pureffi.lua
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-- Copyright (C) Yichun Zhang (agentzh) -- Copyright (C) Shuxin Yang --[[ This module implements a key/value cache store. We adopt LRU as our replace/evict policy. Each key/value pair is tagged with a Time-to-Live (TTL); from user's perspective, stale pairs are automatically removed from the cache. Why FFI ------- In Lua, expression "table[key] = nil" does not *PHYSICALLY* remove the value associated with the key; it just set the value to be nil! So the table will keep growing with large number of the key/nil pairs which will be purged until resize() operator is called. This "feature" is terribly ill-suited to what we need. Therefore we have to rely on FFI to build a hash-table where any entry can be physically deleted immediately. Under the hood: -------------- In concept, we introduce three data structures to implement the cache store: 1. key/value vector for storing keys and values. 2. a queue to mimic the LRU. 3. hash-table for looking up the value for a given key. Unfortunately, efficiency and clarity usually come at each other cost. The data strucutres we are using are slightly more complicated than what we described above. o. Lua does not have efficient way to store a vector of pair. So, we use two vectors for key/value pair: one for keys and the other for values (_M.key_v and _M.val_v, respectively), and i-th key corresponds to i-th value. A key/value pair is identified by the "id" field in a "node" (we shall discuss node later) o. The queue is nothing more than a doubly-linked list of "node" linked via lrucache_pureffi_queue_s::{next|prev} fields. o. The hash-table has two parts: - the _M.bucket_v[] a vector of bucket, indiced by hash-value, and - a bucket is a singly-linked list of "node" via the lrucache_pureffi_queue_s::conflict field. A key must be a string, and the hash value of a key is evaluated by: crc32(key-cast-to-pointer) % size(_M.bucket_v). We mandate size(_M.bucket_v) being a power-of-two in order to avoid expensive modulo operation. At the heart of the module is an array of "node" (of type lrucache_pureffi_queue_s). A node: - keeps the meta-data of its corresponding key/value pair (embodied by the "id", and "expire" field); - is a part of LRU queue (embodied by "prev" and "next" fields); - is a part of hash-table (embodied by the "conflict" field). ]] local ffi = require "ffi" local bit = require "bit" local ffi_new = ffi.new local ffi_sizeof = ffi.sizeof local ffi_cast = ffi.cast local ffi_fill = ffi.fill local ngx_now = ngx.now local uintptr_t = ffi.typeof("uintptr_t") local c_str_t = ffi.typeof("const char*") local int_t = ffi.typeof("int") local int_array_t = ffi.typeof("int[?]") local crc_tab = ffi.new("const unsigned int[256]", { 0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 0x706AF48F, 0xE963A535, 0x9E6495A3, 0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988, 0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91, 0x1DB71064, 0x6AB020F2, 0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7, 0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9, 0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172, 0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B, 0x35B5A8FA, 0x42B2986C, 0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59, 0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423, 0xCFBA9599, 0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924, 0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190, 0x01DB7106, 0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F, 0x9FBFE4A5, 0xE8B8D433, 0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x086D3D2D, 0x91646C97, 0xE6635C01, 0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E, 0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457, 0x65B0D9C6, 0x12B7E950, 0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65, 0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7, 0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0, 0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA, 0xBE0B1010, 0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F, 0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 0x2EB40D81, 0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A, 0xEAD54739, 0x9DD277AF, 0x04DB2615, 0x73DC1683, 0xE3630B12, 0x94643B84, 0x0D6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1, 0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB, 0x196C3671, 0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC, 0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5, 0xD6D6A3E8, 0xA1D1937E, 0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B, 0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55, 0x316E8EEF, 0x4669BE79, 0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236, 0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE, 0xB2BD0B28, 0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D, 0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A, 0x9C0906A9, 0xEB0E363F, 0x72076785, 0x05005713, 0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38, 0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21, 0x86D3D2D4, 0xF1D4E242, 0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777, 0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69, 0x616BFFD3, 0x166CCF45, 0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2, 0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB, 0xAED16A4A, 0xD9D65ADC, 