Carlo Curino commented on YARN-569:

------------- Comments of attached Graphs ----------
The attached graph highlights the need for preemption by means of an example 
designed to highlights this. We run 2 sort jobs over 128GB of data on a 10 
nodes cluster, starting the first job in queue B (20% guaranteed capacity) and 
the second job 400sec later in queue A (80% guaranteed capacity).

We compare three scenarios:
# Default CapacityScheduler with A and B having maximum capacity set to 100%: 
the cluster utilization is high, B runs fast since it can use the entire 
cluster when A is not around, but A needs to wait for very long (almost 20 min) 
before obtaining access to its all of its guaranteed capacity (and over 250 
secs to get any container beside the AM).
# Default CapacityScheduler with A and B have maximum capacity set to 80 and 
20% respectively, A obtains its guaranteed resources immediately, but the 
cluster utilization is very low and jobs in B take over 2X longer since they 
cannot use spare overcapacity.
# CapacityScheduler + preemption: A and B are configured as in 1) but we 
preempt containers. We obtain both high-utilization, short runtimes for B 
(comparable to scenario 1), and prompt resources to A (within 30 sec). 

The second attached graph shows a scenario with 3 queues A, B, C with 40%, 20%, 
40% capacity guaranteed. We show more "internals" of the policy by plotting, 
instantaneous resource utilization as above, total pending request, guaranteed 
capacity, ideal assignment of memory, ideal preemption, actual preemption.
Things to note:
# The idealized memory assignment and instaneous resource utilization are very 
close to each other, i.e., the combination of CapacityScheduler+Preemption 
tightly follows the the ideal distribution of resources
# When only one job is running it gets 100% of the cluster, when B, A are 
running they get 33% and 66% each (which is a fair overcapacity assignment from 
their 20%, 40% guaranteed capacity), when all three jobs are running (and they 
want at least their capacity worth of resources) they obtain their guaranteed 
#actual preemption is a fraction of ideal preemption, this is because we 
account for natural completion of tasks (with a configurable parameter)
#in this experiment we do not bound the total amount of preemption per round 
(i.e., parameter set to 1.0)

> CapacityScheduler: support for preemption (using a capacity monitor)
> --------------------------------------------------------------------
>                 Key: YARN-569
>                 URL: https://issues.apache.org/jira/browse/YARN-569
>             Project: Hadoop YARN
>          Issue Type: Improvement
>          Components: capacityscheduler
>            Reporter: Carlo Curino
>         Attachments: 3queues.pdf, capacity.patch, 
> CapScheduler_with_preemption.pdf
> There is a tension between the fast-pace reactive role of the 
> CapacityScheduler, which needs to respond quickly to 
> applications resource requests, and node updates, and the more introspective, 
> time-based considerations 
> needed to observe and correct for capacity balance. To this purpose we opted 
> instead of hacking the delicate
> mechanisms of the CapacityScheduler directly to add support for preemption by 
> means of a "Capacity Monitor",
> which can be run optionally as a separate service (much like the 
> NMLivelinessMonitor).
> The capacity monitor (similarly to equivalent functionalities in the fairness 
> scheduler) operates running on intervals 
> (e.g., every 3 seconds), observe the state of the assignment of resources to 
> queues from the capacity scheduler, 
> performs off-line computation to determine if preemption is needed, and how 
> best to "edit" the current schedule to 
> improve capacity, and generates events that produce four possible actions:
> # Container de-reservations
> # Resource-based preemptions
> # Container-based preemptions
> # Container killing
> The actions listed above are progressively more costly, and it is up to the 
> policy to use them as desired to achieve the rebalancing goals. 
> Note that due to the "lag" in the effect of these actions the policy should 
> operate at the macroscopic level (e.g., preempt tens of containers
> from a queue) and not trying to tightly and consistently micromanage 
> container allocations. 
> ------------- Preemption policy  (ProportionalCapacityPreemptionPolicy): 
> ------------- 
> Preemption policies are by design pluggable, in the following we present an 
> initial policy (ProportionalCapacityPreemptionPolicy) we have been 
> experimenting with.  The ProportionalCapacityPreemptionPolicy behaves as 
> follows:
> # it gathers from the scheduler the state of the queues, in particular, their 
> current capacity, guaranteed capacity and pending requests (*)
> # if there are pending requests from queues that are under capacity it 
> computes a new ideal balanced state (**)
> # it computes the set of preemptions needed to repair the current schedule 
> and achieve capacity balance (accounting for natural completion rates, and 
> respecting bounds on the amount of preemption we allow for each round)
> # it selects which applications to preempt from each over-capacity queue (the 
> last one in the FIFO order)
> # it remove reservations from the most recently assigned app until the amount 
> of resource to reclaim is obtained, or until no more reservations exits
> # (if not enough) it issues preemptions for containers from the same 
> applications (reverse chronological order, last assigned container first) 
> again until necessary or until no containers except the AM container are left,
> # (if not enough) it moves onto unreserve and preempt from the next 
> application. 
> # containers that have been asked to preempt are tracked across executions. 
> If a containers is among the one to be preempted for more than a certain 
> time, the container is moved in a the list of containers to be forcibly 
> killed. 
> Notes:
> (*) at the moment, in order to avoid double-counting of the requests, we only 
> look at the "ANY" part of pending resource requests, which means we might not 
> preempt on behalf of AMs that ask only for specific locations but not any. 
> (**) The ideal balance state is one in which each queue has at least its 
> guaranteed capacity, and the spare capacity is distributed among queues (that 
> wants some) as a weighted fair share. Where the weighting is based on the 
> guaranteed capacity of a queue, and the function runs to a fix point.  
> Tunables of the ProportionalCapacityPreemptionPolicy:
> #     observe-only mode (i.e., log the actions it would take, but behave as 
> read-only)
> # how frequently to run the policy
> # how long to wait between preemption and kill of a container
> # which fraction of the containers I would like to obtain should I preempt 
> (has to do with the natural rate at which containers are returned)
> # deadzone size, i.e., what % of over-capacity should I ignore (if we are off 
> perfect balance by some small % we ignore it)
> # overall amount of preemption we can afford for each run of the policy (in 
> terms of total cluster capacity)
> In our current experiments this set of tunables seem to be a good start to 
> shape the preemption action properly. More sophisticated preemption policies 
> could take into account different type of applications running, job 
> priorities, cost of preemption, integral of capacity imbalance. This is very 
> much a control-theory kind of problem, and some of the lessons on designing 
> and tuning controllers are likely to apply.
> Generality:
> The monitor-based scheduler edit, and the preemption mechanisms we introduced 
> here are designed to be more general than enforcing capacity/fairness, in 
> fact, we are considering other monitors that leverage the same idea of 
> "schedule edits" to target different global properties (e.g., allocate enough 
> resources to guarantee deadlines for important jobs, or data-locality 
> optimizations, IO-balancing among nodes, etc...).
> Note that by default the preemption policy we describe is disabled in the 
> patch.
> Depends on YARN-45 and YARN-567, is related to YARN-568

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