git / moonbite-core / src/cuckoocache.h · raw
// Copyright (c) 2016 Jeremy Rubin// Distributed under the MIT software license, see the accompanying// file COPYING or http://www.opensource.org/licenses/mit-license.php.#ifndef BITCOIN_CUCKOOCACHE_H#define BITCOIN_CUCKOOCACHE_H#include <algorithm> // std::find#include <array>#include <atomic>#include <cmath>#include <cstring>#include <memory>#include <utility>#include <vector>/** High-performance cache primitives. * * Summary: * * 1. @ref bit_packed_atomic_flags is bit-packed atomic flags for garbage collection * * 2. @ref cache is a cache which is performant in memory usage and lookup speed. It * is lockfree for erase operations. Elements are lazily erased on the next insert. */namespace CuckooCache{/** @ref bit_packed_atomic_flags implements a container for garbage collection flags * that is only thread unsafe on calls to setup. This class bit-packs collection * flags for memory efficiency. * * All operations are `std::memory_order_relaxed` so external mechanisms must * ensure that writes and reads are properly synchronized. * * On setup(n), all bits up to `n` are marked as collected. * * Under the hood, because it is an 8-bit type, it makes sense to use a multiple * of 8 for setup, but it will be safe if that is not the case as well. */class bit_packed_atomic_flags{ std::unique_ptr<std::atomic<uint8_t>[]> mem;public: /** No default constructor, as there must be some size. */ bit_packed_atomic_flags() = delete; /** * bit_packed_atomic_flags constructor creates memory to sufficiently * keep track of garbage collection information for `size` entries. * * @param size the number of elements to allocate space for * * @post bit_set, bit_unset, and bit_is_set function properly forall x. x < * size * @post All calls to bit_is_set (without subsequent bit_unset) will return * true. */ explicit bit_packed_atomic_flags(uint32_t size) { // pad out the size if needed size = (size + 7) / 8; mem.reset(new std::atomic<uint8_t>[size]); for (uint32_t i = 0; i < size; ++i) mem[i].store(0xFF); }; /** setup marks all entries and ensures that bit_packed_atomic_flags can store * at least `b` entries. * * @param b the number of elements to allocate space for * @post bit_set, bit_unset, and bit_is_set function properly forall x. x < * b * @post All calls to bit_is_set (without subsequent bit_unset) will return * true. */ inline void setup(uint32_t b) { bit_packed_atomic_flags d(b); std::swap(mem, d.mem); } /** bit_set sets an entry as discardable. * * @param s the index of the entry to bit_set * @post immediately subsequent call (assuming proper external memory * ordering) to bit_is_set(s) == true. */ inline void bit_set(uint32_t s) { mem[s >> 3].fetch_or(1 << (s & 7), std::memory_order_relaxed); } /** bit_unset marks an entry as something that should not be overwritten. * * @param s the index of the entry to bit_unset * @post immediately subsequent call (assuming proper external memory * ordering) to bit_is_set(s) == false. */ inline void bit_unset(uint32_t s) { mem[s >> 3].fetch_and(~(1 << (s & 7)), std::memory_order_relaxed); } /** bit_is_set queries the table for discardability at `s`. * * @param s the index of the entry to read * @returns true if the bit at index `s` was set, false otherwise * */ inline bool bit_is_set(uint32_t s) const { return (1 << (s & 7)) & mem[s >> 3].load(std::memory_order_relaxed); }};/** @ref cache implements a cache with properties similar to a cuckoo-set. * * The cache is able to hold up to `(~(uint32_t)0) - 1` elements. * * Read Operations: * - contains() for `erase=false` * * Read+Erase Operations: * - contains() for `erase=true` * * Erase Operations: * - allow_erase() * * Write Operations: * - setup() * - setup_bytes() * - insert() * - please_keep() * * Synchronization Free Operations: * - invalid() * - compute_hashes() * * User Must Guarantee: * * 1. Write requires synchronized access (e.g. a lock) * 2. Read requires no concurrent Write, synchronized with last insert. * 3. Erase requires no concurrent Write, synchronized with last insert. * 4. An Erase caller must release all memory before allowing a new