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

[Michael Wolf]

On Wed, 2013-03-06 at 23:30 -0300, Marcelo Tosatti wrote:
> On Wed, Mar 06, 2013 at 10:27:13AM -0600, Michael Wolf wrote:
> > On Tue, 2013-03-05 at 22:41 -0300, 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@xxxxxxxxxxxxxxxxxx>:
> > > > > > > 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?
> > Yes, that is the goal.
> > > 
> > > Probably don't need to report new data to the guest for that.
> > Not sure I understand what you are saying here. Do you mean that I don't
> > need to report the expected steal from the guest?  If I don't do that
> > then I'm not reporting all of the time and changing /proc/stat in a
> > bigger way than adding another catagory.  Also I thought I would need to
> > provide the consigned time and the steal time for debugging purposes.
> > Maybe I'm missing your point.....
> 
> OK so the usefulness of steal time comes from the ability to measure 
> CPU cycles that the guest is being deprived of, relative to some unit
> (implicitly the CPU frequency presented to the VM). That way, it becomes
> easier to properly allocate resources.
> 
> From top man page:
> st : time stolen from this vm by the hypervisor
> 
> Not only its a problem for the lender, it is also confusing for the user
> (who has to subtract from the reported value himself), the hardcapping 
> from reported steal time.
> 
> 
> The problem with the algorithm in the patchset is the following
> (practical example):
> 
> - Hard capping set to 80% of available CPU.
> - vcpu does not exceed its threshold, say workload with 40%
> CPU utilization.
> - Under this scenario it is possible for vcpu to be deprived
> of cycles (because out of the 40% that workload uses, only 30% of
> actual CPU time are being provided).
> - The algorithm in this patchset will not report any stolen time
> because it assumes 20% of stolen time reported via 'run_delay'
> is fixed at all times (which is false), therefore any valid 
> stolen time below 20% will not be reported.
> 
> Makes sense?
> 
> Not sure what the concrete way to report stolen time relative to hard
> capping is (probably easier inside the scheduler, where run_delay is
> calculated).
> 
> Reporting the hard capping to the guest is a good idea (which saves the
> user from having to measure it themselves), but better done separately
> via new field.

didnt respond to this in the previous response.  I'm not sure I'm
following you here.  I thought this is what I was doing by having a
consigned (expected steal) field add to the /proc/stat output.  Are you
looking for something else or a better naming convention?

> 
> 
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