Re: Thoughts on simple scanner approach for free page hinting

[David Hildenbrand]

> With that said I have a few ideas that may help to address the 4
> issues called out above. The basic idea is simple. We use a high water
> mark based on zone->free_area[order].nr_free to determine when to wake
> up a thread to start hinting memory out of a given free area. From
> there we allocate non-"Offline" pages from the free area and assign
> them to the hinting queue up to 64MB at a time. Once the hinting is
> completed we mark them "Offline" and add them to the tail of the
> free_area. Doing this we should cycle the non-"Offline" pages slowly
> out of the free_area. In addition the search cost should be minimal
> since all of the "Offline" pages should be aggregated to the tail of
> the free_area so all pages allocated off of the free_area will be the
> non-"Offline" pages until we shift over to them all being "Offline".
> This should be effective for MAX_ORDER - 1 and MAX_ORDER - 2 pages
> since the only real consumer of add_to_free_area_tail is
> __free_one_page which uses it to place a page with an order less than
> MAX_ORDER - 2 on the tail of a free_area assuming that it should be
> freeing the buddy of that page shortly. The only other issue with
> adding to tail would be the memory shuffling which was recently added,
> but I don't see that as being something that will be enabled in most
> cases so we could probably just make the features mutually exclusive,
> at least for now.
> 
> So if I am not mistaken this would essentially require a couple
> changes to the mm infrastructure in order for this to work.
> 
> First we would need to split nr_free into two counters, something like
> nr_freed and nr_bound. You could use nr_freed - nr_bound to get the
> value currently used for nr_free. When we pulled the pages for hinting
> we would reduce the nr_freed value and then add back to it when the
> pages are returned. When pages are allocated they would increment the
> nr_bound value. The idea behind this is that we can record nr_free
> when we collect the pages and save it to some local value. This value
> could then tell us how many new pages have been added that have not
> been hinted upon.
> 
> In addition we will need some way to identify which pages have been
> hinted on and which have not. The way I believe easiest to do this
> would be to overload the PageType value so that we could essentially
> have two values for "Buddy" pages. We would have our standard "Buddy"
> pages, and "Buddy" pages that also have the "Offline" value set in the
> PageType field. Tracking the Online vs Offline pages this way would
> actually allow us to do this with almost no overhead as the mapcount
> value is already being reset to clear the "Buddy" flag so adding a
> "Offline" flag to this clearing should come at no additional cost.

BTW I like the idea of allocating pages that have already been hinted as
last "choice", allocating pages that have not been hinted yet first.

-- 

Thanks,

David / dhildenb