serialize the most recently used blitbuffer/koptcontext

to speedup koreader startup for PDF/DJVU documents
especially when reflowing

frontend/cache.lua

@@ -1,6 +1,9 @@
 --[[
 A global LRU cache
 ]]--
+require("MD5")
+local DEBUG = require("dbg")
+
 local function calcFreeMem()
     local meminfo = io.open("/proc/meminfo", "r")
     local freemem = 0
@@ -26,6 +29,22 @@ local function calcCacheMemSize()
     return math.min(max, math.max(min, calc))
 end
 
+local cache_path = lfs.currentdir().."/cache/"
+
+--[[
+-- return a snapshot of disk cached items for subsequent check
+--]]
+function getDiskCache()
+    local cached = {}
+    for key_md5 in lfs.dir(cache_path) do
+        local file = cache_path..key_md5
+        if lfs.attributes(file, "mode") == "file" then
+            cached[key_md5] = file
+        end
+    end
+    return cached
+end
+
 local Cache = {
     -- cache configuration:
     max_memsize = calcCacheMemSize(),
@@ -34,7 +53,9 @@ local Cache = {
     -- associative cache
     cache = {},
     -- this will hold the LRU order of the cache
-    cache_order = {}
+    cache_order = {},
+    -- disk Cache snapshot
+    cached = getDiskCache(),
 }
 
 function Cache:new(o)
@@ -64,7 +85,11 @@ function Cache:insert(key, object)
     self.current_memsize = self.current_memsize + object.size
 end
 
-function Cache:check(key)
+--[[
+--  check for cache item for key
+--  if ItemClass is given, disk cache is also checked.
+--]]
+function Cache:check(key, ItemClass)
     if self.cache[key] then
         if self.cache_order[1] ~= key then
             -- put key in front of the LRU list
@@ -76,6 +101,18 @@ function Cache:check(key)
             table.insert(self.cache_order, 1, key)
         end
         return self.cache[key]
+    elseif ItemClass then
+        local cached = self.cached[md5(key)]
+        if cached then
+            local item = ItemClass:new{}
+            local ok, msg = pcall(item.load, item, cached)
+            if ok then
+                self:insert(key, item)
+                return item
+            else
+                DEBUG("discard cache", msg)
+            end
+        end
     end
 end
 
@@ -86,6 +123,34 @@ function Cache:willAccept(size)
     end
 end
 
+function Cache:serialize()
+    -- calculate disk cache size
+    local cached_size = 0
+    local sorted_caches = {}
+    for _,file in pairs(self.cached) do
+        table.insert(sorted_caches, {file=file, time=lfs.attributes(file, "access")})
+        cached_size = cached_size + (lfs.attributes(file, "size") or 0)
+    end
+    table.sort(sorted_caches, function(v1,v2) return v1.time > v2.time end)
+    -- serialize the most recently used cache
+    local cache_size = 0
+    for _, key in ipairs(self.cache_order) do
+        if self.cache[key].dump then
+            cache_size = self.cache[key]:dump(cache_path..md5(key)) or 0
+            if cache_size > 0 then break end
+        end
+    end
+    -- set disk cache the same limit as memory cache
+    while cached_size + cache_size - self.max_memsize > 0 do
+        -- discard the least recently used cache
+        local discarded = table.remove(sorted_caches)
+        cached_size = cached_size - lfs.attributes(discarded.file, "size")
+        os.remove(discarded.file)
+    end
+    -- disk cache may have changes so need to refresh disk cache snapshot
+    self.cached = getDiskCache()
+end
+
 -- blank the cache
 function Cache:clear()
     for k, _ in pairs(self.cache) do