Re: [PATCH] Smarter retry of costly-order allocations

[Mel Gorman <mel@csn.ul.ie>]

On (15/04/08 02:02), Andrew Morton didst pronounce:
> On Tue, 15 Apr 2008 09:51:55 +0100 Mel Gorman <mel@csn.ul.ie> wrote:
> 
> > On (11/04/08 16:35), Nishanth Aravamudan didst pronounce:
> > > Because of page order checks in __alloc_pages(), hugepage (and similarly
> > > large order) allocations will not retry unless explicitly marked
> > > __GFP_REPEAT. However, the current retry logic is nearly an infinite
> > > loop (or until reclaim does no progress whatsoever). For these costly
> > > allocations, that seems like overkill and could potentially never
> > > terminate.
> > > 
> > > Modify try_to_free_pages() to indicate how many pages were reclaimed.
> > > Use that information in __alloc_pages() to eventually fail a large
> > > __GFP_REPEAT allocation when we've reclaimed an order of pages equal to
> > > or greater than the allocation's order. This relies on lumpy reclaim
> > > functioning as advertised. Due to fragmentation, lumpy reclaim may not
> > > be able to free up the order needed in one invocation, so multiple
> > > iterations may be requred. In other words, the more fragmented memory
> > > is, the more retry attempts __GFP_REPEAT will make (particularly for
> > > higher order allocations).
> > > 
> > > Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
> > 
> > Changelog is a lot clearer now. Thanks.
> > 
> > Tested-by: Mel Gorman <mel@csn.ul.ie>
> 
> Tested in what way though?
> 

It was tested as part of the full patchset as hugepage allocations was the
easiest trigger for __GFP_REPEAT usage. It was based on 2.6.25-rc9. Test
was as follows

1. kernbench as a smoke-test
2. hugetlbcap test
	1. Build 6 trees simultaneously on a 512MB laptop
		(should have caught if pagetable allocations getting broken
		 by the change in __GFP_REPEAT semantics)
	2. Allocate hugepages via proc under load
	3. Kill all compile jobs
	4. Allocate hugepages at rest
3. Run hugepages_get test which is the output I posted as part of patch 3

The main check was to see if pagetable allocations were getting messed
up. I didn't notice a problem on the laptop, but it's 1-way so I've
started tests on larger machines just in case.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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