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Large-scale cluster management at Google with Borg

Google built a cluster management system called Borg that admits, schedules, starts, restarts, and monitors the full range of applications that Google runs. The main benefits that Borg provides are: hiding the details of resource management and failure handling so its users can focus on application development; operates with very high reliability and availability, supporting applications that do the same; lets its users to run workloads across tens of thousands of machines effectively.

The user perspective

Users submit work to Borg in the form of jobs, each of which consists of one or more tasks that all run the same program. Each job runs in one Borg cell, a set of machines that are managed as a unit.

The workload

The workload can be heterogeneous and it has two types: the first being long-running services that should “never” go down, and handle short-lived latency-sensitive requests. The second type is batch jobs that take from a few seconds to a few days to complete, and are much less sensitive to short-term performance variation.

Clusters and cells

The machines in a cell belong to a single cluster. A cluster lives inside a single datacenter building, and a collection of buildings makes up a site. A cluster usually hosts one large cell and may have a few smaller-scale test or special-purpose cells.

The machines in a cell are heterogeneous in several aspects, including: sizes (CPU, RAM, disk, network), processor type, performance, and capabilities like external IP address or flash storage. Most of these differences are hidden from the user by Borg.

Jobs and tasks

Each job has properties such as: its name, owner, and number of tasks it has. They can have constraints1 to force its tasks to run on machines with specific attributes like processor architecture, OS version, external IP address, etc.

Each task maps to a set of Linux processes running in a container on a machine and the vast majority does not run inside virtual machines.

The tasks also have properties, such as resource requirements and the task index within the job. These properties can be changed in a running job by pushing a new job configuration, and then instructing Borg to update the tasks. To reduce dependencies on the runtime environment, Borg programs are statically linked, and structured as packages, whose installation is orchestrated by Borg.

Other Borg jobs, monitoring systems, or a command-line tool can be used to operate on jobs via RPCs to Borg.

The following image illustrates the states that jobs and tasks go through their lifetime:

Allocs

A Borg alloc (allocation) is a reserved set of resources on a machine in which one or more tasks can be run; the resources remain assigned whether or not they are used.

Priority, quota, and admission control

Each job has a priority, a small positive integer. Resources for a high-priority task can be obtained at the expense of lower-priority task, even if that means preempting the lower-priority task. A preempted task is rescheduled somewhere else in the cell.

Quota is expressed as a vector of resources quantities (CPU, RAM, disk, etc) at a given priority, for a period of time which is mostly given in months. They are used to decide which jobs to admit for scheduling. Higher-priority quota costs more than lower-priority and quota allocation is handled outside Borg.

Naming and monitoring

Each task has a name that is created using the “Borg name service” (BNS) and this name includes the cell name, job name, and task number. The 50th task in a job jfoo owned by user ubar in cell cc would be reachable via the following address: 50.jfoo.ubar.cc.borg.google.com.

Borg Architecture

A Borg cell consists of a set of machines, a logically centralized controller called the Borgmaster, and an agent process called the Borglet that runs on each machine in a cell.

Borgmaster

The Borgmaster consists of two processes: the main Borgmaster process and a separate scheduler. The main process handles client RPCs that either mutate state (e.g., create job) or provide read-only access to data (e.g., lookup job). It also manages state machines for all of the objects in the system (machines, tasks, allocs, etc.), communicates with Borglets, and offers a web UI as a backup to Sigma2.

The Borgmaster is replicated five times, and each replica maintains an in-memory copy of the state of the cell, along with a copy in a highly-available, distributed, Paxos-based store on the replica’s local disks.

Scheduling

When a job is submitted, the Borgmaster records it in the Paxos store and adds the job’s task to the pending queue. The scheduler scans this queue asynchronously and assigns tasks to machines if there are enough available resources meeting the job’s constraints. The scan is done from high to low, priority, modulated by a round-robin scheme within a priority to ensure fairness.

Borglet

The local Borg agent present on every machine in a cell is called Borglet. Its responsibilities include: starting and stopping tasks; restarting them if they fail; managing local resources by manipulating OS kernel settings; rolling over debug logs; and reporting the state of the machine to the Borgmaster and other monitoring systems.

To avoid the herding effect in case of failure, the Borgmaster controls the rate of communication with the Borglets. It polls the Borglets every few seconds to retrieve the machine’s current state and send any outstanding requests.

A Borglet is considered as down if it does not respond to several poll messages. In this case, its tasks will be rescheduled on other machines and once communication is restored, the Borgmaster will tell the Borglet to kill the tasks that have been rescheduled, avoiding duplicates. If contact with a Borgmaster is lost, the Borglet continues normal operation, which means the currently-running tasks and services will remain up even if all Borgmaster replicas fail.

Availability

To provide availability Borg employs several steps, including:

Borgmaster uses a combination of techniques that enable it to achieve 99.99% availability in practice: replication for machine failures; admission control to avoid overload; and deploying instances using simple, low-level tools to minimize external dependencies. Each cell is independent of the others to minimize the chance of correlated operator errors and failure propagation. These goals, not scalability limitations, are the primary argument against larger cells.

References

Notes


  1. These constraints can be hard or soft; soft constraints act more like preferences rather than requirements.

  2. Sigma is a service that provides a web-based user interface (UI) where users can examine the state of all their jobs, a particular cell, look up details of individual jobs and tasks, etc.

Carlos Galdino
@carlosgaldino
github.com/carlosgaldino
blog.carlosgaldino.com