Re: [mm PATCH v5 0/7] Deferred page init improvements

[Alexander Duyck]

On 11/14/2018 7:07 AM, Michal Hocko wrote:
> On Mon 05-11-18 13:19:25, Alexander Duyck wrote:
> > This patchset is essentially a refactor of the page initialization logic
> > that is meant to provide for better code reuse while providing a
> > significant improvement in deferred page initialization performance.
> >
> > In my testing on an x86_64 system with 384GB of RAM and 3TB of persistent
> > memory per node I have seen the following. In the case of regular memory
> > initialization the deferred init time was decreased from 3.75s to 1.06s on
> > average. For the persistent memory the initialization time dropped from
> > 24.17s to 19.12s on average. This amounts to a 253% improvement for the
> > deferred memory initialization performance, and a 26% improvement in the
> > persistent memory initialization performance.
> >
> > I have called out the improvement observed with each patch.
>
> I have only glanced through the code (there is a lot of the code to look
> at here). And I do not like the code duplication and the way how you
> make the hotplug special. There shouldn't be any real reason for that
> IMHO (e.g. why do we init pfn-at-a-time in early init while we do
> pageblock-at-a-time for hotplug). I might be wrong here and the code
> reuse might be really hard to achieve though.

Actually it isn't so much that hotplug is special. The issue is more 
that the non-hotplug case is special in that you have to perform a 
number of extra checks for things that just aren't necessary for the 
hotplug case.

If anything I would probably need a new iterator that would be able to 
take into account all the checks for the non-hotplug case and then 
provide ranges of PFNs to initialize.

> I am also not impressed by new iterators because this api is quite
> complex already. But this is mostly a detail.

Yeah, the iterators were mostly an attempt at hiding some of the 
complexity. Being able to break a loop down to just an iterator provding 
the start of the range and the number of elements to initialize is 
pretty easy to visualize, or at least I thought so.

> Thing I do not like is that you keep microptimizing PageReserved part
> while there shouldn't be anything fundamental about it. We should just
> remove it rather than make the code more complex. I fell more and more
> guilty to add there actually.

I plan to remove it, but don't think I can get to it in this patch set.

I was planning to submit one more iteration of this patch set early next 
week, and then start focusing more on the removal of the PageReserved 
bit for hotplug. I figure it is probably going to be a full patch set 
onto itself and as you pointed out at the start of this email there is 
already enough code to review without adding that.