Hi Eric,

Actually, it really does not matter much, frankly speaking. I've experimented with dropping caches in Linux so many times on our production web server (LAMP).

Linux does such a great job at using all available memory, and reclaiming it when needed, so it is better to let Linux manage those things.

When you drop the cache (or caches), you will see the CPU load go up (sometimes way up) because the cache is gone. Available RAM goes up, but it does not matter because performance is slower because the cache is empty.

Then over time, if you leave the caches off, the performance will suffer because you are not taking advantage of the cache.

Linux tries to use all available RAM, so the caches will fill over time if you don't instruct Linux to drop the caches, and this is a good thing. You will see available RAM go down, but don't worry, it is available for applications when needed because applications take higher priority than cache.

You want Linux to use all the RAM. That is a good thing because the kernel is basically using all available RAM that is not used by applications (and the OS) for cache. Dropping caches has little positive effect on performance. In fact, it tends to have a negative effect. The reason is that you are not really making more RAM available to the apps, because the apps already have been given the RAM needed. You simply are dropping the cache, which degrades performance.

Regarding, swappiness, the same is basically true. Linux will dump the cache before swapping, as I recall, but I would need to read up on that again to see exactly how it works.

We have experimented with swappiness, and ours is currently set to:

Code:

# cat swappiness
20

... and FYI:

Code:

# cat drop_caches
0