Re: [DISCUSSION] proposed mctl() API

On Tue, Jun 10, 2025 at 04:30:43PM +0100, Usama Arif wrote:

If we have 2 workloads on the same server, For e.g. one is database where THPs 
just dont do well, but the other one is AI where THPs do really well. How
will the kernel monitor that the database workload is performing worse
and the AI one isnt?

It can monitor the allocation/access patterns and see who's getting
the benefit.  The two workloads are in competition for memory, and
we can tell which pages are hot and which cold.

And I don't believe it's a binary anyway.  I bet there are some
allocations where the database benefits from having THPs (I mean, I know
a database which invented the entire hugetlbfs subsystem so it could
use PMD entries and avoid one layer of TLB misses!)

I added THP shrinker to hopefully try and do this automatically, and it does
really help. But unfortunately it is not a complete solution.
There are severely memory bound workloads where even a tiny increase
in memory will lead to an OOM. And if you colocate the container thats running
that workload with one in which we will benefit with THPs, we unfortunately
can't just rely on the system doing the right thing.

Then maybe THP aren't for you.  If your workloads are this sensitive,
perhaps you should be using a mechanism which gives you complete control
like hugetlbfs.