On Thu, Sep 06, 2012 at 02:31:12PM +0900, Minchan Kim wrote:

Hi Mel,

On Wed, Sep 05, 2012 at 11:56:11AM +0100, Mel Gorman wrote:

quoted

On Wed, Sep 05, 2012 at 05:11:13PM +0900, Minchan Kim wrote:

quoted

This patch introudes MIGRATE_DISCARD mode in migration.
It drops *clean cache pages* instead of migration so that
migration latency could be reduced by avoiding (memcpy + page remapping).
It's useful for CMA because latency of migration is very important rather
than eviction of background processes's workingset. In addition, it needs
less free pages for migration targets so it could avoid memory reclaiming
to get free pages, which is another factor increase latency.

Bah, this was released while I was reviewing the older version. I did
not read this one as closely but I see the enum problems have gone away
at least. I'd still prefer if CMA had an additional helper to discard
some pages with shrink_page_list() and migrate the remaining pages with
migrate_pages(). That would remove the need to add a MIGRATE_DISCARD
migrate mode at all.

I am not convinced with your point. What's the benefit on separating
reclaim and migration? For just removing MIGRATE_DISCARD mode?

Maintainability. There are reclaim functions and there are migration
functions. Your patch takes migrate_pages() and makes it partially a
reclaim function mixing up the responsibilities of migrate.c and vmscan.c.

I don't think it's not bad because my implementation is very simple(maybe
it's much simpler than separating reclaim and migration) and
could be used by others like memory-hotplug in future.

They could also have used the helper function from CMA that takes a list
of pages, reclaims some and migrates other.

If you're not strong against with me, I would like to insist on my implementation.

I'm not very strongly against it but I'm also very unhappy.

-- 
Mel Gorman
SUSE Labs

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