Re: [PATCH 30/34] mm: vmscan: Do not force kswapd to scan small targets

Greg KH
2012-07-25T19:59:54Z

On Mon, Jul 23, 2012 at 02:38:43PM +0100, Mel Gorman wrote:
> commit ad2b8e601099a23dffffb53f91c18d874fe98854 upstream - WARNING: this is a substitute patch.
>
> Stable note: Not tracked in Bugzilla. This is a substitute for an
> upstream commit addressing a completely different issue that
> accidentally contained an important fix. The workload this patch
> helps was memcached when IO is started in the background. memcached
> should stay resident but without this patch it gets swapped more
> than it should. Sometimes this manifests as a drop in throughput
> but mostly it was observed through /proc/vmstat.
>
> Commit [246e87a9: memcg: fix get_scan_count() for small targets] was
> meant to fix a problem whereby small scan targets on memcg were ignored
> causing priority to raise too sharply. It forced scanning to take place
> if the target was small, memcg or kswapd.
>
> >From the time it was introduced it cause excessive reclaim by kswapd
> with workloads being pushed to swap that previously would have stayed
> resident. This was accidentally fixed by commit [ad2b8e60: mm: memcg:
> remove optimization of keeping the root_mem_cgroup LRU lists empty] but
> that patchset is not suitable for backporting.
>
> The original patch came with no information on what workloads it benefits
> but the cost of it is obvious in that it forces scanning to take place
> on lists that would otherwise have been ignored such as small anonymous
> inactive lists. This patch partially reverts 246e87a9 so that small lists
> are not force scanned which means that IO-intensive workloads with small
> amounts of anonymous memory will not be swapped.
>
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
> mm/vmscan.c | 3 ---
> 1 file changed, 3 deletions(-)

I don't understand this patch. The original
ad2b8e601099a23dffffb53f91c18d874fe98854 commit touched the file
mm/memcontrol.c and seemed to do something quite different from what you
have done below.

I'm all for fixing things in a different way than what was done in
Linus's tree, IF there is a reason to, but the comparison between these
two patches (yours and upstream) are not making any sense at all.

confused,

greg k-h