Cc: Andrew Morton <akpm@...>, Joern Engel <joern@...>, Nick Piggin <nickpiggin@...>, Christoph Lameter <clameter@...>, <andrea@...>, <torvalds@...>, <linux-fsdevel@...>, <linux-kernel@...>, Christoph Hellwig <hch@...>, William Lee Irwin III <wli@...>, David Chinner <dgc@...>, Jens Axboe <jens.axboe@...>, Badari Pulavarty <pbadari@...>, Maxim Levitsky <maximlevitsky@...>, Fengguang Wu <fengguang.wu@...>, swin wang <wangswin@...>, <totty.lu@...>, <hugh@...>
On (15/09/07 14:14), Goswin von Brederlow didst pronounce:
It is actually really easy to force regions to never share. At the
moment, there is a fallback list that determines a preference for what
block to mix.
The reason why this isn't enforced is the cost of moving. On x86 and
x86_64, a block of interest is usually 2MB or 4MB. Clearing out one of
those pages to prevent any mixing would be bad enough. On PowerPC, it's
potentially 16MB. On IA64, it's 1GB.
As this was fragmentation avoidance, not guarantees, the decision was
made to not strictly enforce the types of pages within a block as the
cost cannot be made back unless the system was making agressive use of
large pages. This is not the case with Linux.
This is easily achieved, just really really expensive because of the
amount of copying that would have to take place. It would also compel
that min_free_kbytes be at least one free PAGEBLOCK_NR_PAGES and likely
MIGRATE_TYPES * PAGEBLOCK_NR_PAGES to reduce excessive copying. That is
a lot of free memory to keep around which is why fragmentation avoidance
doesn't do it.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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