record_access_tracker.h

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#pragma once

#include <memory>

#include "mongo/util/concurrency/mutex.h"
#include "mongo/util/time_support.h"

namespace mongo {

class ClockSource;
class Date_t;
class MmapV1RecordHeader;

class RecordAccessTracker {
    MONGO_DISALLOW_COPYING(RecordAccessTracker);

public:
    RecordAccessTracker(ClockSource* cs);

    enum Constants {
        SliceSize = 1024,
        MaxChain = 20,  // intentionally very low
        NumSlices = 10,
        RotateTimeSecs = 90,
        BigHashSize = 128
    };

    void markAccessed(const void* record);

    bool checkAccessedAndMark(const void* record);

    void reset();

    //
    // For testing.
    //

    void disableSystemBlockInMemCheck();

private:
    enum State { In, Out, Unk };

    struct Entry {
        size_t region;
        unsigned long long value;
    };

    class Slice {
    public:
        Slice();

        void reset();

        State get(int regionHash, size_t region, short offset);

        bool put(int regionHash, size_t region, short offset);

    private:
        Entry* _get(int start, size_t region, bool add);

        Entry _data[SliceSize];
    };

    class Rolling {
    public:
        Rolling() = default;

        bool access(size_t region, short offset, bool doHalf, ClockSource* cs);

        void updateLastRotate(ClockSource* cs);

    private:
        void _rotate(ClockSource* cs);

        int _curSlice = 0;
        Date_t _lastRotate;
        Slice _slices[NumSlices];

        SimpleMutex _lock;
    };

    // Should this record tracker fallback to making a system call?
    bool _blockSupported;
    ClockSource* _clock;

    // An array of Rolling instances for tracking record accesses.
    std::unique_ptr<Rolling[]> _rollingTable;
};

}  // namespace