Re: [RFC] memory_hotplug: Free pages as pageblock_order

[Arun KS]

On 2018-09-14 14:40, Michal Hocko wrote:
> On Wed 12-09-18 20:12:30, Arun KS wrote:
> > On 2018-09-12 18:47, Michal Hocko wrote:
> > > On Wed 12-09-18 22:57:43, Balbir Singh wrote:
> > > > On Wed, Sep 12, 2018 at 12:38:53PM +0200, Michal Hocko wrote:
> > > > > On Wed 12-09-18 14:56:45, Arun KS wrote:
> > > > > > When free pages are done with pageblock_order, time spend on
> > > > > > coalescing pages by buddy allocator can be reduced. With
> > > > > > section size of 256MB, hot add latency of a single section
> > > > > > shows improvement from 50-60 ms to less than 1 ms, hence
> > > > > > improving the hot add latency by 60%.
> > > > >
> > > > > Where does the improvement come from? You are still doing the same
> > > > > amount of work except that the number of callbacks is lower. Is this the
> > > > > real source of 60% improvement?
> > > > >
> > > >
> > > > It looks like only the first page of the pageblock is initialized, is
> > > > some of the cost amortized in terms of doing one initialization for
> > > > the page with order (order) and then relying on split_page and helpers
> > > > to do the rest? Of course the number of callbacks reduce by a
> > > > significant
> > > > number as well.
> > >
> > > Ohh, I have missed that part. Now when re-reading I can see the reason
> > > for the perf improvement. It is most likely the higher order free which
> > > ends up being much cheaper. This part makes some sense.
> > >
> > > How much is this feasible is another question. Do not forget we have
> > > those external providers of the online callback and those would need to
> > > be updated as well.
> > Sure Michal, I ll look into this.
> >
> > >
> > > Btw. the normal memmap init code path does the same per-page free as
> > > well. If we really want to speed the hotplug path then I guess the init
> > > one would see a bigger improvement and those two should be in sync.
> > Thanks for pointers, Will look further.
>
> I haven't looked closer and I will be travelling next week so just 
> hint.
> Have a look at the nobootmem and how it frees pages to the page
> allocator in __free_pages_boot_core. Seems exactly what you want and it
> also answers your question about reference counting.

Thanks Michal. Will send a new version after testing.

Regards,
Arun