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

Michael Wolf
2013-03-07T21:10:13Z

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@linux.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?
> > 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?

I understand the scenerio. I will have to go back and look at the
CFS bandwidth code and run some tests. The question I have to look at is
how is everything reported in your scenerio above.

This will depend on how the cfs bandwidth is configured, are there
uncapped processes on the system and how cpu intensive are they.

I will run some tests and report back.

>
> 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.

I looked at doing something like this. If bandwidth controls are
configured there is a throttled flag. So in effect if the throttled
flag is set, don't add the time spent on the runqueue. But this will
fail to work in some cases.

For example
you sent up cfs bandwidth controls. set up the group to get 50% of the
processor

Have 1 physical cpu

Have 2 guests each with 1 vcpu.

Have each guest running to its full entitlement.

So in this case each guest will have time on the runqueue but neither
will ever be throttled since they will not exceed their quota in the
defined period. So now just trying to do this in the scheduler doesn't
work because you cannot rely on the throttled flag. In either case the
time is accumulated as time on the runqueue.

This is why my patchset had included a timer. It was basically
mimicking the bandwidth controller by using a timer set to the same
period. So in a given period of time a fixed quota of time on the
runqueue can be expected. If the amount of time on the runqueue exceeds
the expected, then report it.

>
>
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