Writer. * * * Note on function names: * - The name "allow_erase" is used because the real discard happens later. * - The name "please_keep" is used because elements may be erased anyways on insert. * * @tparam Element should be a movable and copyable type * @tparam Hash should be a function/callable which takes a template parameter * hash_select and an Element and extracts a hash from it. Should return * high-entropy uint32_t hashes for `Hash h; h<0>(e) ... h<7>(e)`. */template <typename Element, typename Hash>class cache{private: /** table stores all the elements */ std::vector<Element> table; /** size stores the total available slots in the hash table */ uint32_t size; /** The bit_packed_atomic_flags array is marked mutable because we want * garbage collection to be allowed to occur from const methods */ mutable bit_packed_atomic_flags collection_flags; /** epoch_flags tracks how recently an element was inserted into * the cache. true denotes recent, false denotes not-recent. See insert() * method for full semantics. */ mutable std::vector<bool> epoch_flags; /** epoch_heuristic_counter is used to determine when an epoch might be aged * & an expensive scan should be done. epoch_heuristic_counter is * decremented on insert and reset to the new number of inserts which would * cause the epoch to reach epoch_size when it reaches zero. */ uint32_t epoch_heuristic_counter; /** epoch_size is set to be the number of elements supposed to be in a * epoch. When the number of non-erased elements in an epoch * exceeds epoch_size, a new epoch should be started and all * current entries demoted. epoch_size is set to be 45% of size because * we want to keep load around 90%, and we support 3 epochs at once -- * one "dead" which has been erased, one "dying" which has been marked to be * erased next, and one "living" which new inserts add to. */ uint32_t epoch_size; /** depth_limit determines how many elements insert should try to replace. * Should be set to log2(n). */ uint8_t depth_limit; /** hash_function is a const instance of the hash function. It cannot be * static or initialized at call time as it may have internal state (such as * a nonce). */ const Hash hash_function; /** compute_hashes is convenience for not having to write out this * expression everywhere we use the hash values of an Element. * * We need to map the 32-bit input hash onto a hash bucket in a range [0, size) in a * manner which preserves as much of the hash's uniformity as possible. Ideally * this would be done by bitmasking but the size is usually not a power of two. * * The naive approach would be to use a mod -- which isn't perfectly uniform but so * long as the hash is much larger than size it is not that bad. Unfortunately, * mod/division is fairly slow on ordinary microprocessors (e.g. 90-ish cycles on * haswell, ARM doesn't even have an instruction for it.); when the divisor is a * constant the compiler will do clever tricks to turn it into a multiply+add+shift, * but size is a run-time value so the compiler can't do that here. * * One option would be to implement the same trick the compiler uses and compute the * constants for exact division based on the size, as described in "{N}-bit Unsigned * Division via {N}-bit Multiply-Add" by Arch D. Robison in 2005. But that code is * somewhat complicated and the result is still slower than other options: * * Instead we treat the 32-bit random number as a Q32 fixed-point number in the range * [0, 1) and simply multiply it by the size. Then we just shift the result down by * 32-bits to get our bucket number. The result has non-uniformity the same as a * mod, but it is much faster to compute. More about this technique can be found at * http://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/ . * * The resulting non-uniformity is also more equally distributed which would be * advantageous for something like linear probing, though it shouldn't matter * one way or the other for a cuckoo table. * * The primary disadvantage of this approach is increased intermediate precision is * required but for a 32-bit random number we only need the high 32 bits of a * 32*32->64 multiply, which