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Distributed systems: which cluster do I obey?

April 17th, 2017 • Comments Off on Distributed systems: which cluster do I obey?

The topic of cluster formation in itself is not new. There are plenty of methods around to form cluster on the fly [1]. They mostly follow methods which make use of gossip protocols. Implementation can wildly be found, even in products like Akka.

Once a cluster is formed the question becomes wehter control (see this post for some background) is centralized or decentralized. Or something in between with a (hierarchical) set of leaders managing – in a distributed fashion – a cluster of entities that are actuating. Both, centralized and decentralized approaches, have their advantages and disadvantages. Centralized approaches are sometimes more efficient as they might have more knowledge to base their decisions upon. Note that even in centralized management approaches a gossip protocols might be in use. There are also plenty of algorithms around to support consensus and leader election [2], [3].

Especially in Fog and Fdge computing use cases, entities which move (e.g. a car, plane, flying/sailing drone) have to decide to which cluster they want to belong and obey the actions initiated by those. Graph representations are great for this – and yet again because of a rich set of algorithms in the graph theory space, the solution might be quite simple to implement. Note that in Fog/Edge use cases there most likely will always be very (geographic) static entities like Cloudlets in the mix – which could be elected as leaders.

For example, the following diagram shows two clusters: The red nodes’ size indicates who is the current leader. The blue dot – marks an entity that is connected to both clusters. The width of the edge connecting it to the cluster indicates to which cluster it ‘belongs’ (aka it’s affinity).

(Click to enlarge)

Now as the blue entity moves (geographically) the connections/edges to the clusters change. Now based on algorithms like this, the entity can make a decision to hand-off control to another cluster and obey the directions given from it.

(Click to enlarge)

Example 2: Intelligent Orchestration & Scheduling with OpenLava

January 7th, 2017 • Comments Off on Example 2: Intelligent Orchestration & Scheduling with OpenLava

This is the second post in a series (the first post can be found here) about how to insert smarts into a resource manager. So let’s look how a job scheduler or distributed resource management system (DRMS) — in a HPC use case — with OpenLava can be used. For the rationale behind all this check the background section of the last post.

The basic principle about this particular example is simple: each host in a cluster will report a “rank”; the rank will be used to make a decision on where to place a job. The rank could be defined as: a rank is high when the sub-systems of the hosts are heavily used; and the rank is low when none or some sub-system are utilized. How the individual sub-systems usage influences the rank value, is something that can be machine learned.

Let’s assume the OpenLava managed cluster is up and running and a couple of hosts are available. The concept of elims can be used to get the job done. The first step is, to teach the system what the rank is. This is done in the lsf.shared configuration file. The rank is defined to be a numeric value which will be updated every 10 seconds (while not increasing):

Begin Resource
RESOURCENAME  TYPE    INTERVAL INCREASING  DESCRIPTION
   ...
   rank       Numeric 10       N           (A rank for this host.)
End Resource

Next OpenLava needs to know for which hosts this rank should be determined. This is done through a concept of ‘resource mapping’ in the lsf.cluster.* configuration file. For now the rank should be used for all hosts by default:

Begin ResourceMap
RESOURCENAME LOCATION
rank ([default])
End ResourceMap

Next an external load information manager (LIM) script which will report the rank to OpenLava needs to be written. OpenLava expects that the script writes to stdout with the following format: <number of resources to report on> <first resource name> <first resource value> <second resource name> <second resource value> … . So in this case it should spit out ‘1 rank 42.0‘ every 10 seconds. The following python script will do just this – place this script in the file elim in $LSF_SERVERDIR:

#!/usr/bin/python2.7 -u

import time

INTERVAL = 10


def _calc_rank():
    # TODO calc value here...
    return {'rank': value}

while True:
    RES = _calc_rank()
    TMP = [k + ' ' + str(v) for k, v in RES.items()]
    print(\"%s %s\" % (len(RES), ' '.join(TMP)))
    time.sleep(INTERVAL)

Now a special job queue in the lsb.queues configuration file can be used which makes use of the rank. See the RES_REQ parameter in which it is defined that the candidate hosts for a job request are ordered by the rank:

Begin Queue
QUEUE_NAME = special
DESCRIPTION = Special queue using the rank coming from the elim.
RES_REQ = order[rank]
End Queue

Submitting a job to this queue is as easy as: bsub -q special sleep 1000. Or the rank can be passed along as a resource requirements on job submission (for any other queue): bsub -R “order[-rank]” -q special sleep 1000. By adding the ‘-‘ it is said that the submitter request the candidate hosts to be sorted for hosts with a high rank first.

