Date: Fri, 1 Apr 2011 23:34:55 +0530
From: Balbir Singh <balbir@linux.vnet.ibm.com>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: linux-mm@kvack.org, akpm@linux-foundation.org, npiggin@kernel.dk,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org, cl@linux.com,
        kamezawa.hiroyu@jp.fujitsu.com, Mel Gorman <mel@csn.ul.ie>,
        Minchan Kim <minchan.kim@gmail.com>
Subject: Re: [PATCH 0/3] Unmapped page cache control (v5)
Message-ID: <20110401180455.GU2879@balbir.in.ibm.com>
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* KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [2011-04-01 22:21:26]:

> > * KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [2011-04-01 16:56:57]:
> > 
> > > Hi
> > > 
> > > > > 1) zone reclaim doesn't work if the system has multiple node and the
> > > > >    workload is file cache oriented (eg file server, web server, mail server, et al). 
> > > > >    because zone recliam make some much free pages than zone->pages_min and
> > > > >    then new page cache request consume nearest node memory and then it
> > > > >    bring next zone reclaim. Then, memory utilization is reduced and
> > > > >    unnecessary LRU discard is increased dramatically.
> > > > > 
> > > > >    SGI folks added CPUSET specific solution in past. (cpuset.memory_spread_page)
> > > > >    But global recliam still have its issue. zone recliam is HPC workload specific 
> > > > >    feature and HPC folks has no motivation to don't use CPUSET.
> > > > 
> > > > I am afraid you misread the patches and the intent. The intent to
> > > > explictly enable control of unmapped pages and has nothing
> > > > specifically to do with multiple nodes at this point. The control is
> > > > system wide and carefully enabled by the administrator.
> > > 
> > > Hm. OK, I may misread.
> > > Can you please explain the reason why de-duplication feature need to selectable and
> > > disabled by defaut. "explicity enable" mean this feature want to spot corner case issue??
> > 
> > Yes, because given a selection of choices (including what you
> > mentioned in the review), it would be nice to have
> > this selectable.
> 
> It's no good answer. :-/

I am afraid I cannot please you with my answers

> Who need the feature and who shouldn't use it? It this enough valuable for enough large
> people? That's my question point.
> 

You can see the use cases documented, including when running Linux as
a guest under other hypervisors, today we have a choice of not using
host page cache with cache=none, but nothing the other way round.
There are other use cases for embedded folks (in terms of controlling
unmapped page cache), please see previous discussions.

-- 
	Three Cheers,
	Balbir
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