interpreting top

[Edward Ned Harvey]

It is normal for a Linux kernel to grow the cache and buffers infinitely to
consume all available ram, over time.  It will then opt to sometimes swap
out idle processes, or throw away cached files from memory, as processes
request more memory for active processing.

You don't get valuable information from the free/used.
But if you see the free staying very small ... and you see the cache &
buffers becoming very small ... then you don't have enough ram.  In your
case, it's quite the opposite.

I am guessing this server is either httpd, or a fileserver.  Active
processes consume almost no memory, processor almost totally idle, and the
kernel is using all the ram for caching.