linux-kernel - Re: [DISCUSSION] proposed mctl() API

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Message-ID: <aEhTYkzsTsaBua40@casper.infradead.org>
Date: Tue, 10 Jun 2025 16:46:42 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	Arnd Bergmann <arnd@...db.de>,
	Christian Brauner <brauner@...nel.org>,
	SeongJae Park <sj@...nel.org>, Mike Rapoport <rppt@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Barry Song <21cnbao@...il.com>, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, Pedro Falcato <pfalcato@...e.de>
Subject: 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.