Re: [LSF/MM/BPF TOPIC] 64k (or 16k) base page size on x86

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Kiryl Shutsemau <kas@kernel.org> [260220 07:33]:
> On Thu, Feb 19, 2026 at 10:28:20PM -0500, Liam R. Howlett wrote:
> > * Kiryl Shutsemau <kas@kernel.org> [260219 17:05]:
> > > On Thu, Feb 19, 2026 at 09:08:57AM -0800, Dave Hansen wrote:
> > > > On 2/19/26 07:08, Kiryl Shutsemau wrote:
> > > > >   - The order-0 page size cuts struct page overhead by a factor of 16. From
> > > > >     ~1.6% of RAM to ~0.1%;
> > > > ...
> > > > But, it will mostly be getting better performance at the _cost_ of
> > > > consuming more RAM, not saving RAM.
> > > 
> > > That's fair.
> > > 
> > > The problem with struct page memory consumption is that it is static and
> > > cannot be reclaimed. You pay the struct page tax no matter what.
> > > 
> > > Page cache rounding overhead can be large, but a motivated userspace can
> > > keep it under control by avoiding splitting a dataset into many small
> > > files. And this memory is reclaimable.
> > > 
> > 
> > But we are in reclaim a lot more these days.  As I'm sure you are aware,
> > we are trying to maximize the resources (both cpu and ram) of any
> > machine powered on.  Entering reclaim will consume the cpu time and will
> > affect other tasks.
> > 
> > Especially with multiple workload machines, the tendency is to have a
> > primary focus with the lower desired work being killed, if necessary.
> > Reducing the overhead just means more secondary tasks, or a bigger
> > footprint of the ones already executing.
> > 
> > Increasing the memory pressure will degrade the primary workload more
> > frequently, even if we recover enough to avoid OOMing the secondary.
> > 
> > While in the struct page tax world, the secondary task would be killed
> > after a shorter (and less frequently executed) reclaim comes up short.
> > So, I would think that we would be degrading the primary workload in an
> > attempt to keep the secondary alive?  Maybe I'm over-simplifying here?
> 
> I am not sure I fully follow your point.
> 
> Sizing tasks and scheduling tasks between machines is hard in general.
> I don't think the balance between struct page tax and page cache
> rounding overhead is going to be the primary factor.

I think there are more trade offs than what you listed.  It's still
probably worth doing, but I wanted to know if you though that this would
cause us to spend more time in reclaim, which seems to be implied above.
So, another trade-off might be all the reclaim penalty being paid more
frequently?

...

Thanks,
Liam