config: memtx section for instance config

This section describes the instance memtx engine configuration.

Part of #8778

NO_DOC=will be added later
NO_CHANGELOG=will be added later

src/box/lua/config/instance_config.lua

@@ -361,6 +361,44 @@ return schema.new('instance_config', schema.record({
             default = 5 * 1024 * 1024,
         }),
     }),
+    memtx = schema.record({
+        memory = schema.scalar({
+            type = 'integer',
+            box_cfg = 'memtx_memory',
+            default = 256 * 1024 * 1024,
+        }),
+        allocator = schema.enum({
+            'small',
+            'system',
+        }, {
+            box_cfg = 'memtx_allocator',
+            box_cfg_nondynamic = true,
+            default = 'small',
+        }),
+        slab_alloc_granularity = schema.scalar({
+            type = 'integer',
+            box_cfg = 'slab_alloc_granularity',
+            box_cfg_nondynamic = true,
+            default = 8,
+        }),
+        slab_alloc_factor = schema.scalar({
+            type = 'number',
+            box_cfg = 'slab_alloc_factor',
+            box_cfg_nondynamic = true,
+            default = 1.05,
+        }),
+        min_tuple_size = schema.scalar({
+            type = 'integer',
+            box_cfg = 'memtx_min_tuple_size',
+            box_cfg_nondynamic = true,
+            default = 16,
+        }),
+        max_tuple_size = schema.scalar({
+            type = 'integer',
+            box_cfg = 'memtx_max_tuple_size',
+            default = 1024 * 1024,
+        }),
+    }),
 }, {
     -- Any configuration data should contain a version of the
     -- config schema for which it is written.

test/config-luatest/instance_config_schema_test.lua

@@ -282,3 +282,29 @@ g.test_sql = function()
     local res = instance_config:apply_default({}).sql
     t.assert_equals(res, exp)
 end
+
+g.test_memtx = function()
+    local iconfig = {
+        memtx = {
+            memory = 1,
+            allocator = 'small',
+            slab_alloc_granularity = 1,
+            slab_alloc_factor = 1,
+            min_tuple_size = 1,
+            max_tuple_size = 1,
+        },
+    }
+    instance_config:validate(iconfig)
+    validate_fields(iconfig.memtx, instance_config.schema.fields.memtx)
+
+    local exp = {
+        memory = 268435456,
+        allocator = 'small',
+        slab_alloc_granularity = 8,
+        slab_alloc_factor = 1.05,
+        min_tuple_size = 16,
+        max_tuple_size = 1048576,
+    }
+    local res = instance_config:apply_default({}).memtx
+    t.assert_equals(res, exp)
+end