Re: [PATCH 1/3] page-allocator: Under memory pressure, wait on pressure to relieve instead of congestion

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

On Tue, Mar 09, 2010 at 11:11:55AM -0600, Christoph Lameter wrote:
> On Tue, 9 Mar 2010, Mel Gorman wrote:
> 
> > Until it's timeout at least. It's still better than the current
> > situation of sleeping on congestion.
> 
> Congestion may clear if memory becomes available in other zones.
> 

I understand that.

> > The ideal would be waiting on a per-node basis. I'm just not liking having
> > to look up the node structure when freeing a patch of pages and making a
> > cache line in there unnecessarily hot.
> 
> The node structure (pgdat) contains the zone structures. If you know the
> type of zone then you can calculate the pgdat address.
> 

I know you can lookup the pgdat from the zone structure. The concern is that
the suggestion requires adding fields to the node structure that then become
hot in the free_page path when the per-cpu lists are being drained. This patch
also adds a hot cache line to the zone but at least it can be eliminated by
using zone->flags. The same optimisation does not apply to working on a
per-node basis.

Adding such a hot line is a big minus and the gain is that processes may
wake up slightly faster when under memory pressure. It's not a good trade-off.

> > > But then an overallocated node may stall processes. If that node is full
> > > of unreclaimable memory then the process may never wake up?
> >
> > Processes wake after a timeout.
> 
> Ok that limits it but still we may be waiting for no reason.
> 

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