memcached: add a new module function with slab info

Before switching to our own slab arena user can control usage and
information about slab arena using Tarantool Lua API (specifically
box.slab.{check,info,stat}() functions), see [1]. Example of output
box.slab.info() is below:

tarantool> box.slab.info()
---
- items_size: 146896
  items_used_ratio: 26.05%
  quota_size: 268435456
  quota_used_ratio: 12.50%
  arena_used_ratio: 3.7%
  items_used: 38264
  quota_used: 33554432
  arena_size: 33554432
  arena_used: 1234296
...

With our own slab arena we need a way to obtain information about arena.
It's a reason why new module function memcached.slab.info() was
introduced. Output of memcached.slab.info() is similar to output
produced by `box.slab.info()`, but without items_size, items_used,
items_used_ratio and arena_used.

tarantool> mc = require('memcached')
creating arena with 4294967295 bytes...
allocate slab cache with slab size 4194304
---
...

tarantool> mc.slab.info()
---
- quota_used: 0
  quota_size: 4294967296
  quota_used_ratio: 0%
...

1. https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_slab/slab_info/

Follows up #96

.luacheckrc

@@ -1,6 +1,7 @@
 std = "luajit"
 globals = {
     "box",
+    "tonumber64",
 }
 ignore = {
     -- Accessing an undefined field of a global variable <package>.

README.md

@@ -79,6 +79,7 @@ END
 ## API
 
 * `local memcached = require('memcached')` - acquire a library handle
+* `local slab = memcached.slab.info()` - show information about slab arena and quota.
 * `local instance = memcached.create(<name>, <uri>, <opts>)` - create a new instance, register it and run
   - `name` - a string with instance name
   - `uri`  - a string with uri to bind to, for example: `0.0.0.0:11211` (only TCP is supported now)

memcached/init.lua

@@ -54,6 +54,12 @@ struct memcached_stat {
     uint64_t      auth_errors;
 };
 
+struct slab_arena_info {
+	size_t quota_size;
+	size_t quota_used;
+	double quota_used_ratio;
+};
+
 void
 memcached_set_opt (struct memcached_service *srv, int opt, ...);
 
@@ -86,6 +92,9 @@ void memcached_slab_arena_create();
 
 void memcached_slab_cache_create();
 
+struct slab_arena_info *
+memcached_slab_arena_info();
+
 int    memcached_start     (struct memcached_service *);
 void   memcached_stop      (struct memcached_service *);
 void   memcached_free      (struct memcached_service *);
@@ -237,6 +246,12 @@ local RUNNING = 'r'
 local STOPPED = 's'
 local ERRORED = 'e'
 
+local slab_arena_info_table = {
+    'quota_size',
+    'quota_used',
+    'quota_used_ratio',
+}
+
 local stat_table = {
     'total_items', 'curr_items',
     'curr_conns', 'total_conns',
@@ -390,6 +405,17 @@ local function memcached_get(name)
     return memcached_services[name]
 end
 
+local function memcached_slab_info()
+    local slab_arena_info = memcached_internal.memcached_slab_arena_info()
+    local slab_info = {}
+    for _, v in pairs(slab_arena_info_table) do
+        slab_info[v] = tonumber64(slab_arena_info[0][v])
+    end
+    slab_info['quota_used_ratio'] = string.format('%0.2f%%', slab_info['quota_used_ratio'])
+
+    return slab_info
+end
+
 local function memcached_init()
     memcached_internal.memcached_slab_arena_create()
     memcached_internal.memcached_slab_cache_create()
@@ -399,6 +425,9 @@ memcached_init()
 
 return {
     create = memcached_create_instance,
+    slab   = {
+        info = memcached_slab_info,
+    },
     get    = memcached_get,
     server = setmetatable({}, {
         __index = memcached_services

memcached/internal/alloc.c

@@ -6,6 +6,35 @@ static struct slab_arena arena;
 static struct slab_cache slabc;
 static struct quota quota;
 
