Re: [DISCUSSION] proposed mctl() API

On Tue, Jun 10, 2025 at 05:00:47PM +0100, Usama Arif wrote:

On 10/06/2025 16:46, Matthew Wilcox wrote:

quoted

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

quoted

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!)

Sure, but this is just an example. Workload owners are not going to spend time
trying to see how each allocation works and if its hot, they put it in hugetlbfs.

No, they're not.  It should be automatic.  There are many deficiencies
in the kernel; this is one of them.

Ofcourse hugetlbfs has its own drawbacks of reserving pages.

Drawback or advantage?  It's a feature.  You're being very strange about
this.  First you want to reserve THPs for some workloads only, then when
given a way to do that you complain that ... you have to reserve hugetlb
pages.  You can't possibly mean both of these things sincerely.