Re: [PATCH 02/12] mm: supporting variables and functions for balanced zone aging

On Thu, Dec 01, 2005 at 09:49:31PM -0800, Andrew Morton wrote:
> From: Andrew Morton <akpm <at> osdl.org>
> 
> Revert a patch which went into 2.6.8-rc1.  The changelog for that patch was:
> 
>   The shrink_zone() logic can, under some circumstances, cause far too many
>   pages to be reclaimed.  Say, we're scanning at high priority and suddenly
>   hit a large number of reclaimable pages on the LRU.
> 
>   Change things so we bale out when SWAP_CLUSTER_MAX pages have been
>   reclaimed.
> 
> Problem is, this change caused significant imbalance in inter-zone scan
> balancing by truncating scans of larger zones.
> 
> Suppose, for example, ZONE_HIGHMEM is 10x the size of ZONE_NORMAL.  The zone
> balancing algorithm would require that if we're scanning 100 pages of
> ZONE_HIGHMEM, we should scan 10 pages of ZONE_NORMAL.  But this logic will
> cause the scanning of ZONE_HIGHMEM to bale out after only 32 pages are
> reclaimed.  Thus effectively causing smaller zones to be scanned relatively
> harder than large ones.
> 
> Now I need to remember what the workload was which caused me to write this
> patch originally, then fix it up in a different way...

Maybe it's a situation like this:

__|____|________|________________|________________________________|________________________________________________________________|________________________________________________________________________________________________________________________________|________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 ----------------------------------------------------------------------------------------------------------------------------------
        _: pinned chunk
        -: reclaimable chunk
        |: shrink_zone() invocation

First we run into a large range of pinned chunks, which lowered the scan
priority.  And then there are plenty of reclaimable chunks, bomb...

Thanks,
Wu