0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9, 0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693, 0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94, 0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D }); local setmetatable = setmetatable local tonumber = tonumber local tostring = tostring local type = type local brshift = bit.rshift local bxor = bit.bxor local band = bit.band local new_tab do local ok ok, new_tab = pcall(require, "table.new") if not ok then new_tab = function(narr, nrec) return {} end end end -- queue data types -- -- this queue is a double-ended queue and the first node -- is reserved for the queue itself. -- the implementation is mostly borrowed from nginx's ngx_queue_t data -- structure. ffi.cdef[[ /* A lrucache_pureffi_queue_s node hook together three data structures: * o. the key/value store as embodied by the "id" (which is in essence the * indentifier of key/pair pair) and the "expire" (which is a metadata * of the corresponding key/pair pair). * o. The LRU queue via the prev/next fields. * o. The hash-tabble as embodied by the "conflict" field. */ typedef struct lrucache_pureffi_queue_s lrucache_pureffi_queue_t; struct lrucache_pureffi_queue_s { /* Each node is assigned a unique ID at construction time, and the * ID remain immutatble, regardless the node is in active-list or * free-list. The queue header is assigned ID 0. Since queue-header * is a sentinel node, 0 denodes "invalid ID". * * Intuitively, we can view the "id" as the identifier of key/value * pair. */ int id; /* The bucket of the hash-table is implemented as a singly-linked list. * The "conflict" refers to the ID of the next node in the bucket. */ int conflict; uint32_t user_flags; double expire; /* in seconds */ lrucache_pureffi_queue_t *prev; lrucache_pureffi_queue_t *next; }; ]] local queue_arr_type = ffi.typeof("lrucache_pureffi_queue_t[?]") --local queue_ptr_type = ffi.typeof("lrucache_pureffi_queue_t*") local queue_type = ffi.typeof("lrucache_pureffi_queue_t") local NULL = ffi.null --======================================================================== -- -- Queue utility functions -- --======================================================================== -- Append the element "x" to the given queue "h". local function queue_insert_tail(h, x) local last = h[0].prev x.prev = last last.next = x x.next = h h[0].prev = x end --[[ Allocate a queue with size + 1 elements. Elements are linked together in a circular way, i.e. the last element's "next" points to the first element, while the first element's "prev" element points to the last element. ]] local function queue_init(size) if not size then size = 0 end local q = ffi_new(queue_arr_type, size + 1) ffi_fill(q, ffi_sizeof(queue_type, size + 1), 0) if size == 0 then q[0].prev = q q[0].next = q else local prev = q[0] for i = 1, size do local e = q[i] e.id = i e.user_flags = 0 prev.next = e e.prev = prev prev = e end local last = q[size] last.next = q q[0].prev = last end return q end local function queue_is_empty(q) -- print("q: ", tostring(q), "q.prev: ", tostring(q), ": ", q == q.prev) return q == q[0].prev end local function queue_remove(x) local prev = x.prev local next = x.next next.prev = prev prev.next = next -- for debugging purpose only: x.prev = NULL x.next = NULL end -- Insert the element "x" the to the given queue "h" local function queue_insert_head(h, x) x.next = h[0].next x.next.prev = x x.prev = h h[0].next = x end local function queue_last(h) return h[0].prev end local function queue_head(h) return h[0].next end --======================================================================== -- -- Miscellaneous Utility Functions -- --======================================================================== local function ptr2num(ptr) return tonumber(ffi_cast(uintptr_t, ptr)) end local function crc32_ptr(ptr) local p = brshift(ptr2num(ptr), 3) local b = band(p, 255) local crc32 = crc_tab[b] b = band(brshift(p, 8), 255) crc32 = bxor(brshift(crc32, 8), crc_tab[band(bxor(crc32, b), 255)]) b = band(brshift(p, 16), 255) crc32 = bxor(brshift(crc32, 8), crc_tab[band(bxor(crc32, b), 255)]) --b = band(brshift(p, 24), 255) --crc32 = bxor(brshift(crc32, 8), crc_tab[band(bxor(crc32, b), 255)]) return crc32 end --======================================================================== -- -- Implementation of "export" functions -- --======================================================================== local _M = { _VERSION = '0.13' } local mt = { __index = _M } -- "size" specifies the maximum number of entries in the LRU queue, and the -- "load_factor" designates the 'load factor' of the hash-table we are using -- internally. The default value of load-factor is 0.5 (i.e. 50%); if the -- load-factor is specified, it will be clamped to the range of [0.1, 1](i.e. -- if load-factor is greater than 1, it will be saturated to 1, likewise, -- if load-factor is smaller than 0.1, it will be clamped to 0.1). function _M.new(size, load_factor) if size < 1 then return nil, "size too small" end -- Determine bucket size, which must be power of two. local load_f = load_factor if not load_factor then load_f = 0.5 elseif load_factor > 1 then load_f = 1 elseif load_factor < 0.1 then load_f = 0.1 end local bs_min = size / load_f -- The bucket_sz *MUST* be a power-of-two. See the hash_string(). local bucket_sz = 1 repeat bucket_sz = bucket_sz * 2 until bucket_sz >= bs_min local self = { size = size, bucket_sz = bucket_sz, free_queue = queue_init(size), cache_queue = queue_init(0), node_v = nil, key_v = new_tab(size, 0), val_v = new_tab(size, 0), bucket_v = ffi_new(int_array_t, bucket_sz), num_items = 0, } -- "node_v" is an array of all the nodes used in the LRU queue. Exprpession -- node_v[i] evaluates to the element of ID "i". self.node_v = self.free_queue -- Allocate the array-part of the key_v, val_v, bucket_v. --local key_v = self.key_v --local val_v = self.val_v --local bucket_v = self.bucket_v ffi_fill(self.bucket_v, ffi_sizeof(int_t, bucket_sz), 0) return setmetatable(self, mt) end function _M.count(self) return self.num_items end function _M.capacity(self) return self.size end local function hash_string(self, str) local c_str = ffi_cast(c_str_t, str) local hv = crc32_ptr(c_str) hv = band(hv, self.bucket_sz - 1) -- Hint: bucket is 0-based return hv end -- Search the node associated with the key in the bucket, if found returns -- the the id of the node, and the id of its previous node in the conflict list. -- The "bucket_hdr_id" is the ID of the first node in the bucket local function _find_node_in_bucket(key, key_v, node_v, bucket_hdr_id) if bucket_hdr_id ~= 0 then local prev = 0 local cur = bucket_hdr_id while cur ~= 0 and key_v[cur] ~= key do prev = cur cur = node_v[cur].conflict end if cur ~= 0 then return cur, prev end end end -- Return the node corresponding to the key/val. local function find_key(self, key) local key_hash = hash_string(self, key) return _find_node_in_bucket(key, self.key_v, self.node_v, self.bucket_v[key_hash]) end --[[ This function tries to 1. Remove the given key and the associated value from the key/value store, 2. Remove the entry associated with the key from the hash-table. NOTE: all queues remain intact. If there was a node bound to the key/val, return that node; otherwise, nil is returned. ]] local function remove_key(self, key) local key_v = self.key_v local val_v = self.val_v local node_v = self.node_v local bucket_v = self.bucket_v local key_hash = hash_string(self, key) local cur, prev = _find_node_in_bucket(key, key_v, node_v, bucket_v[key_hash]) if cur then -- In an attempt to make key and val dead. key_v[cur] = nil val_v[cur] = nil self.num_items = self.num_items - 1 -- Remove the node from the hash table local next_node = node_v[cur].conflict if prev ~= 0 then node_v[prev].conflict = next_node else bucket_v[key_hash] = next_node end node_v[cur].conflict = 0 return cur end end --[[ Bind the key/val with the given node, and insert the node into the Hashtab. NOTE: this function does not touch any queue ]] local function insert_key(self, key, val, node) -- Bind the key/val with the node local node_id = node.id self.key_v[node_id] = key self.val_v[node_id] = val -- Insert the node into the hash-table local key_hash = hash_string(self, key) local bucket_v = self.bucket_v node.conflict = bucket_v[key_hash] bucket_v[key_hash] = node_id self.num_items = self.num_items + 1 end function _M.get(self, key) if type(key) ~= "string" then key = tostring(key) end local node_id = find_key(self, key) if not node_id then return nil end -- print(key, ": moving node ", tostring(node), " to cache queue head") local cache_queue = self.cache_queue local node = self.node_v + node_id queue_remove(node) queue_insert_head(cache_queue, node) local expire = node.expire if expire >= 0 and expire < ngx_now() then -- print("expired: ", node.expire, " > ", ngx_now()) return nil, self.val_v[node_id], node.user_flags end return self.val_v[node_id], nil, node.user_flags end function _M.delete(self, key) if type(key) ~= "string" then key = tostring(key) end local node_id = remove_key(self, key); if not node_id then return false end local node = self.node_v + node_id queue_remove(node) queue_insert_tail(self.free_queue, node) return true end function _M.set(self, key, value, ttl, flags) if type(key) ~= "string" then key = tostring(key) end local node_id = find_key(self, key) local node if not node_id then local free_queue = self.free_queue if queue_is_empty(free_queue) then -- evict the least recently used key -- assert(not queue_is_empty(self.cache_queue)) node = queue_last(self.cache_queue) remove_key(self, self.key_v[node.id]) else -- take a free queue node node = queue_head(free_queue) -- print(key, ": get a new free node: ", tostring(node)) end -- insert the key insert_key(self, key, value, node) else node = self.node_v + node_id self.val_v[node_id] = value end queue_remove(node) queue_insert_head(self.cache_queue, node) if ttl then node.expire = ngx_now() + ttl else node.expire = -1 end if type(flags) == "number" and flags >= 0 then node.user_flags = flags else node.user_flags = 0 end end function _M.get_keys(self, max_count, res) if not max_count or max_count == 0 then max_count = self.num_items end if not res then res = new_tab(max_count + 1, 0) -- + 1 for trailing hole end local cache_queue = self.cache_queue local key_v = self.key_v local i = 0 local node = queue_head(cache_queue) while node ~= cache_queue do if i >= max_count then break end i = i + 1 res[i] = key_v[node.id] node = node.next end res[i + 1] = nil return res end function _M.flush_all(self) local cache_queue = self.cache_queue local key_v = self.key_v local node = queue_head(cache_queue) while node ~= cache_queue do local key = key_v[node.id] node = node.next _M.delete(self, key) end end return _M
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