Re: [PATCH 5/7] Add /proc trigger for memory compaction

[David Rientjes <rientjes@google.com>]

On Thu, 21 Jan 2010, Mel Gorman wrote:

> > It would be helpful to be able to determine what is "compactable" at the 
> > same time by adding both global and per-node "compact_order" tunables that 
> > would default to HUGETLB_PAGE_ORDER. 
> 
> Well, rather than having a separate tunable, writing a number to
> /proc/sys/vm/compact could indicate the order if that trigger is now
> working system-wide. Would that be suitable?
> 

Do you think you'll eventually find a need to call try_to_compact_pages() 
with a higher order than the one passed to the page allocator to limit 
"compaction thrashing" from fragmented frees to a zone where we're 
constantly compacting order-1 pages, for instance?  I agree that memory 
compaction should always be used before direct reclaim for higher order 
allocations, but it may be more efficient to define a compact_min_order, 
tunable from userspace, that would avoid the need for constant order-1 
compactions from subsequent page allocations.

If that's a possibility, we may find a need for "compact_order", now 
renamed "compact_min_order", outside of the explicit trigger.

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