linux-kernel - Re: [PATCH 07/11] Memory compaction core

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Message-Id: <20100319152102.876C.A69D9226@jp.fujitsu.com>
Date:	Fri, 19 Mar 2010 15:21:52 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 07/11] Memory compaction core

> > > > V1 did compaction per pageblock. but current patch doesn't.
> > > > so, Is COMPACTBLOCKS still good name?
> > > 
> > > It's not such a minor nit. I wondered about that myself but it's still a
> > > block - just not a pageblock. Would COMPACTCLUSTER be a better name as it's
> > > related to COMPACT_CLUSTER_MAX?
> > 
> > I've looked at this code again. honestly I'm a abit confusing even though both your
> > suggestions seems reasonable.  
> > 
> > now COMPACTBLOCKS is tracking #-of-called-migrate_pages. but I can't imazine
> > how to use it. can you please explain this ststics purpose? probably this is only useful
> > when conbination other stats, and the name should be consist with such combination one.
> > 
> 
> It is intended to count how many steps compaction took, the fewer the
> better so minimally, the lower this number is the better. Specifically, the
> "goodness" is related to the number of pages that were successfully allocated
> due to compaction. Assuming the only high-order allocation was huge pages,
> one possible calculation for "goodness" is;
> 
> hugepage_clusters = (1 << HUGE HUGETLB_PAGE_ORDER) / COMPACT_CLUSTER_MAX
> goodness = (compactclusters / hugepage_clusters) / compactsuccess
> 
> The value of goodness is undefined if "compactsuccess" is 0.
> 
> Otherwise, the closer the "goodness" is to 1, the better. A value of 1
> implies that compaction is selecting exactly the right blocks for migration
> and the minimum number of pages are being moved around. The greater the value,
> the more "useless" work compaction is doing.
> 
> If there are a mix of high-orders that are resulting in compaction, calculating
> the goodness is a lot harder and compactcluster is just a rule of thumb as
> to how much work compaction is doing.
> 
> Does that make sense?

Sure! then, now I fully agree with COMPACTCLUSTER.

Thanks.


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