what are pagecache, dentries, inodes?

Question

Just learned these 3 new techniques from https://unix.stackexchange.com/questions/87908/how-do-you-empty-the-buffers-and-cache-on-a-linux-system:

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To free pagecache

Answer 1

I am trying to understand what exactly are pagecache, dentries and inodes. What exactly are they?

user3344003 already gave an exact answer to that specific question, but it's still important to note those memory structures are dynamically allocated.

When there's no better use for "free memory", memory will be used for those caches, but automatically purged and freed when some other "more important" application wants to allocate memory.

No, those caches don't affect any caches maintained by any applications (including redis and memcached).

My Amazon EC2 server RAM was getting filled up over the days - from 6% to up to 95% in a matter of 7 days. I am having to run a bi-weekly cronjob to remove these cache. Then memory usage drops to 6% again.

Probably you're mis-interpreting the situation: your system may just be making efficient usage of its ressources.

To simplify things a little bit: "free" memory can also be seen as "unused", or even more dramatic - a waste of resources: you paid for it, but don't make use of it. That's a very un-economic situation, and the linux kernel tries to make some "more useful" use of your "free" memory.

Part of its strategy involves using it to save various kinds of disk I/O by using various dynamically sized memory caches. A quick access to cache memory saves "slow" disk access, so that's often a useful idea.

As soon as a "more important" process wants to allocate memory, the Linux kernel voluntarily frees those caches and makes the memory available to the requesting process. So there's usually no need to "manually free" those caches.

The Linux kernel may even decide to swap out memory of an otherwise idle process to disk (swap space), freeing RAM to be used for "more important" tasks, probably also including to be used as some cache.

So as long as your system is not actively swapping in/out, there's little reason to manually flush caches.

A common case to "manually flush" those caches is purely for benchmark comparison: your first benchmark run may run with "empty" caches and so give poor results, while a second run will show much "better" results (due to the pre-warmed caches). By flushing your caches before any benchmark run, you're removing the "warmed" caches and so your benchmark runs are more "fair" to be compared with each other.