On Fri, Oct 19, 2018 at 09:54:35AM +0100, Mel Gorman wrote: > On Fri, Oct 19, 2018 at 01:57:03PM +0800, Aaron Lu wrote: > > > > > > I don't think this is the right way of thinking about it because it's > > > possible to have the system split in such a way so that the migration > > > scanner only encounters unmovable pages before it meets the free scanner > > > where unmerged buddies were in the higher portion of the address space. > > > > Yes it is possible unmerged pages are in the higher portion. > > > > My understanding is, when the two scanners meet, all unmerged pages will > > be either used by the free scanner as migrate targets or sent to merge > > by the migration scanner. > > > > It's not guaranteed if the lower portion of the address space consisted > entirely of pages that cannot migrate (because they are unmovable or because > migration failed due to pins). It's actually a fundamental limitation > of compaction that it can miss migration and compaction opportunities > due to how the scanners are implemented. It was designed that way to > avoid pageblocks being migrated unnecessarily back and forth but the > downside is missed opportunities. > > > > You either need to keep unmerged buddies on a separate list or search > > > the order-0 free list for merge candidates prior to compaction. > > > > > > > > It's needed to form them efficiently but excessive reclaim or writing 3 > > > > > to drop_caches can also do it. Be careful of tying lazy buddy too > > > > > closely to compaction. > > > > > > > > That's the current design of this patchset, do you see any immediate > > > > problem of this? Is it that you are worried about high-order allocation > > > > success rate using this design? > > > > > > I've pointed out what I see are the design flaws but yes, in general, I'm > > > worried about the high order allocation success rate using this design, > > > the reliance on compaction and the fact that the primary motivation is > > > when THP is disabled. > > > > When THP is in use, zone lock contention is pretty much nowhere :-) > > > > I'll see what I can get with 'address space range' lock first and will > > come back to 'lazy buddy' if it doesn't work out. With the address space range idea, wouldn't the zone free_area require changes too? I can't see how locking by address range could synchronize it as it exists now otherwise, with per order/mt list heads. One idea is to further subdivide the free area according to how the locking works and find some reasonable way to handle having to search for pages of a given order/mt in multiple places.