Let’s assume a couple of hosts are up & running and they have different ranks (see the last column):

openlava@242e2f1f935a:/tmp$ lsload -l
HOST_NAME               status  r15s   r1m  r15m   ut    pg    io  ls    it   tmp   swp   mem   rank
45cf955541cf                ok   0.2   0.2   0.3   2%   0.0     0   0 2e+08  159G   16G   11G    9.0
b7245f8e6d0d                ok   0.2   0.2   0.3   2%   0.0     0   0 2e+08  159G   16G   11G    8.0
242e2f1f935a                ok   0.2   0.2   0.3   3%   0.0     0   0 2e+08  159G   16G   11G   98.0

When checking the earlier submitted job, the execution host (EXEC_HOST) is indeed the hosts with the lowest rank as expected:

openlava@242e2f1f935a:/tmp$ bjobs
JOBID   USER    STAT  QUEUE      FROM_HOST   EXEC_HOST   JOB_NAME   SUBMIT_TIME
101     openlav RUN   special    242e2f1f935 b7245f8e6d0 sleep 1000 Jan  7 10:06

The rank can also be seen in web interface like the one available for the OpenLava Mesos framework. What was described in this post is obviously just an example – other methods to integrate your smarts into the OpenLava resource manager can be realized as well.

Example 1: Intelligent Orchestration & Scheduling with Kubernetes

September 18th, 2016 • Comments Off on Example 1: Intelligent Orchestration & Scheduling with Kubernetes

In the last blog I suggested that analytical capabilities need to move to the core of resource managers. This is very much needed for autonomous controlled large scale systems which figure out the biggest chunk of decisions to be made themselves. While the benefits from this might be obvious, the how to inject the insights/intelligence back into the resource manager might not be. Hence this blog post series documenting a bit how to let systems (they are just examples – trying to cover most domains :-)) like Kubernetes, OpenStackMesos, YARN and OpenLava make smarter decisions.

Background

The blog posts are going to cover some generic concepts as well as point to specific documentation bits of the individual resource managers. Some of this is already covered in past blog posts but to recap let’s look at the 5(+1) Ws  for resource managers decision making (click to skip to the technical details):

Decision most of the time cannot be made generic – e.g. decisions made in HPC/HTC systems do not necessarily apply to a telco environments in which the workloads and resource are different. Hence the context of workloads and resource in place play a huge role. Ultimately Analytics which embraces the context (in all sorts and forms: deep/machine learning, statistical modelling, artificial intelligence, …) of the environment can drive the intelligence in the decision making through insights. This can obviously in multiple places/flows (see the foreground and background flow concepts here) and ultimately enables autonomous control.

Enhancing Kubernetes

For the Kubernetes examples let’s focus on a crucial decision point – doing the initial placement of a workloads (aka a POD in Kubernetes language) in a cluster. Although much of today’s research focuses on initial placement I’d urge everybody not to forget about all the other decisions that can be made more intelligent.

Like most Orchestrators and Schedulers Kubernetes follows a simple approach of filtering and ranking. After shortlisting possible candidates, the first step involves filtering those resource which do not meet the workloads demands. The second step involves prioritization (or ranking) of the resources best suited.

