scylladb/utils/logalloc.hh

/*
 * Copyright 2015 Cloudius Systems
 */

#pragma once

#include <bits/unique_ptr.h>
#include <seastar/core/scollectd.hh>
#include <seastar/core/memory.hh>
#include "allocation_strategy.hh"

namespace logalloc {

struct occupancy_stats;

using eviction_fn = std::function<void()>;

// Controller for all LSA regions. There's one per shard.
class tracker {
public:
    class impl;
private:
    std::unique_ptr<impl> _impl;
    memory::reclaimer _reclaimer;
    friend class region;
public:
    tracker();
    ~tracker();

    //
    // Tries to reclaim given amount of bytes in total using all compactible
    // and evictable regions. Returns the number of bytes actually reclaimed.
    // That value may be smaller than requested when evictable pools are empty
    // and compactible pools can't compact any more.
    //
    // Invalidates references to objects in all compactible and evictable regions.
    //
    size_t reclaim(size_t bytes);

    // Compacts as much as possible. Very expensive, mainly for testing.
    // Invalidates references to objects in all compactible and evictable regions.
    void full_compaction();

    // Returns aggregate statistics for all pools.
    occupancy_stats occupancy() const;
};

tracker& shard_tracker();

// Monoid representing pool occupancy statistics.
// Naturally ordered so that sparser pools come fist.
// All sizes in bytes.
class occupancy_stats {
    size_t _free_space;
    size_t _total_space;
public:
    occupancy_stats() : _free_space(0), _total_space(0) {}

    occupancy_stats(size_t free_space, size_t total_space)
        : _free_space(free_space), _total_space(total_space) { }

    bool operator<(const occupancy_stats& other) const {
        return used_fraction() < other.used_fraction();
    }

    friend occupancy_stats operator+(const occupancy_stats& s1, const occupancy_stats& s2) {
        occupancy_stats result(s1);
        result += s2;
        return result;
    }

    friend occupancy_stats operator-(const occupancy_stats& s1, const occupancy_stats& s2) {
        occupancy_stats result(s1);
        result -= s2;
        return result;
    }

    occupancy_stats& operator+=(const occupancy_stats& other) {
        _total_space += other._total_space;
        _free_space += other._free_space;
        return *this;
    }

    occupancy_stats& operator-=(const occupancy_stats& other) {
        _total_space -= other._total_space;
        _free_space -= other._free_space;
        return *this;
    }

    size_t used_space() const {
        return _total_space - _free_space;
    }

    size_t free_space() const {
        return _free_space;
    }

    size_t total_space() const {
        return _total_space;
    }

    float used_fraction() const {
        return _total_space ? float(used_space()) / total_space() : 0;
    }

    friend std::ostream& operator<<(std::ostream&, const occupancy_stats&);
};

//
// Log-structured allocator region.
//
// Objects allocated using this region are said to be owned by this region.
// Objects must be freed only using the region which owns them. Ownership can
// be transferred across regions using the merge() method. Region must be live
// as long as it owns any objects.
//
// Each region has separate memory accounting and can be compacted
// independently from other regions. To reclaim memory from all regions use
// shard_tracker().
//
// Region is automatically added to the set of
// compactible regions when constructed.
//
class region {
public:
    class impl;
private:
    std::unique_ptr<impl> _impl;
public:
    region();
    ~region();
    region(region&& other) = default;
    region& operator=(region&& other) = default;
    region(const region& other) = delete;

    occupancy_stats occupancy() const;

    allocation_strategy& allocator();

    // Merges another region into this region. The other region is left empty.
    // Doesn't invalidate references to allocated objects.
    void merge(region& other);

    // Compacts everything. Mainly for testing.
    // Invalidates references to allocated objects.
    void full_compaction();

    // Changes the compactibility state of this region. When region is not
    // compactible, it won't be considered by tracker::reclaim(). By default region is
    // compactible after construction.
    void set_compactible(bool);

    // Makes this region an evictable region. Supplied function will be called
    // when data from this region needs to be evicted in order to reclaim space.
    // The function should free some space from this region.
    void make_evictable(eviction_fn);
};

}