linux-kernel - Re: [PATCH RFC 0/3] Add guest cpu

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Date:	Sat, 25 Aug 2012 19:36:19 -0400
From:	Glauber Costa <glommer@...allels.com>
To:	<mjw@...ux.vnet.ibm.com>
CC:	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<peterz@...radead.org>, <mtosatti@...hat.com>, <mingo@...hat.com>,
	<avi@...hat.com>
Subject: Re: [PATCH RFC 0/3] Add guest cpu_entitlement reporting

On 08/24/2012 11:11 AM, Michael Wolf wrote:
> On Fri, 2012-08-24 at 08:53 +0400, Glauber Costa wrote:
>> On 08/24/2012 03:14 AM, Michael Wolf wrote:
>>> This is an RFC regarding the reporting of stealtime.  In the case of
>>> where you have a system that is running with partial processors such as
>>> KVM the user may see steal time being reported in accounting tools such
>>> as top or vmstat.  This can cause confusion for the end user.  To
>>> ease the confusion this patch set adds a sysctl interface to set the
>>> cpu entitlement.  This is the percentage of cpu that the guest system is
>>>  expected to receive.  As long as the steal time is within its expected
>>> range it will show up as 0 in /proc/stat.  The user will then see in the
>>> accounting tools that they are getting a full utilization of the cpu
>>> resources assigned to them.
>>>
>>
>> And how is such a knob not confusing?
>>
>> Steal time is pretty well defined in meaning and is shown in top for
>> ages. I really don't see the point for this.
> 
> Currently you can see the steal time but you have no way of knowing if
> the cpu utilization you are seeing on the guest is the expected amount.
> I decided on making it a knob because a guest could be migrated to
> another system and it's entitlement could change because of hardware or 
> load differences.  It could simply be a /proc file and report the
> current entitlement if needed.   As things are currently implemented I 
> don't see how someone knows if the guest is running as expected or
> whether there is a problem.
> 

Turning off steal time display won't get even close to displaying the
information you want. What you probably want is a guest-visible way to
say how many miliseconds you are expected to run each second. Right?
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