Re: [patch 5/5]thp: split huge page if head page is isolated

[Shaohua Li <shaohua.li@intel.com>]

On Fri, 2011-10-28 at 15:30 +0800, Minchan Kim wrote:
> On Fri, Oct 28, 2011 at 01:11:55PM +0800, Shaohua Li wrote:
> > On Fri, 2011-10-28 at 07:34 +0800, Minchan Kim wrote:
> > > On Tue, Oct 25, 2011 at 10:59:40AM +0800, Shaohua Li wrote:
> > > > With current logic, if page reclaim finds a huge page, it will just reclaim
> > > > the head page and leave tail pages reclaimed later. Let's take an example,
> > > > lru list has page A and B, page A is huge page:
> > > > 1. page A is isolated
> > > > 2. page B is isolated
> > > > 3. shrink_page_list() adds page A to swap page cache. so page A is split.
> > > > page A+1, page A+2, ... are added to lru list.
> > > > 4. shrink_page_list() adds page B to swap page cache.
> > > > 5. page A and B is written out and reclaimed.
> > > > 6. page A+1, A+2 ... is isolated and reclaimed later.
> > > > So the reclaim order is A, B, ...(maybe other pages), A+1, A+2 ...
> > > 
> > > I don't see your code yet but have a question.
> > > You mitigate this problem by 4/5 which could add subpages into lru tail
> > > so subpages would reclaim next interation of reclaim.
> > > 
> > > What do we need 5/5?
> > > Do I miss something?
> > Both patches are required. without this patch, current page reclaim will
> > only reclaim the first page of a huge page, because the hugepage isn't
> > split yet. The hugepage is split when the first page is being written to
> > swap, which is too later and page reclaim might already isolated a lot
> > of pages.
> 
> When split happens, subpages would be located in tail of LRU by your 4/5.
> (Assume tail of LRU is old age).
yes, but a lot of other pages already isolated. we will reclaim those
pages first. for example, reclaim huge page A, B. current reclaim order
is A, B, A+1, ... B+1, because we will isolated A and B first, all tail
pages are not isolated yet. While with my patch, the order is A, A
+1, ... B, B+1,.... with my patch, we can avoid unnecessary page split
or page isolation. This is exactly why my patch reduces the thp_split
count.

> In addtion, isolation happens 32 page chunk so the subpages would be isolated
> and reclaimed in next iteration. I think 32 pages are not too many.
> 
> What do you think about it?
since headpage and tailpages are in different list, the 32 chunk will
not include tailpages.