means the operation is reasonably fast even on a * typical 32-bit processor. * * @param e The element whose hashes will be returned * @returns Deterministic hashes derived from `e` uniformly mapped onto the range [0, size) */ inline std::array<uint32_t, 8> compute_hashes(const Element& e) const { return {{(uint32_t)(((uint64_t)hash_function.template operator()<0>(e) * (uint64_t)size) >> 32), (uint32_t)(((uint64_t)hash_function.template operator()<1>(e) * (uint64_t)size) >> 32), (uint32_t)(((uint64_t)hash_function.template operator()<2>(e) * (uint64_t)size) >> 32), (uint32_t)(((uint64_t)hash_function.template operator()<3>(e) * (uint64_t)size) >> 32), (uint32_t)(((uint64_t)hash_function.template operator()<4>(e) * (uint64_t)size) >> 32), (uint32_t)(((uint64_t)hash_function.template operator()<5>(e) * (uint64_t)size) >> 32), (uint32_t)(((uint64_t)hash_function.template operator()<6>(e) * (uint64_t)size) >> 32), (uint32_t)(((uint64_t)hash_function.template operator()<7>(e) * (uint64_t)size) >> 32)}}; } /** invalid returns a special index that can never be inserted to * @returns the special constexpr index that can never be inserted to */ constexpr uint32_t invalid() const { return ~(uint32_t)0; } /** allow_erase marks the element at index `n` as discardable. Threadsafe * without any concurrent insert. * @param n the index to allow erasure of */ inline void allow_erase(uint32_t n) const { collection_flags.bit_set(n); } /** please_keep marks the element at index `n` as an entry that should be kept. * Threadsafe without any concurrent insert. * @param n the index to prioritize keeping */ inline void please_keep(uint32_t n) const { collection_flags.bit_unset(n); } /** epoch_check handles the changing of epochs for elements stored in the * cache. epoch_check should be run before every insert. * * First, epoch_check decrements and checks the cheap heuristic, and then does * a more expensive scan if the cheap heuristic runs out. If the expensive * scan succeeds, the epochs are aged and old elements are allow_erased. The * cheap heuristic is reset to retrigger after the worst case growth of the * current epoch's elements would exceed the epoch_size. */ void epoch_check() { if (epoch_heuristic_counter != 0) { --epoch_heuristic_counter; return; } // count the number of elements from the latest epoch which // have not been erased. uint32_t epoch_unused_count = 0; for (uint32_t i = 0; i < size; ++i) epoch_unused_count += epoch_flags[i] && !collection_flags.bit_is_set(i); // If there are more non-deleted entries in the current epoch than the // epoch size, then allow_erase on all elements in the old epoch (marked // false) and move all elements in the current epoch to the old epoch // but do not call allow_erase on their indices. if (epoch_unused_count >= epoch_size) { for (uint32_t i = 0; i < size; ++i) if (epoch_flags[i]) epoch_flags[i] = false; else allow_erase(i); epoch_heuristic_counter = epoch_size; } else // reset the epoch_heuristic_counter to next do a scan when worst // case behavior (no intermittent erases) would exceed epoch size, // with a reasonable minimum scan size. // Ordinarily, we would have to sanity check std::min(epoch_size, // epoch_unused_count), but we already know that `epoch_unused_count // < epoch_size` in this branch epoch_heuristic_counter = std::max(1u, std::max(epoch_size / 16, epoch_size - epoch_unused_count)); }public: /** You must always construct a cache with some elements via a subsequent * call to setup or setup_bytes, otherwise operations may segfault. */ cache() : table(), size(), collection_flags(0), epoch_flags(), epoch_heuristic_counter(), epoch_size(), depth_limit(0), hash_function() { } /** setup initializes the container to store no more than new_size * elements. * * setup should only be called once. * * @param new_size the desired number of elements to store * @returns the maximum number of elements storable */ uint32_t setup(uint32_t new_size) { // depth_limit must be at least one otherwise errors can occur. depth_limit = static_cast<uint8_t>(std::log2(static_cast<float>(std::max((uint32_t)2, new_size)))); size = std::max<uint32_t>(2, new_size); table.resize(size); collection_flags.setup(size); epoch_flags.resize(size); // Set to 45% as described above epoch_size = std::max((uint32_t)1, (45 * size) / 100); // Initially set to wait for a whole epoch epoch_heuristic_counter = epoch_size; return size; } /** setup_bytes is a convenience function which accounts for internal memory * usage when deciding how many elements to store. It isn't perfect because * it doesn't account for any overhead (struct size, MallocUsage, collection * and epoch flags). This was done to simplify selecting a power of two * size. In the expected use case, an extra two bits per entry should be * negligible compared to the size of the elements. * * @param bytes the approximate number of bytes to use for this data * structure * @returns the maximum number of elements storable (see setup() * documentation for more detail) */ uint32_t setup_bytes(size_t bytes) { return setup(bytes/sizeof(Element)); } /** insert loops at most depth_limit times trying to insert a hash * at various locations in the table via a variant of the Cuckoo Algorithm * with eight hash locations. * * It drops the last tried element if it runs out of depth before * encountering an open slot. * * Thus: * * ``` * insert(x); * return contains(x, false); * ``` * * is not guaranteed to return true. * * @param e the element to insert * @post one of the following: All previously inserted elements and e are * now in the table, one previously inserted element is evicted from the * table, the entry attempted to be inserted is evicted. */ inline void insert(Element e) { epoch_check(); uint32_t last_loc = invalid(); bool last_epoch = true; std::array<uint32_t, 8> locs = compute_hashes(e); // Make sure we have not already inserted this element // If we have, make sure that it does not get deleted for (const uint32_t loc : locs) if (table[loc] == e) { please_keep(loc); epoch_flags[loc] = last_epoch; return; } for (uint8_t depth = 0; depth < depth_limit; ++depth) { // First try to insert to an empty slot, if one exists for (const uint32_t loc : locs) { if (!collection_flags.bit_is_set(loc)) continue; table[loc] = std::move(e); please_keep(loc); epoch_flags[loc] = last_epoch; return; } /** Swap with the element at the location that was * not the last one looked at. Example: * * 1. On first iteration, last_loc == invalid(), find returns last, so * last_loc defaults to locs[0]. * 2. On further iterations, where last_loc == locs[k], last_loc will * go to locs[k+1 % 8], i.e., next of the 8 indices wrapping around * to 0 if needed. * * This prevents moving the element we just put in. * * The swap is not a move -- we must switch onto the evicted element * for the next iteration. */ last_loc = locs[(1 + (std::find(locs.begin(), locs.end(), last_loc) - locs.begin())) & 7]; std::swap(table[last_loc], e); // Can't std::swap a std::vector<bool>::reference and a bool&. bool epoch = last_epoch; last_epoch = epoch_flags[last_loc]; epoch_flags[last_loc] = epoch; // Recompute the locs -- unfortunately happens one too many times! locs = compute_hashes(e); } } /** contains iterates through the hash locations for a given element * and checks to see if it is present. * * contains does not check garbage collected state (in other words, * garbage is only collected when the space is needed), so: * * ``` * insert(x); * if (contains(x, true)) * return contains(x, false); * else * return true; * ``` * * executed on a single thread will always return true! * * This is a great property for re-org performance for example. * * contains returns a bool set true if the element was found. * * @param e the element to check * @param erase whether to attempt setting the garbage collect flag * * @post if erase is true and the element is found, then the garbage collect * flag is set * @returns true if the element is found, false otherwise */ inline bool contains(const Element& e, const bool erase) const { std::array<uint32_t, 8> locs = compute_hashes(e); for (const uint32_t loc : locs) if (table[loc] == e) { if (erase) allow_erase(loc); return true; } return false; }};} // namespace CuckooCache#endif // BITCOIN_CUCKOOCACHE_H