UNIX for Beginners Questions & Answers

Good question.
kernel.org developers claim that freeing buffers and cache is fast.
But under some workloads this is not true.
The problem is not memory fragmentation. But a concurrent I/O that tries to fill cache and buffers significantly slows down the clearing.

No difference. The kernel won't stop everything and deliver 8 gigs when you ask, just marks that you're owed it. It will assign memory to you piecemeal when you actually start using it.

The difference is the speed.
It takes a second to clear the 5GB buffers&cache.
If there is high concurrent I/O it takes a minute.
In the second scenario the filling of 8 GB of RAM happens faster (or much faster).

There is a difference between filling (actual usage) and requesting. Linux serves RAM requests immediately without checking if this is really available, "unlimited overcommitment". For example a java application that claims 25 GB RAM will start, and later crash when it fills it with data.

In contrast, Solaris 10 has "limited overcommitment". For example only commits up to twice the available RAM; a java application that claims 25 GB RAM will only start when there is >13 GB available.