How to allocate memory to memcached

The short answer: Either one is what you are looking for, but my first choice would be memcache (the first one you listed), purely based on its correct use of nomenclature.

Now here's how I came to that conclusion:

Here is a quick backgrounder in naming conventions (for those unfamiliar), which explains the frustration by the question asker: For many *nix applications, the piece that does the backend work is called a "daemon" (think "service" in Windows-land), while the interface or client application is what you use to control or access the daemon. The daemon is most often named the same as the client, with the letter "d" appended to it. For example "imap" would be a client that connects to the "imapd" daemon.

This naming convention is clearly being adhered to by memcache when you read the introduction to the memcache module (notice the distinction between memcache and memcached in this excerpt):

Memcache module provides handy procedural and object oriented interface to memcached, highly effective caching daemon, which was especially designed to decrease database load in dynamic web applications.

The Memcache module also provides a session handler (memcache).

More information about memcached can be found at » http://www.danga.com/memcached/.

The frustration here is caused by the author of the PHP extension which was badly named memcached, since it shares the same name as the actual daemon called memcached. Notice also that in the introduction to memcached (the php module), it makes mention of libmemcached, which is the shared library (or API) that is used by the module to access the memcached daemon:

memcached is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.

This extension uses libmemcached library to provide API for communicating with memcached servers. It also provides a session handler (memcached).

Information about libmemcached can be found at » http://tangent.org/552/libmemcached.html.

In summary, both are functionally the same, but they simply have different authors, and the one is simply named more appropriately than the other.

Well, you have practically all the time over 8G either free (yellow, "unused") or as a cache (dark blue). In my opinion, your upper limit to allocate for memcached is maybe 6GB (give or take), which will still allow quite a lot of headroom for other processes and some disk cache without real risk of swapping. One could also argue you could allocate more. It's a matter of taste and real performance benefits will depend on your specific application.

Keep in mind that if you tell memcached to use up to 6GB memory, it won't use that much unless you store 6G in it. It will only consume 1GB if you only store 1GB in it, even if the limit is 6GB. I'd recommend you add memcached to your monitoring, and keep an eye on memcached's bytes statistic and limit the memory usage accordingly to make sure the system will not do anything unexpected if there's surprise surge of users.

From shell you can use free to determine memory usage, and it'll tell something like this:

             total       used       free     shared    buffers     cached
Mem:       4060096    3775976     284120          0     270828    1899720
-/+ buffers/cache:    1605428    2454668
Swap:       577528          0     577528

Free is unused memory, cached is disk cache. Free memory will get used as a disk cache over time, so it's quite safe to consider memory used as a cache as free, even if it's being used for something useful at the moment.