Re: [DISCUSSION] proposed mctl() API

[Matthew Wilcox <willy@infradead.org>]

On Thu, May 29, 2025 at 10:54:34AM -0700, Shakeel Butt wrote:
> On Thu, May 29, 2025 at 04:28:46PM +0100, Matthew Wilcox wrote:
> > People should put more effort into allocating THPs automatically and
> > monitoring where they're helping performance and where they're hurting
> > performance,
> 
> Can you please expand on people putting more effort? Is it about
> workloads or maybe malloc implementations (tcmalloc, jemalloc) being
> more intelligent in managing their allocations/frees to keep more used
> memory in hugepage aligned regions? And conveying to kernel which
> regions they prefer hugepage backed and which they do not? Or something
> else?

We need infrastructure inside the kernel to monitor whether a task is
making effective use of the THPs that it has, and if it's not then move
those THPs over to where they will be better used.

I don't necessarily object to userspace giving hints like "I think I'm
going to use all of this 20MB region quite heavily", but the kernel should
treat those hints with the appropriate skepticism, otherwise it's just
a turbo button that nobody would ever _not_ press.

> > instead of coming up with these baroque reasons to blame
> > the sysadmin for not having tweaked some magic knob.
> 
> To me this is not about blaming sysadmin but more about sysadmin wanting
> more fine grained control on THP allocation policies for different
> workloads running in a multi-tenant environment.

That's the same thing.  Linux should be auto-tuning, not relying on some
omniscient sysadmin to fix it up.