This general part is described nicely in the Kubernetes documentation here: https://github.com/kubernetes/kubernetes/blob/master/docs/devel/scheduler.md

This filtering part is mostly done based on capacities, while the second can involve information like the utilization. If you want to see this code have a look at the generic scheduling implementation: here. The available algorithms for filtering (aka predicates) and prioritization can be found here. The default methods that Kubernetes filters upon can be seen here: here – the default prioritization algorithms here: here. Note that weights can be applied to the algorithms based on your own needs as a provider. This is a nice way to tune and define how the resource under the control of the provider can be used.

While the process and the defaults already do a great job – let’s assume you’ve found a way on when and how to use an accelerator. Thankfully like most scheduling systems the scheduler in Kubernetes is extendable. Documentation for this can be found here. 3 ways are possible:

  1. recompile and alter the scheduler code,
  2. implement your own scheduler completely and run it in parallel,
  3. or implement an extension which the default scheduler calls when needed.

The first option is probably hard to manage in the long term, the second option requires you to deal with the messiness or concurrency while the third option is interesting (although adds latency to the process of scheduling due to an extra HTTP(s) call made). The default scheduler can basically call an external process to either ‘filter’ or ‘prioritize’. In the first case a list of possible candidate hosts is returned, in the the second case a prioritized list if returned. Now unfortunately the documentation get’s a bit vague, but luckily some code is available from the integration tests. For example here you can see some external filtering code, and here the matching prioritization code. Now that just needs to be served up over HTTP and you are ready to go, next to adding some configurations documented here.

So now an external scheduler extension can make a decisions if an accelerator should be assigned to a workload or not. The intelligent decision implemented in this extender could e.g. decide if an SR-IOV port is needed based on a bandwidth requirement, or if it is even a good idea to assign a Accelerator to a workload par the previous example.

Corrections, feedback and additional info are more then welcome. I’ve some scheduler extender code running here – but that is not shareable yet. I will update the post once I’ve completed this. In the next posts OpenStack (e.g. service like Nova, Watcher, Heat and Neutron), Mesos (how e.g. allocator modules can be used to inject smarts) and OpenLava (for which e.g. elims can be used to make better scheduling decisions) and obviously others will be introduced 🙂

 

Insight driven resource management & scheduling

July 25th, 2016 • Comments Off on Insight driven resource management & scheduling

Future data center resource and workload managers – and their [distributed]schedulers – will require a new key integrate capability: analytics. Reason for this is the the pure scale and the complexity of the disaggregation of resources and workloads which requires getting deeper insights to make better actuation decisions.

For data center management two major factors play a role: the workload (processes, tasks, containers, VMs, …) and the resources (CPUs, MEM, disks, power supplies, fans, …) under control. These form the service and resource landscape and are specific to the context of the individual data center. Different providers use different (heterogeneous) hardware (revisions) resource and have different customer groups running different workloads. The landscape overall describes how the entities in the data center are spatially connected. Telemetry systems allow for observing how they behave over time.

The following diagram can be seen as a metaphor on how the two interact: the workload create a impact on the landscape. The box represent a simple workload having an impact on the resource landscape. The landscape would be composed of all kind of different entities in the data center: from the air conditioning facility all the way to the CPU. Obviously the model taken here is very simple and in real-life a service would span multiple service components (such as load-balancers, DBs, frontends, backends, …). Different kinds of workloads impact the resource landscape in different ways.

landscape_gravity

(Click to enlarge)

Current data center management systems are too focused on understanding resources behavior only and while external analytics capabilities exists, it becomes crucial that these capabilities need to move to the core of it and allow for observing and deriving insights for both the workload and resource behavior:

Deriving insights on how workloads behave during the life-cycle, and how resources react to that impact, as well as how they can enhance the service delivery is ultimately key to finding the best match between service components over space and time. Better matching (aka actually playing Tetris – and smartly placing the title on the playing field) allows for optimized TCO given a certain business objective. Hence it is key that the analytical capabilities for getting insights on workload and resource behavior move to the very core of the workload and resource management systems in future to make better insightful decisions. This btw is key on all levels of the system hierarchy: on single resource, hosts, resource group and cluster level.

Note: parts of this were discussed during the 9th workshop on cloud control.

