diff options
Diffstat (limited to 'src/cuckoocache.h')
| -rw-r--r-- | src/cuckoocache.h | 457 |
1 files changed, 457 insertions, 0 deletions
diff --git a/src/cuckoocache.h b/src/cuckoocache.h new file mode 100644 index 0000000000..efd6a820b5 --- /dev/null +++ b/src/cuckoocache.h @@ -0,0 +1,457 @@ +// 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 <array> +#include <algorithm> +#include <atomic> +#include <cstring> +#include <cmath> +#include <memory> +#include <vector> + + +/** namespace CuckooCache provides high performance cache primitives + * + * Summary: + * + * 1) bit_packed_atomic_flags is bit-packed atomic flags for garbage collection + * + * 2) 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 +{ +/** 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. + */ + 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 size 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 if the bit at index s was set. + * */ + inline bool bit_is_set(uint32_t s) const + { + return (1 << (s & 7)) & mem[s >> 3].load(std::memory_order_relaxed); + } +}; + +/** 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(*, false) + * + * Read+Erase Operations: + * - contains(*, 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 the 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 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 a 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 a 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; + + /** hash_mask should be set to appropriately mask out a hash such that every + * masked hash is [0,size), eg, if floor(log2(size)) == 20, then hash_mask + * should be (1<<20)-1 + */ + uint32_t hash_mask; + + /** 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. + * + * @param e the element whose hashes will be returned + * @returns std::array<uint32_t, 8> of deterministic hashes derived from e + */ + inline std::array<uint32_t, 8> compute_hashes(const Element& e) const + { + return {{hash_function.template operator()<0>(e) & hash_mask, + hash_function.template operator()<1>(e) & hash_mask, + hash_function.template operator()<2>(e) & hash_mask, + hash_function.template operator()<3>(e) & hash_mask, + hash_function.template operator()<4>(e) & hash_mask, + hash_function.template operator()<5>(e) & hash_mask, + hash_function.template operator()<6>(e) & hash_mask, + hash_function.template operator()<7>(e) & hash_mask}}; + } + + /* end + * @returns a 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 suceeds, 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 rounds down to a power of two size. + * + * 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 = 1 << depth_limit; + hash_mask = size-1; + 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 (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 (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 indicies 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 + * + * @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 (uint32_t loc : locs) + if (table[loc] == e) { + if (erase) + allow_erase(loc); + return true; + } + return false; + } +}; +} // namespace CuckooCache + +#endif |