Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

[David Hildenbrand]

On 06.05.21 21:30, Matthew Wilcox wrote:
> On Thu, May 06, 2021 at 09:10:52PM +0200, David Hildenbrand wrote:
> > I have to admit that I am not really a friend of that. I still think our
> > target goal should be to have gigantic THP *in addition to* ordinary THP.
> > Use gigantic THP where enabled and possible, and just use ordinary THP
> > everywhere else. Having one pageblock granularity is a real limitation IMHO
> > and requires us to hack the system to support it to some degree.
>
> You're thinking too small with only two THP sizes ;-)  I'm aiming to

Well, I raised in my other mail that we will have multiple different use 
cases, including multiple different THP e.g., on aarch64 ;)

> support arbitrary power-of-two memory allocations.  I think there's a
> fruitful discussion to be had about how that works for anonymous memory --
> with page cache, we have readahead to tell us when our predictions of use
> are actually fulfilled.  It doesn't tell us what percentage of the pages

Right, and I think we have to think about a better approach than just 
increasing the pageblock_order.

> allocated were actually used, but it's a hint.  It's a big lift to go from
> 2MB all the way to 1GB ... if you can look back to see that the previous
> 1GB was basically fully populated, then maybe jump up from allocating
> 2MB folios to allocating a 1GB folio, but wow, that's a big step.
>
> This goal really does mean that we want to allocate from the page
> allocator, and so we do want to grow MAX_ORDER.  I suppose we could
> do somethig ugly like
>
> 	if (order <= MAX_ORDER)
> 		alloc_page()
> 	else
> 		alloc_really_big_page()
>
> but that feels like unnecessary hardship to place on the user.

I had something similar for the sort term in mind, relying on 
alloc_contig_pages() (and maybe ZONE_MOVABLE to make allocations more 
likely to succeed). Devil's in the details (page migration, ...).


--
Thanks,

David / dhildenb