What is compression for in PHP's Memcache?

Question

On PHP.net I am looking at the Memcache::set function and it has this optional flag to use compression...

Use MEMCACHE_COMPRESSED to store the item compressed (uses zlib).

$memcache_obj->set('var_key',
                   'some really big variable',
                    MEMCACHE_COMPRESSED,
                    50);

I am curious, what would be the benefit of this, just using less space? It seems like this would slow the process down?

Answer 1

You're right, it's a trade off between taking up less space or eating up more CPU cycles.

You've too choose which resource is more precious to you, RAM or CPU.

Answer 2

1. memcached can be used to keep a distributed cache between machines. In that case updates to the cache will need to be sent through the network or other connection between the machines. Compression reduces the amount of data that needs to be sent therefore reduce time needed to transfer and can therefore improve system performance.
2. even within one host, compressing is orders of magnitude faster than swapping memory pages to/from disks, so if you are swapping due memory shortage, compression will reduce the amount that needs to be swapped, and thereby improve performance. Under the right corner conditions, compression might reduce the size of data enough so that you don't have to swap at all - a big win in performance.

Answer 3

Compressing and decompressing on the servers that are running PHP can be quick -- depending on your network and the load on your servers, it can be quicker than transferring more (not compressed) data to and from the memcached servers via the network.

Compressing your data also means using less memory on the memcached servers.

As often, you have to choose between CPU, network, and RAM -- and you have to take into consideration your servers and network, their load, and available resources, to be able to decide.

What will be the "quickiest" solution in your particular situation ? Only you can tell... And the only way to know is probably by testing both solutions in "real conditions".

Also, memcached entries cannot be bigger than 1 MB ; I suppose, in some ways, that compression can help put entries a bit bigger than 1 MB (when not compressed yet) into memcached.