From: "<>" 
Reply-To: "OpenStack Development Mailing List (not for usage questions)" 
Date: Wednesday, February 25, 2015 at 8:53 AM
To: "OpenStack Development Mailing List (not for usage questions)" 
Subject: Re: [openstack-dev] [Congress][Delegation] Initial workflow design

Hi Tim, All,

1)     Step 3: The VM-placement engine is also a “datalog engine” . Right?

When policies are delegated:

when policies are inserted? When the VM-placement engine has already registered 
itself all policies are given to it?

“In our example, this would mean the domain-specific policy engine executes the 
following API call over the DSE”

ð  “domain-agnostic” ….


2)     Step 4:


But finally: if Congress will likely “delegate”

Not sure what you’re suggesting here.

3)     Step 5:  Compilation of subpolicy to LP in VM-placement engine

For the PoC, it is likely that the LP program ( in PuLP or some other ML) is 
*not* completely generated by compiler/translator.

ð  Right?

Where does the rest of the program originate?  I’m not saying the entire LP 
program is generated from the Datalog constraints; some of it is generated by 
the solver independent of the Datalog.  In the text, I gave the example of 
defining hMemUse[j].

     You also indicate that some category of constraints (“the LP solver 
doesn’t know what the relationship between assign[i][j], hMemUse[j], and 
vMemUse[i] actually is, so the VM-placement engine must also include 
constraints”) .
     These constraints must be “explicitly” written?  (e.g. max_ram_allocation 
etc that are constraints used in the solver-scheduler’s package).

The VM-placement engine does 2 things: (I) translates Datalog to LP and (ii) 
generates additional LP constraints.  (Both work items could leverage any 
constraints that are builtin to a specific solver, e.g. the solver-scheduler.  
The point is that there are 2 distinct, conceptual origins of the LP 
constraints: those that represent the Datalog and those that codify the domain.

         So what “parts” will be generated:
            Cost function :
            Constraint from Policy : memory usage < 75%

         Then the rest should be “filled” up?

         Could we convene on an intermediary “modeling language”?
            @Yathi: do you think we could use some thing like AMPL ? Is this 

    A detail: the example “Y[host1] = hMemUse[host1] > 0.75 * hMemCap[host1]”

ð  To be changed to a linear form (mi – Mi > 0 then Yi = 1 else Yi = 0) so 
something like (mi – Mi) < 100 yi

Each domain-specific solver can do whatever it wants, so it’s not clear to me 
what the value of choosing a modeling language actually is—unless we want to 
build a library of common functionality that makes the construction of 
domain-specific engine (wrappers) easier.  I’d prefer to spend our energy 
understanding whether the proposed workflow/interface works for a couple of 
different domain-specific policy engines OR to flush this one out and build it.

4)     Step 6: This is completely internal to the VM-placement engine (and we 
could say this is “transparent” to Congress)

We should allow configuration of a solver (this itself could be a policy ☺ )

How to invoke the solver API ?

The domain-specific placement engine could send out to DSE (action_handler: 

I had always envisioned the solver being just a library of code—not an entity 
that sits on the DSE itself.

3)   Step 7 : Perform the migrations (according to the assignments computed in 
the step 6)

     This part invokes OpenStack API (to perform migrations).
     We may suppose that there are services implementing “action handlers”?
     It can listen on the DSE and execute the action.

That interface is supposed to exist by the Kilo release.  I’ll check up on the 

5)     Nova tables to use
Policy warning(id) :-
    nova:host(id, name, service, zone, memory_capacity),
    ceilometer:statistics(id, "memory", avg, count, duration,
     durstart, durend, max, min, period,
perstart, perend, sum, unit),
    avg > 0.75 * memory_capacity

I believe that ceilometer gives usage of VMs and not hosts. The host table 
(ComputeNode table) should give current used capacity.

Good to know.

6)     One of the issues highlighted in OpenStack (scheduler) and also 
elsewhere (e.g. Omega scheduler by google) is :

Reading “host utilization” state from the data bases and DB (nova:host table) 
updates and overhead of maintaining in-memory state uptodate.

ð  This is expensive and current nova-scheduler does face this issue (many 

      While the first goal is a PoC, this will likely become a concern in terms 
of adoption.

So you’re saying we won’t have fresh enough data to make policy decisions?  If 
the data changes so frequently that we can’t get an accurate view, then I’m 
guessing we shouldn’t be migrating based on that data anyway.

Could you point me to some of these discussions?

7)     While in this document you have changed the “example” policy, could we 
drill down the set of policies for the PoC (the server under utilization ?)

ð  As a reference

Sure.  The only reason I chose this policy was because it doesn’t have 
aggregation.  I’m guessing we’ll want to avoid aggregation for the POC because 
we don’t yet have it in Congress, and it complicates the problem of translating 
Datalog to LP substantially.



De : Yathiraj Udupi (yudupi) []
Envoyé : mardi 24 février 2015 20:01
À : OpenStack Development Mailing List (not for usage questions); Tim Hinrichs
Cc : Debo Dutta (dedutta)
Objet : Re: [openstack-dev] [Congress][Delegation] Initial workflow design

Hi Tim,

Thanks for your updated doc on Delegation from Congress to a domain-specific 
policy engine, in this case, you are planning to build a LP-based VM-Placement 
engine to be the domain specific policy engine.
I agree your main goal is to first get the delegation interface sorted out.  It 
will be good so that external services (like Solver-Scheduler) can also easily 
integrate to the delegation model.

From the Solver-Scheduler point of view,  we would actually want to start 
working on a PoC effort to start integrating Congress and the Solver-Scheduler.
We believe rather than pushing this effort to a long-term,  it would add value 
to both the Solver Scheduler effort, as well as the Congress effort to try some 
early integration now, as most of the LP solver work for VM placements is ready 
available now in Solver scheduler, and we need to spend some time thinking 
about translating your domain-agnostic policy to constraints that the Solver 
scheduler can use.

I would definitely need your help from the Congress interfaces and I hope you 
will share your early interfaces for the delegation, so I can start the effort 
from the Solver scheduler side for integration.
I will reach out to you to get some initial help for integration w.r.t. 
Congress, and also keep you posted about the progress from our side.


On 2/23/15, 11:28 AM, "Tim Hinrichs" 
<<>> wrote:

Hi all,

I made a heavy editing pass of the Delegation google doc, incorporating many of 
your comments and my latest investigations into VM-placement.  I left the old 
stuff in place at the end of the doc and put the new stuff at the top.

My goal was to propose an end-to-end workflow for a PoC that we could put 
together quickly to help us explore the delegation interface.  We should 
iterate on this design until we have something that we think is workable.   And 
by all means pipe up if you think we need a totally different starting point to 
begin the iteration.

(BTW I'm thinking of the integration with solver-scheduler as a long-term 
solution to VM-placement, once we get the delegation interface sorted out.)<>



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