Attempt to improve merge selecting algorithm [#METR-23632].

dbms/include/DB/Storages/MergeTree/SimpleMergeSelector.h

@@ -11,6 +11,9 @@ class SimpleMergeSelector : public IMergeSelector
 public:
 	struct Settings
 	{
+		/// Zero means unlimited.
+		size_t max_parts_to_merge_at_once = 100;
+
 		/** Minimum ratio of size of one part to all parts in set of parts to merge (for usual cases).
 		  * For example, if all parts have equal size, it means, that at least 'base' number of parts should be merged.
 		  * If parts has non-uniform sizes, then minumum number of parts to merge is effectively increased.
@@ -35,7 +38,7 @@ public:
 		  * 3. Sum size of parts participating in merge. If higher - then more age is required to lower base. So, base is lowered slower.
 		  * It means: for small parts, it's worth to merge faster, even not so wide or balanced.
 		  *
-		  * We have multivariative dependency. Let it be logarithmic of size and multi-linear by other variables,
+		  * We have multivariative dependency. Let it be logarithmic of size and somewhat multi-linear by other variables,
 		  *  between some boundary points, and constant outside.
 		  */
 
@@ -53,8 +56,19 @@ public:
 		/// Add this to size before all calculations. It means: merging even very small parts has it's fixed cost.
 		size_t size_fixed_cost_to_add = 5 * 1024 * 1024;
 
-		/// Zero means unlimited.
-		size_t max_parts_to_merge_at_once = 100;
+		/** Heuristic:
+		  * Make some preference for ranges, that sum_size is like (in terms of ratio) to part previous at left.
+		  */
+		bool enable_heuristic_to_align_parts = true;
+		double heuristic_to_align_parts_min_ratio_of_sum_size_to_prev_part = 0.9;
+		double heuristic_to_align_parts_max_absolute_difference_in_powers_of_two = 0.5;
+		double heuristic_to_align_parts_max_score_adjustment = 0.75;
+
+		/** Heuristic:
+		  * From right side of range, remove all parts, that size is less than specified ratio of sum_size.
+		  */
+		bool enable_heuristic_to_remove_small_parts_at_right = true;
+		double heuristic_to_remove_small_parts_at_right_max_ratio = 0.01;
 	};
 
 	SimpleMergeSelector(const Settings & settings) : settings(settings) {}

dbms/src/Storages/MergeTree/SimpleMergeSelector.cpp

@@ -17,24 +17,21 @@ struct Estimator
 {
 	using Iterator = SimpleMergeSelector::PartsInPartition::const_iterator;
 
-	void consider(Iterator begin, Iterator end, size_t sum_size, size_t sum_size_fixed_cost, size_t size_prev_at_left)
+	void consider(Iterator begin, Iterator end, size_t sum_size, size_t size_prev_at_left, const SimpleMergeSelector::Settings & settings)
 	{
-		double current_score = score(end - begin, sum_size, sum_size_fixed_cost);
+		double current_score = score(end - begin, sum_size, settings.size_fixed_cost_to_add);
 
-		/** Heuristic:
-		  * Make some preference for ranges, that sum_size is like (in terms of ratio) to part previous at left.
-		  */
-		if (size_prev_at_left > sum_size * 0.9)
+		if (settings.enable_heuristic_to_align_parts
+			&& size_prev_at_left > sum_size * settings.heuristic_to_align_parts_min_ratio_of_sum_size_to_prev_part)
 		{
 			double difference = std::abs(log2(static_cast<double>(sum_size) / size_prev_at_left));
-			if (difference < 0.5)
-				current_score *= 0.75 + difference * 0.5;
+			if (difference < settings.heuristic_to_align_parts_max_absolute_difference_in_powers_of_two)
+				current_score *= interpolateLinear(settings.heuristic_to_align_parts_max_score_adjustment, 1,
+					difference / settings.heuristic_to_align_parts_max_absolute_difference_in_powers_of_two);
 		}
 
-		/** Heuristic:
-		  * From right side of range, remove all parts, that size is less than 1% of sum_size.
-		  */
-		while (end >= begin + 3 && (end - 1)->size < 0.01 * sum_size)
+		if (settings.enable_heuristic_to_remove_small_parts_at_right)
+			while (end >= begin + 3 && (end - 1)->size < settings.heuristic_to_remove_small_parts_at_right_max_ratio * sum_size)
 				--end;
 
 		if (!min_score || current_score < min_score)
@@ -175,8 +172,8 @@ void selectWithinPartition(
 					parts.begin() + begin,
 					parts.begin() + end,
 					sum_size,
-					settings.size_fixed_cost_to_add,
-					begin == 0 ? 0 : parts[begin - 1].size);
+					begin == 0 ? 0 : parts[begin - 1].size,
+					settings);
 		}
 	}
 }

dbms/src/Storages/tests/merge_selector2.cpp

@@ -41,25 +41,34 @@ int main(int argc, char ** argv)
 		IMergeSelector::Part part;
 		in >> part.size >> "\t" >> part.age >> "\t" >> part.level >> "\t" >> part_names.back() >> "\n";
 		part.data = part_names.back().data();
+		part.level = 0;
 		parts.emplace_back(part);
 		sum_parts_size += part.size;
 	}
 
 	size_t sum_size_written = sum_parts_size;
 	size_t num_merges = 1;
+	size_t age_passed = 0;
 
 	while (parts.size() > 1)
 	{
-		IMergeSelector::PartsInPartition selected_parts = selector.select(partitions, 0);
+		IMergeSelector::PartsInPartition selected_parts = selector.select(partitions, 0 /*100ULL * 1024 * 1024 * 1024*/);
 
 		if (selected_parts.empty())
 		{
-			std::cout << '.';
+			++age_passed;
 			for (auto & part : parts)
 				++part.age;
+
+			if (age_passed > 60 * 86400)
+				break;
+
+			if (age_passed % 86400 == 0)
+				std::cout << ".";
+
 			continue;
 		}
-		std::cout << '\n';
+		std::cout << "Time passed: " << age_passed << '\n';
 
 		size_t sum_merged_size = 0;
 		size_t start_index = 0;
@@ -75,7 +84,7 @@ int main(int argc, char ** argv)
 				start_index = i;
 			}
 
-			std::cout << parts[i].size;
+			std::cout << (parts[i].size / 1024) << "_" << parts[i].level;
 			if (in_range)
 			{
 				sum_merged_size += parts[i].size;
@@ -101,6 +110,14 @@ int main(int argc, char ** argv)
 
 		sum_size_written += sum_merged_size;
 		++num_merges;
+
+		double time_to_merge = sum_merged_size / (1048576 * 10.0);
+
+		age_passed += time_to_merge;
+		for (auto & part : parts)
+			part.age += time_to_merge;
+
+		std::cout << "Time passed: " << age_passed << ", num parts: " << parts.size() << '\n';
 	}
 
 	std::cout << std::fixed << std::setprecision(2)