Graph stitching

January 2nd, 2016 • Comments Off on Graph stitching

Graph stitching describes a way to merge two graphs by adding relationships/edges between them. To determine which edges to add, a notion of node types is used (based on node naming would be easy :-)). Nodes with a certain type can be “stitched” to a node with a certain other type. As multiple mappings are possible, multiple result/candidate graphs are possible. A good stitch is defined by:

So based on node types two graphs are stitched together, and than a set of candidate result graphs will be validated, to especially satisfy the second bullet.

Let’s use an example to explain this concept a bit further. Assume the electrical “grid” in a house can be described by a graph with nodes like the power outlets and fuses, as well as edges describe the wiring. Some home appliances might be in place and connected to this graph as well. Hence a set of nodes describing for example a microwave (the power supply & magnetron), are in this graph as well. The edge between the power supply and the power outlet describe the power cable. The edge between the power supply and the magnetron is the internal cabling within the microwave. This graph can be seen in the following diagram.

(Click on enlarge)

(Click on enlarge)

The main fuse is connected to the fuses 1 & 2. Fuse 1 has three connected power outlets, of which outlet #2 is used by the microwave. Fuse 2 has two connected power outlets. Let’s call this graph the container from now on.

Now let’s assume a new HiFi installation (consisting of a blu-ray player and an amplifier) needs to be placed within this existing container. The installation itself can again be described using a simple graph, as shown in the following diagram.

(Click to enlarge)

(Click to enlarge)

Placing this request graph into the container graph now only requires that the power supplies of the player and amplifier are connected to the power outlets in the wall using a power cord. Hence edges/relationships are added to the container to stitch it to the request. This is done using the following mapping defition (The power_supply and power_outlet are values for the attribute “type” in the request & container graph):

{
    "power_supply": "power_outlet"
}

As there is more than one possible results for stitching two graphs, candidates (there are 2 power supplies and 5 power outlets in the mix) need to be examined to see if they make sense (e.g. the fuse to which the microwave is connected might blow up if another “consumer” is added). But before getting to the validation, the number of candidates graphs should be limited using conditions.

For example the HiFi installation should be placed in the living room and not the kitchen. Hence a condition as follows (The power outlet nodes in the container graph have an attribute which is either set to ‘kitchen’ or ‘living’) can be defined:

condition = {
    'attributes': [
        ('eq', ('bluray_p', ('room', 'living'))),
        ('eq', ('amp_p', ('room', 'living'))),
    ]
}

Also the amplifier should not be placed in the kitchen while the blu-ray player is placed in the living room. Hence the four nodes describing the request should share the same value for the room attribute. Also it can be defined that the power supplies of the player & amplifier should not be connected to the same power outlet:

condition = {
    'compositions': [
        ('share', ('room', ['amp', 'amp_p', 'bluray', 'bluray_p'])),
        ('diff', ('amp_p', 'bluray_p'))
    ]
}

This already limits the number of candidate resulting graphs which need to validated further. During validation it is determined if a graph resulting out of a possible stitching falls under the definition of a good stitch (see earlier on). Within the container – shown early – the nodes are ranked – red indicating the power outlet or fuse heavy loaded; while green means the power outlet/fuse is doing fine. Now let’s assume no more “consumers” should be added to the second outlet connected to the first fuse as the load (rank) is to high. The high load might be caused by the microwave.

All possible candidate graphs (given the second condition described earlier) are shown in the following diagram. The titles of the graphs describe the outcome of the validation, indicating that adding any more consumers to outlet_2 will cause problems:

(Click to enlarge)

(Click to enlarge)

The container and request are represented as shown earlier, while the stitches for each candidate resulting graph are shown as dotted lines.

Graph stitcher is a simple tool implements a simple a stitching algorithm which generates the possible graphs (while adhering all kinds of conditions). These graphs can than be validated further based on different validators. For example by looking at number of incoming edges, node rank like described before, or any other algorithm. The tool hence can be seen as a simple framework (with basic visualisation support) to validate the concepts & usefulness of graph stitching.

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