Add CuckooCache implementation and replace the sigcache map_type with it

SQUASHME: Change cuckoocache to only work for powers of two, to avoid mod operator
SQUASHME: Update Documentation and simplify logarithm logic
SQUASHME: OSX Build Errors
SQUASHME: minor Feedback from sipa + bluematt
SQUASHME: DOCONLY: Clarify a few comments.

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