Since the recent slashdotting of our website (we held up okay, but there’s always room for improvement), I’ve been investigating the possibility of moving from Cache_Lite (actually, Cache_Lite_Function) to memcached in our PHP code.
Much discussion comparing these solutions focuses on raw performance in benchmarks. In the real world, though, not all things outside the benchmark are equal. On a VPS, disk I/O times are notorious for being highly variable. This makes memcached all the more attractive. Yes, memory is faster than disk in almost every environment, but also, avoiding disk access conserves a precious resource so fewer processes must block for it.
A public mailing list post by one Brian Moon points this out exactly:
If you rolled your own caching system on the local filesystem, benchmarks would show that it is faster. However, what you do not see in benchmarks is what happens to your FS under load. Your kernel has to use a lot of resources to do all that file IO. [...]
So, enter memcached. It scales much better than a file based cache. Sure, its slower. I have even seen some tests where its slower than the database. But, tests are not the real world. In the real world, memcached does a great job.
Okay, great. memcached is better when you take into account overall resources. But there’s a very useful Cache_Lite_Function feature that memcached doesn’t seem to have.
When you initialize a Cache_Lite_Function object, you set a “lifeTime” parameter, then use the call() method to wrap your regular function calls. If the output of the function hasn’t been cached within that time period, the call gets made and its results replaced in the cache with a new timestamp.
The cool thing about it is that you can create different cache objects pointing to the same directory store without a problem. Pages can increase and decrease the lifetime of the cache dynamically as load changes, so you can serve slightly older data from cache if necessary, keeping the site responsive while saving database queries. On a site where content changes relatively infrequently, this is a great feature to have: serve it fresh when load is low, serve from cache when load is high.
memcached, on the other hand, requires that you specify an expiration time at the time you place data in the cache. A retrieval call doesn’t let you specify a time period, so you can’t do the above. If data has expired, it’s expired.
It’d be interesting to hack Cache_Lite_Function to use memcached as its store, so you could get the best of both worlds. It would involve storing things in memcached with no expiration, tacking on a timestamp in the data, and doing the checking manually. But it might work.