Re: [PATCH 10/11] Direct compact when a high-order allocation fails

[KAMEZAWA Hiroyuki]

On Thu, 25 Mar 2010 10:16:54 +0000
Mel Gorman <mel@xxxxxxxxx> wrote:

> On Thu, Mar 25, 2010 at 06:50:21PM +0900, KAMEZAWA Hiroyuki wrote:
> > On Thu, 25 Mar 2010 09:48:26 +0000
> > Mel Gorman <mel@xxxxxxxxx> wrote:
> > 
> > > > In that case, compact_finished() can't
> > > > find there is a free chunk and do more work.  How about using a function like
> > > > 	 free_pcppages_bulk(zone, pcp->batch, pcp);
> > > > to bypass pcp list and freeing pages at once ?
> > > > 
> > > 
> > > I think you mean to drain the PCP lists while compaction is happening
> > > but is it justified? It's potentially a lot of IPI calls just to check
> > > if compaction can finish a little earlier. If the pages on the PCP lists
> > > are making that much of a difference to high-order page availability, it
> > > implies that the zone is pretty full and it's likely that compaction was
> > > avoided and we direct reclaimed.
> > > 
> > Ah, sorry for my short word again. I mean draining "local" pcp list because
> > a thread which run direct-compaction freed pages. IPI is not necessary and
> > overkill.
> > 
> 
> Ah, I see now. There are two places that pages get freed.  release_freepages()
> at the end of compaction when it's too late for compact_finished() to be
> helped and within migration itself. Migration frees with either
> free_page() or more commonly put_page() with put_page() being the most
> frequently used. As free_page() is called on failure to migrate (rare),
> there is little help in changing it and I'd rather not modify how
> put_page() works.
> 
> I could add a variant of drain_local_pages() that drains just the local PCP of
> a given zone before compact_finished() is called. The cost would be a doubling
> of the number of times zone->lock is taken to do the drain. Is it
> justified? It seems overkill to me to take the zone->lock just in case
> compaction can finish a little earlier. It feels like it would be adding
> a guaranteed cost for a potential saving.
> 
If you want to keep code comapct, I don't ask more.

I worried about that just because memory hot-unplug were suffered by pagevec
and pcp list before using  MIGRATE_ISOLATE and proper lru_add_drain().

Thanks,
-Kame



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