Date: Fri, 17 Nov 2006 14:05:13 +0000
From: Alan <alan@lxorguk.ukuu.org.uk>
To: Patrick.Le-Dot@bull.net (Patrick.Le-Dot)
Cc: balbir@in.ibm.com, ckrm-tech@lists.sourceforge.net, dev@openvz.org,
       haveblue@us.ibm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       rohitseth@google.com
Subject: Re: [ckrm-tech] [RFC][PATCH 5/8] RSS controller task migration
 support
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On Fri, 17 Nov 2006 14:25:33 +0100 (CET)
> For a customer the main reason to use guarantee is to be sure that
> some pages of a job remain in memory when the system is low on free
> memory. This should be true even for a job in group/container A with

That actually doesn't appear a very useful definition.

There are two reasons for wanting memory guarantees

#1	To be sure a user can't toast the entire box but just their own
	compartment (eg web hosting)

#2	To ensure all apps continue to make progress

The simple approach doesn't seem to work for either. There is a threshold
above which #1 and #2 are the same thing, below that trying to keep a few
pages in memory will thrash not make progress and will harm overall
behaviour thus failing to solve #1 or #2. At that point you have to
decide whether what you have is a misconfiguration or whether the system
should be prepared to do temporary cycling overcommits so containers take
it in turn to make progress when overcommitted.

> If the limit is a "hard limit" then we have implemented reservation and
> this is too strict.

Thats fundamentally a judgement based on your particular workload and
constraints. If I am web hosting then I don't generally care if my end
users compartment blows up under excess load, I care that the other 200
customers using the box don't suffer and all phone me to complain.

Alan
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