[TriLUG] Growing OS cache in RAM causing high system CPU usage

[Michael Peters]

On Tue, Jul 8, 2014 at 1:28 PM, Bill Farrow <bill at arrowsreach.com> wrote:

> This process is "seamless" to your application but there is still
> going to be a CPU overhead involved here.

I know there is some overhead, but this seems really excessive. 8 CPUs
pegged 96-99% for 3 minutes?

> The memory pages being used
> for OS cache need to be flushed from the CPU caches, and then possibly
> written out to disk if they are dirty (have changes in them).

There's only a tiny amount of dirty memory in the cache (2060 kB0 and
no increase in disk IO during these CPU spikes.

> Then
> they need to be marked as unused and the OS table/list/hash etc needs
> to be updated accordingly.
>
> Once the memory is "unused", it probably has to be initialized to zero
> before being handed to the application to prevent data leakage.
>
> So it is not CPU "free" even if it is seamless to your application.

So given that the memory management overhead of the OS using too much
cache seems to be causing this how can I limit that? I don't see
anything in the virtual memory settings to control this. And while I
can force the cache to be cleared with /proc/sys/vm/drop_caches that's
a pretty blunt tool that clears everything and will impact performance
as the OS tries to refill the most needed things. Is there a way to
clear out the older parts of the cache explicitly?