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

[Mike Rapoport <rppt@linux.ibm.com>]

On Wed, Nov 14, 2018 at 05:55:12PM -0800, Mike Rapoport wrote:
> On Wed, Nov 14, 2018 at 04:50:23PM -0800, Alexander Duyck wrote:
> > 
> > 
> > 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.
> 
> Just recently we had a discussion about overlapping for_each_mem_range()
> and for_each_mem_pfn_range(), but unfortunately it appears that no mailing
> list was cc'ed by the original patch author :(
> In short, there was a spelling fix in one of them and Michal pointed out
> that their functionality overlaps.
> 
> I have no objection for for_each_free_mem_pfn_range_in_zone() and
> __next_mem_pfn_range_in_zone(), but probably we should consider unifying
> the older iterators before we introduce a new one? 

Another thing I realized only now is that
for_each_free_mem_pfn_range_in_zone() can be used only relatively late in
the memblock life-span because zones are initialized far later than
setup_arch() in many cases.

At the very least this should be documented.
 
> > >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.
> > 
> > 
> 
> -- 
> Sincerely yours,
> Mike.

-- 
Sincerely yours,
Mike.