ClickHouse/src/Storages/MergeTree/MergeTreeReadPool.h

#pragma once

#include <Core/NamesAndTypes.h>
#include <Storages/MergeTree/RangesInDataPart.h>
#include <Storages/MergeTree/MergeTreeBlockReadUtils.h>
#include <Storages/MergeTree/MergeTreeData.h>
#include <Storages/SelectQueryInfo.h>
#include <mutex>


namespace DB
{

using MergeTreeReadTaskPtr = std::unique_ptr<MergeTreeReadTask>;

/**   Provides read tasks for MergeTreeThreadSelectProcessor`s in fine-grained batches, allowing for more
 *    uniform distribution of work amongst multiple threads. All parts and their ranges are divided into `threads`
 *    workloads with at most `sum_marks / threads` marks. Then, threads are performing reads from these workloads
 *    in "sequential" manner, requesting work in small batches. As soon as some thread has exhausted
 *    it's workload, it either is signaled that no more work is available (`do_not_steal_tasks == false`) or
 *    continues taking small batches from other threads' workloads (`do_not_steal_tasks == true`).
 */
class MergeTreeReadPool : private boost::noncopyable
{
public:
    /** Pull could dynamically lower (backoff) number of threads, if read operation are too slow.
      * Settings for that backoff.
      */
    struct BackoffSettings
    {
        /// Pay attention only to reads, that took at least this amount of time. If set to 0 - means backoff is disabled.
        size_t min_read_latency_ms = 1000;
        /// Count events, when read throughput is less than specified bytes per second.
        size_t max_throughput = 1048576;
        /// Do not pay attention to event, if not enough time passed since previous event.
        size_t min_interval_between_events_ms = 1000;
        /// Number of events to do backoff - to lower number of threads in pool.
        size_t min_events = 2;
        /// Try keeping the minimal number of threads in pool.
        size_t min_concurrency = 1;

        /// Constants above is just an example.
        explicit BackoffSettings(const Settings & settings)
            : min_read_latency_ms(settings.read_backoff_min_latency_ms.totalMilliseconds()),
            max_throughput(settings.read_backoff_max_throughput),
            min_interval_between_events_ms(settings.read_backoff_min_interval_between_events_ms.totalMilliseconds()),
            min_events(settings.read_backoff_min_events),
            min_concurrency(settings.read_backoff_min_concurrency)
        {
        }

        BackoffSettings() : min_read_latency_ms(0) {}
    };

    BackoffSettings backoff_settings;

private:
    /** State to track numbers of slow reads.
      */
    struct BackoffState
    {
        size_t current_threads;
        Stopwatch time_since_prev_event {CLOCK_MONOTONIC_COARSE};
        size_t num_events = 0;

        explicit BackoffState(size_t threads) : current_threads(threads) {}
    };

    BackoffState backoff_state;

public:
    MergeTreeReadPool(
        size_t threads_, size_t sum_marks_, size_t min_marks_for_concurrent_read_,
        RangesInDataParts && parts_, const MergeTreeData & data_, const StorageSnapshotPtr & storage_snapshot_,
        const PrewhereInfoPtr & prewhere_info_,
        const Names & column_names_,
        const BackoffSettings & backoff_settings_, size_t preferred_block_size_bytes_,
        bool do_not_steal_tasks_ = false);

    MergeTreeReadTaskPtr getTask(size_t min_marks_to_read, size_t thread, const Names & ordered_names);

    /** Each worker could call this method and pass information about read performance.
      * If read performance is too low, pool could decide to lower number of threads: do not assign more tasks to several threads.
      * This allows to overcome excessive load to disk subsystem, when reads are not from page cache.
      */
    void profileFeedback(ReadBufferFromFileBase::ProfileInfo info);

    Block getHeader() const;

private:
    std::vector<size_t> fillPerPartInfo(const RangesInDataParts & parts);

    void fillPerThreadInfo(
        size_t threads, size_t sum_marks, std::vector<size_t> per_part_sum_marks,
        const RangesInDataParts & parts, size_t min_marks_for_concurrent_read);

    const MergeTreeData & data;
    StorageSnapshotPtr storage_snapshot;
    const Names column_names;
    bool do_not_steal_tasks;
    bool predict_block_size_bytes;

    struct PerPartParams
    {
        MergeTreeReadTaskColumns task_columns;
        NameSet column_name_set;
        MergeTreeBlockSizePredictorPtr size_predictor;
    };

    std::vector<PerPartParams> per_part_params;

    PrewhereInfoPtr prewhere_info;

    struct Part
    {
        MergeTreeData::DataPartPtr data_part;
        size_t part_index_in_query;
    };

    std::vector<Part> parts_with_idx;

    struct ThreadTask
    {
        struct PartIndexAndRange
        {
            size_t part_idx;
            MarkRanges ranges;
        };

        std::vector<PartIndexAndRange> parts_and_ranges;
        std::vector<size_t> sum_marks_in_parts;
    };

    std::vector<ThreadTask> threads_tasks;

    std::set<size_t> remaining_thread_tasks;

    RangesInDataParts parts_ranges;

    mutable std::mutex mutex;

    Poco::Logger * log = &Poco::Logger::get("MergeTreeReadPool");

    std::vector<bool> is_part_on_remote_disk;
};

using MergeTreeReadPoolPtr = std::shared_ptr<MergeTreeReadPool>;

}