Date:	Wed, 28 Nov 2012 12:45:38 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michael Wolf <mjw@...ux.vnet.ibm.com>
CC:	<linux-kernel@...r.kernel.org>, <riel@...hat.com>,
	<kvm@...r.kernel.org>, <peterz@...radead.org>,
	<mtosatti@...hat.com>, <mingo@...hat.com>, <anthony@...emonkey.ws>,
	"gleb@...hat.com >> Gleb Natapov" <gleb@...hat.com>
Subject: Re: [PATCH 0/5] Alter steal time reporting in KVM

On 11/27/2012 07:10 PM, Michael Wolf wrote:
> On 11/27/2012 02:48 AM, Glauber Costa wrote:
>> Hi,
>>
>> On 11/27/2012 12:36 AM, Michael Wolf wrote:
>>> 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.  To ease the confusion this patch set
>>> adds the idea of consigned (expected steal) time.  The host will
>>> separate
>>> the consigned time from the steal time.  The consignment limit passed
>>> to the
>>> host will be the amount of steal time expected within a fixed period of
>>> time.  Any other steal time accruing during that period will show as the
>>> traditional steal time.
>> If you submit this again, please include a version number in your series.
> Will do.  The patchset was sent twice yesterday by mistake.  Got an
> error the first time and didn't
> think the patches went out.  This has been corrected.
>>
>> It would also be helpful to include a small changelog about what changed
>> between last version and this version, so we could focus on that.
> yes, will do that.  When I took the RFC off the patches I was looking at
> it as a new patchset which was
> a mistake.  I will make sure to add a changelog when I submit again.
>>
>> As for the rest, I answered your previous two submissions saying I don't
>> agree with the concept. If you hadn't changed anything, resending it
>> won't change my mind.
>>
>> I could of course, be mistaken or misguided. But I had also not seen any
>> wave of support in favor of this previously, so basically I have no new
>> data to make me believe I should see it any differently.
>>
>> Let's try this again:
>>
>> * Rik asked you in your last submission how does ppc handle this. You
>> said, and I quote: "In the case of lpar on POWER systems they simply
>> report steal time and do not alter it in any way.
>> They do however report how much processor is assigned to the partition
>> and that information is in /proc/ppc64/lparcfg."
> Yes, but we still get questions from users asking what is steal time?
> why am I seeing this?
>>
>> Now, that is a *way* more sensible thing to do. Much more. "Confusing
>> users" is something extremely subjective. This is specially true about
>> concepts that are know for quite some time, like steal time. If you out
>> of a sudden change the meaning of this, it is sure to confuse a lot more
>> users than it would clarify.
> Something like this could certainly be done.  But when I was submitting
> the patch set as
> an RFC then qemu was passing a cpu percentage that would be used by the
> guest kernel
> to adjust the steal time. This percentage was being stored on the guest
> as a sysctl value.
> Avi stated he didn't like that kind of coupling, and that the value
> could get out of sync.  Anthony stated "The guest shouldn't need to know
> it's entitlement. Or at least, it's up to a management tool to report
> that in a way that's meaningful for the guest."
> 
> So perhaps I misunderstood what they were suggesting, but I took it to
> mean that they did not
> want the guest to know what the entitlement was.  That the host should
> take care of it and just
> report the already adjusted data to the guest.  So in this version of
> the code the host would use a set
> period for a timer and be passed essentially a number of ticks of
> expected steal time.  The host
> would then use the timer to break out the steal time into consigned and
> steal buckets which would be
> reported to the guest.
> 
> Both the consigned and the steal would be reported via /proc/stat. So
> anyone needing to see total
> time away could add the two fields together.  The user, however, when
> using tools like top or vmstat
> would see the usage based on what the guest is entitled to.
> 
> Do you have suggestions for how I can build consensus around one of the
> two approaches?
> 

Before I answer this, can you please detail which mechanism are you
using to enforce the entitlement? Is it the cgroup cpu controller, or
something else?

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