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

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Date:	Wed, 06 Mar 2013 10:29:12 -0600
From:	Michael Wolf <mjw@...ux.vnet.ibm.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, riel@...hat.com, gleb@...hat.com,
	kvm@...r.kernel.org, peterz@...radead.org, mingo@...hat.com,
	anthony@...emonkey.ws
Subject: Re: [PATCH 0/4] Alter steal-time reporting in the guest

On Wed, 2013-03-06 at 12:13 +0400, Glauber Costa wrote:
> On 03/06/2013 05:41 AM, Marcelo Tosatti wrote:
> > 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.
> > 
> If we take into account that 1 second always have one second, I believe
> that you can just subtract the consigned time from the steal time the
> host passes to the guest.
> 
> During each second, the numbers won't sum up to 100. The delta to 100 is
> the consigned time, if anyone cares.
> 
> Adopting this would simplify this a lot. All you need to do, really, is
> to get your calculation right from the bandwidth given by the cpu
> controller. Subtract it in the host, and voila.

I looked at doing that once but was told that I was changing the
interface in an unacceptable way, because now I was not reporting all of
the elapsed time.  I agree it would make things simpler.
> 
> 


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