Re: [RFC PATCH 00/16] 1GB THP support on x86_64

From: Michal Hocko
Date: Wed Sep 09 2020 - 03:04:56 EST

On Tue 08-09-20 10:41:10, Rik van Riel wrote:
> On Tue, 2020-09-08 at 16:35 +0200, Michal Hocko wrote:
> > A global knob is insufficient. 1G pages will become a very precious
> > resource as it requires a pre-allocation (reservation). So it really
> > has
> > to be an opt-in and the question is whether there is also some sort
> > of
> > access control needed.
> The 1GB pages do not require that much in the way of
> pre-allocation. The memory can be obtained through CMA,
> which means it can be used for movable 4kB and 2MB
> allocations when not
> being used for 1GB pages.

That CMA has to be pre-reserved, right? That requires a configuration.

> That makes it relatively easy to set aside
> some fraction
> of system memory in every system for 1GB and movable
> allocations, and use it for whatever way it is needed
> depending on what workload(s) end up running on a system.

I was not talking about how easy or hard it is. My main concern is that
this is effectively a pre-reserved pool and a global knob is a very
suboptimal way to control access to it. I (rather) strongly believe this
should be an explicit opt-in and ideally not 1GB specific but rather
something to allow large pages to be created as there is a fit. See
other subthread for more details.

Michal Hocko