+struct slab_arena_info {
+	uint64_t quota_size;
+	uint64_t quota_used;
+	double quota_used_ratio;
+};
+
+struct slab_arena_info slab_info;
+
+/*
+ * Function produce an output with information of total quota size, used quota
+ * size and quota used ratio. Note that function relies on a fact that
+ * memcached uses a single slab cache for all allocations it made.
+ */
+struct slab_arena_info *
+memcached_slab_arena_info()
+{
+	/*
+	 * How much quota has been booked - reflects the total
+	 * size of slabs in various slab caches.
+	 */
+	slab_info.quota_used = (uint64_t)quota_used(arena.quota);
+
+	slab_info.quota_size = (uint64_t)quota_total(arena.quota);
+	slab_info.quota_used_ratio = 100 * ((double) slab_info.quota_used /
+				     ((double) slab_info.quota_size + 0.0001));
+
+	return &slab_info;
+}
+
 void
 memcached_slab_arena_create()
 {

memcached/internal/alloc.h

@@ -16,4 +16,7 @@ memcached_slab_cache_destroy();
 struct slab_cache *
 memcached_slab_cache();
 
+struct slab_arena_info *
+memcached_slab_arena_info();
+
 #endif /* ALLOC_H_INCLUDED */

test/common/module_api.test.lua

@@ -2,9 +2,11 @@
 
 local socket = require('socket')
 local tap = require('tap')
+local ffi = require('ffi')
 package.cpath = './?.so;' .. package.cpath
 local memcached = require('memcached')
 local test = tap.test('memcached module api')
+local abs = require('math').abs
 
 -- 1. Open connection to memcached instance.
 -- 2. Set a key 'key' to value 5.
@@ -46,13 +48,26 @@ if type(box.cfg) == 'function' then
     box.schema.user.grant('guest', 'read,write,execute', 'universe')
 end
 
-test:plan(15)
+test:plan(31)
 
 -- memcached.server: module is initialized, no instances
 
 test:istable(memcached.server, 'type of memcached.server is a table')
 test:is(#memcached.server, 0, 'memcached.server is empty')
 
+-- memcached.slab(): memcached module is initialized, no instances
+
+test:istable(memcached.slab, 'memcached.slab (no instances) returns a table')
+test:isfunction(memcached.slab.info, 'memcached.slab.info() (no instances) returns a function')
+local slab_info = memcached.slab.info()
+test:iscdata(slab_info.quota_size, ffi.typeof('uint64_t'),
+    'memcached.slab.info().quota_size (no instances) returns a cdata')
+test:is(slab_info.quota_size, 4398046510080, 'memcached.slab.info().quota_size (no instances) is correct')
+test:iscdata(slab_info.quota_used, ffi.typeof('uint64_t'),
+    'memcached.slab.info().quota_used (no instances) returns a cdata')
+test:is(slab_info.quota_used, 0, 'memcached.slab.info().quota_used (no instances) is correct')
+test:isstring(slab_info.quota_used_ratio, 'memcached.slab.info().quota_used_ratio (no instances) returns a string')
+
 -- memcached.create(): instance 1
 
 local mc_1_port = 11211
@@ -108,4 +123,26 @@ test:is(instance_2_info.name, mc_2.name, 'memcached.get(): name of instance 2 is
 test:is(instance_2_info.space_name, mc_2.space_name, 'memcached.get(): space name of instance 2 is correct')
 test:is(instance_2_info.space.id, mc_2.space.id, 'memcached.get(): space id of instance 2 is correct')
 
+-- memcached.slab(): check numbers with created memcached instances
+
+mc_1:start()
+mc_2:start()
+
+test:istable(memcached.slab, 'memcached.slab returns a table')
+test:isfunction(memcached.slab.info, 'memcached.slab.info() returns a function')
+local slab_info = memcached.slab.info()
+test:istable(slab_info, 'memcached.slab.info() returns a table')
+test:iscdata(slab_info.quota_size, ffi.typeof('uint64_t'), 'memcached.slab.info().quota_size returns a cdata')
+test:is(slab_info.quota_size, 4398046510080, 'memcached.slab.info().quota_size is correct')
+test:iscdata(slab_info.quota_used, ffi.typeof('uint64_t'), 'memcached.slab.info().quota_used returns a cdata')
+test:is(slab_info.quota_used, 4194304, 'memcached.slab.info().quota_used is correct')
+test:isstring(slab_info.quota_used_ratio, 'memcached.slab.info().quota_used_ratio returns a string')
+local expected_ratio = tonumber(100 * slab_info.quota_used / slab_info.quota_size)
+local actual_ratio = tonumber(slab_info.quota_used_ratio:match('^(%d.%d+)%%'))
+local eps_is_correct = abs(actual_ratio - expected_ratio) < 1
+test:is(eps_is_correct, true, 'memcached.slab.info().quota_used_ratio is correct')
+
+mc_1:stop()
+mc_2:stop()
+
 os.exit(test:check() and 0 or 1)