cpp	2019-11-20 14:56:34.536946729 +0100
+++ new/src/hotspot/share/gc/g1/g1Analytics.cpp	2019-11-20 14:56:34.202936391 +0100
@@ -244,7 +244,7 @@
 }
 
 size_t G1Analytics::predict_scan_card_num(size_t rs_length, bool for_young_gc) const {
-  if (for_young_gc || _mixed_card_merge_to_scan_ratio_seq->num() < 3) {
+  if (for_young_gc || !enough_samples_available(_mixed_card_merge_to_scan_ratio_seq)) {
     return (size_t) (rs_length * predict_young_card_merge_to_scan_ratio());
   } else {
     return (size_t) (rs_length * get_new_prediction(_mixed_card_merge_to_scan_ratio_seq));
@@ -252,7 +252,7 @@
 }
 
 double G1Analytics::predict_card_merge_time_ms(size_t card_num, bool for_young_gc) const {
-  if (for_young_gc || _mixed_cost_per_card_merge_ms_seq->num() < 3) {
+  if (for_young_gc || !enough_samples_available(_mixed_cost_per_card_merge_ms_seq->num())) {
     return card_num * get_new_prediction(_young_cost_per_card_merge_ms_seq);
   } else {
     return card_num * get_new_prediction(_mixed_cost_per_card_merge_ms_seq);
@@ -260,7 +260,7 @@
 }
 
 double G1Analytics::predict_card_scan_time_ms(size_t card_num, bool for_young_gc) const {
-  if (for_young_gc || _mixed_cost_per_card_scan_ms_seq->num() < 3) {
+  if (for_young_gc || !enough_samples_available(_mixed_cost_per_card_scan_ms_seq->num())) {
     return card_num * get_new_prediction(_young_cost_per_card_scan_ms_seq);
   } else {
     return card_num * get_new_prediction(_mixed_cost_per_card_scan_ms_seq);
@@ -268,7 +268,7 @@
 }
 
 double G1Analytics::predict_object_copy_time_ms_during_cm(size_t bytes_to_copy) const {
-  if (_cost_per_byte_ms_during_cm_seq->num() < 3) {
+  if (!enough_samples_available(_cost_per_byte_ms_during_cm_seq)) {
     return (1.1 * bytes_to_copy) * get_new_prediction(_copy_cost_per_byte_ms_seq);
   } else {
     return bytes_to_copy * get_new_prediction(_cost_per_byte_ms_during_cm_seq);
--- old/src/hotspot/share/gc/g1/g1Analytics.hpp	2019-11-20 14:56:35.809986129 +0100
+++ new/src/hotspot/share/gc/g1/g1Analytics.hpp	2019-11-20 14:56:35.461975358 +0100
@@ -80,6 +80,10 @@
   double _recent_avg_pause_time_ratio;
   double _last_pause_time_ratio;
 
+  // Returns whether the sequence have enough samples to get a "good" prediction.
+  // The constant used is random but "small".
+  bool enough_samples_available(TruncatedSeq const* seq) const { return seq->num() >= 3; }
+
   double get_new_prediction(TruncatedSeq const* seq) const;
   size_t get_new_size_prediction(TruncatedSeq const* seq) const;
 
--- old/src/hotspot/share/gc/g1/g1Policy.cpp	2019-11-20 14:56:37.077025344 +0100
+++ new/src/hotspot/share/gc/g1/g1Policy.cpp	2019-11-20 14:56:36.734014727 +0100
@@ -722,7 +722,11 @@
                                       p->sum_thread_work_items(G1GCPhaseTimes::OptMergeRS, G1GCPhaseTimes::MergeRSDirtyCards) +
                                       total_log_buffer_cards;
 
-    if (total_cards_merged > 10) {
+    // The threshold for the number of cards in a given sampling which we consider
+    // large enough so that the impact from setup and other costs is negligible.
+    size_t const CardsNumSamplingThreshold = 10;
+    
+    if (total_cards_merged > CardsNumSamplingThreshold) {
       double avg_time_merge_cards = average_time_ms(G1GCPhaseTimes::MergeER) +
                                     average_time_ms(G1GCPhaseTimes::MergeRS) +
                                     average_time_ms(G1GCPhaseTimes::MergeHCC) +
@@ -735,7 +739,7 @@
     size_t const total_cards_scanned = p->sum_thread_work_items(G1GCPhaseTimes::ScanHR, G1GCPhaseTimes::ScanHRScannedCards) +
                                        p->sum_thread_work_items(G1GCPhaseTimes::OptScanHR, G1GCPhaseTimes::ScanHRScannedCards);
 
-    if (total_cards_scanned > 10) {
+    if (total_cards_scanned > CardsNumSamplingThreshold) {
       double avg_time_dirty_card_scan = average_time_ms(G1GCPhaseTimes::ScanHR) +
                                         average_time_ms(G1GCPhaseTimes::OptScanHR);