Messages in this thread |  | | | Date | Mon, 16 Mar 2009 16:56:28 +0000 | | From | Mel Gorman <> | | Subject | Re: [PATCH 00/35] Cleanup and optimise the page allocator V3 |
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On Mon, Mar 16, 2009 at 04:53:42PM +0100, Nick Piggin wrote: > On Mon, Mar 16, 2009 at 01:32:32PM +0000, Mel Gorman wrote: > > On Mon, Mar 16, 2009 at 01:25:05PM +0100, Nick Piggin wrote: > > > > Well, buddy always uses the smallest available page first. Even with > > > > deferred coalescing, it will merge up to order-5 at least. Lets say they > > > > could have merged up to order-10 in ordinary circumstances, they are > > > > still avoided for as long as possible. Granted, it might mean that an > > > > order-5 is split that could have been merged but it's hard to tell how > > > > much of a difference that makes. > > > > > > But the kinds of pages *you* are interested in are order-10, right? > > > > > > > Yes, but my expectation is that multiple free order-5 pages can be > > merged to make up an order-10. > > Yes, but lazy buddy will give out part of an order-10 free area > to an order-5 request even when there are genuine order-5,6,7,8,9 > free areas available. >
True.
> Now it could be assumed that not too much else in the kernel > asks for anything over order-3, so you are unlikely to get these > kinds of requests.
Which is an assumption I was working with.
> But it's worse than that actually, because > lazy buddy will also split half of an order-10 free area in order > to satisfy an order-0 allocation in cases that there are no smaller > orders than 5 available. >
Also true. In movable areas it probably makes no difference but it might if large high-order unmovable allocations were common.
> So yes definitely I think there should be a very real impact on > higher order coalescing no matter what you do. >
Because this is not straight-forward at all, I'll put lazy buddy onto the back-burner and exhaust all other possibilities before revisiting it again.
> > > If they can't, then lumpy reclaim kicks > > in as normal. My expectation actually is that order-10 allocations often > > end up using lumpy reclaim and the pages are not automatically > > available. > > movable zone is less interesting, although it will make it harder > to allocate these guys from movable zone. But the pages are > movable so eventually they should be able to be reclaimed. >
Exactly.
> unmovable zone fragmentation is more important point because it > eventually can destroy the movable zone. >
Which is why rmqueue_fallback() also merges up all buddies before making any decisions but I accept your points. This is hard enough to mind-experiment with that it should be considered last.
-- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab
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