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

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Date:	Mon, 27 Aug 2012 11:50:22 -0700
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 08/27/2012 08:50 AM, Michael Wolf wrote:
> On Sat, 2012-08-25 at 19:36 -0400, Glauber Costa wrote:
>> 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?
> 
> It is not clear to me how knowing how many milliseconds you are
> expecting to run will help the user.  Currently the users will run top
> to see how well the guest is running.  If they see _any_ steal time some
> users think they are not getting the full use of their processor
> entitlement.
>

And your plan is just to selectively lie about it, but disabling it with
a knob?

> Maybe I'm missing what you are proposing, but even if you knew the
> milliseconds that you were expecting for your processor you would have
> to adjust the top output in your head so to speak.  You would see the
> utilization and then say 'ok that matches the number of milliseconds I
> expected to run..."   If we take away the steal time (as long as it is
> equal to or less than the expected amount of steal time) then the user
> running top will see the 100% utilization.
> 

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