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85 lines
3.5 KiB
C++
85 lines
3.5 KiB
C++
#pragma once
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#include <DB/Storages/MergeTree/MergeSelector.h>
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namespace DB
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{
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class SimpleMergeSelector : public IMergeSelector
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{
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public:
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struct Settings
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{
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/// Zero means unlimited.
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size_t max_parts_to_merge_at_once = 100;
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/** Minimum ratio of size of one part to all parts in set of parts to merge (for usual cases).
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* For example, if all parts have equal size, it means, that at least 'base' number of parts should be merged.
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* If parts has non-uniform sizes, then minumum number of parts to merge is effectively increased.
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* This behaviour balances merge-tree workload.
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* It called 'base', because merge-tree depth could be estimated as logarithm with that base.
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*
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* If base is higher - then tree gets more wide and narrow, lowering write amplification.
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* If base is lower - then merges occurs more frequently, lowering number of parts in average.
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*
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* We need some balance between write amplification and number of parts.
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*/
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double base = 5;
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/** Base is lowered until 1 (effectively means "merge any two parts") depending on several variables:
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*
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* 1. Total number of parts in partition. If too much - then base is lowered.
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* It means: when too much parts - do merges more urgently.
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*
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* 2. Minimum age of parts participating in merge. If higher age - then base is lowered.
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* It means: do less wide merges only rarely.
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*
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* 3. Sum size of parts participating in merge. If higher - then more age is required to lower base. So, base is lowered slower.
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* It means: for small parts, it's worth to merge faster, even not so wide or balanced.
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*
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* We have multivariative dependency. Let it be logarithmic of size and somewhat multi-linear by other variables,
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* between some boundary points, and constant outside.
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*/
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size_t min_size_to_lower_base = 1024 * 1024;
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size_t max_size_to_lower_base = 100ULL * 1024 * 1024 * 1024;
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time_t min_age_to_lower_base_at_min_size = 10;
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time_t min_age_to_lower_base_at_max_size = 10;
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time_t max_age_to_lower_base_at_min_size = 3600;
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time_t max_age_to_lower_base_at_max_size = 30 * 86400;
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size_t min_parts_to_lower_base = 10;
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size_t max_parts_to_lower_base = 50;
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/// Add this to size before all calculations. It means: merging even very small parts has it's fixed cost.
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size_t size_fixed_cost_to_add = 5 * 1024 * 1024;
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/** Heuristic:
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* Make some preference for ranges, that sum_size is like (in terms of ratio) to part previous at left.
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*/
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bool enable_heuristic_to_align_parts = true;
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double heuristic_to_align_parts_min_ratio_of_sum_size_to_prev_part = 0.9;
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double heuristic_to_align_parts_max_absolute_difference_in_powers_of_two = 0.5;
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double heuristic_to_align_parts_max_score_adjustment = 0.75;
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/** Heuristic:
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* From right side of range, remove all parts, that size is less than specified ratio of sum_size.
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*/
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bool enable_heuristic_to_remove_small_parts_at_right = true;
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double heuristic_to_remove_small_parts_at_right_max_ratio = 0.01;
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};
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SimpleMergeSelector(const Settings & settings) : settings(settings) {}
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PartsInPartition select(
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const Partitions & partitions,
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const size_t max_total_size_to_merge) override;
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private:
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const Settings settings;
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};
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}
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