nginx+lua+memcached，报错：module 'resty.mem?报错

2015/02/04 12:04:23 [error] 31306#0: *1 lua entry thread aborted: runtime error: /usr/local/nginx/conf/vhost/cache.lua:25: module 'resty.memcached' not found:
        no field package.preload['resty.memcached']
        no file './resty/memcached.lua'
        no file '/usr/local/share/luajit-2.0.3/resty/memcached.lua'
        no file '/usr/local/share/lua/5.1/resty/memcached.lua'
        no file '/usr/local/share/lua/5.1/resty/memcached/init.lua'
        no file './resty/memcached.so'
        no file '/usr/local/lib/lua/5.1/resty/memcached.so'
        no file '/usr/local/lib/lua/5.1/loadall.so'
        no file './resty.so'
        no file '/usr/local/lib/lua/5.1/resty.so'
        no file '/usr/local/lib/lua/5.1/loadall.so'
stack traceback:
coroutine 0:
        [C]: in function 'require'
        /usr/local/nginx/conf/vhost/cache.lua:25: in function </usr/local/nginx/conf/vhost/cache.lua:1>, client: 192.168.1.10, server: localhost, request: "GET /?cmd=get&key=123ab HTTP/1.1", host: "192.168.1.251"