linux-kernel - Re: [PATCH 0/4] Alter steal-time reporting in the guest

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Message-ID: <20130306014151.GB11481@amt.cnet>
Date:	Tue, 5 Mar 2013 22:41:51 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Michael Wolf <mjw@...ux.vnet.ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, riel@...hat.com, gleb@...hat.com,
	kvm@...r.kernel.org, peterz@...radead.org, glommer@...allels.com,
	mingo@...hat.com, anthony@...emonkey.ws
Subject: Re: [PATCH 0/4] Alter steal-time reporting in the guest

On Tue, Mar 05, 2013 at 02:22:08PM -0600, Michael Wolf wrote:
> Sorry for the delay in the response.  I did not see the email
> right away.
> 
> On Mon, 2013-02-18 at 22:11 -0300, Marcelo Tosatti wrote:
> > On Mon, Feb 18, 2013 at 05:43:47PM +0100, Frederic Weisbecker wrote:
> > > 2013/2/5 Michael Wolf <mjw@...ux.vnet.ibm.com>:
> > > > In the case of where you have a system that is running in a
> > > > capped or overcommitted environment the user may see steal time
> > > > being reported in accounting tools such as top or vmstat.  This can
> > > > cause confusion for the end user.
> > > 
> > > Sorry, I'm no expert in this area. But I don't really understand what
> > > is confusing for the end user here.
> > 
> > I suppose that what is wanted is to subtract stolen time due to 'known
> > reasons' from steal time reporting. 'Known reasons' being, for example,
> > hard caps. So a vcpu executing instructions with no halt, but limited to
> > 80% of available bandwidth, would not have 20% of stolen time reported.
> 
> Yes exactly and the end user many times did not set up the guest and is
> not aware of the capping.  The end user is only aware of the performance
> level that they were told they would get with the guest.  
> > But yes, a description of the scenario that is being dealt with, with
> > details, is important.
> 
> I will add more detail to the description next time I submit the
> patches.  How about something like,"In a cloud environment the user of a
> kvm guest is not aware of the underlying hardware or how many other
> guests are running on it.  The end user is only aware of a level of
> performance that they should see."   or does that just muddy the picture
> more??

So the feature aims for is to report stolen time relative to hard
capping. That is: stolen time should be counted as time stolen from
the guest _beyond_ hard capping. Yes?

Probably don't need to report new data to the guest for that.

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