[rfc] Fair Share Replication Scheduler (#617)

Discussed on the [ML](https://lists.apache.org/thread.html/rebba9a43bfdf9696f2ce974b0fc7550a631c7b835e4c14e51cd27a87%40%3Cdev.couchdb.apache.org%3E) Based on the Fair Share Scheduler paper by [Judy Kay and Piers Lauder](https://proteusmaster.urcf.drexel.edu/urcfwiki/images/KayLauderFairShare.pdf)

Author: nickva

rfcs/017-fair-share-scheduling.md

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+---
+name: Formal RFC
+about: Submit a formal Request For Comments for consideration by the team.
+title: 'Fair Share Job Scheduling for CouchDB 3.x Replicator'
+labels: rfc, discussion
+assignees: 'vatamane@apache.org'
+
+---
+
+# Introduction
+
+This document describes an improvement to the CouchDB 3.x replicator to
+introduce fair resource sharing between replication jobs in different
+_replicator databases.
+
+## Abstract
+
+Currently CouchDB replicator 3.x schedules jobs without any regard to what
+database they originated from. If there are multiple `_replicator` dbs then
+replication jobs from dbs with most jobs will consume most of the scheduler's
+resources. The proposal is to implement a fair sharing scheme as described in
+[A Fair Share Scheduler][2] paper by Judy Kay and Piers Lauder. It would allow
+sharing replication scheduler resources fairly amongst `_replicator` dbs.
+
+The idea was originally discussed on the [couchdb-dev][1] mailing list and the
+use of the Fair Share algorithm suggested by Joan Touzet.
+
+## Requirements Language
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
+"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
+interpreted as described in [RFC
+2119](https://www.rfc-editor.org/rfc/rfc2119.txt).
+
+## Terminology
+
+`_replicator` databases : A database that is either named `_replicator` or ends
+with the `/_replicator` suffix.
+
+`shares` : An abstract representation of entitlement to run on the replication
+scheduler.
+
+`usage` : A measure of resource usage by jobs from a particular `_replicator`
+db. For the scheduling replicator this will be the total time spent running.
+
+`continuous` replications : Replication jobs created with the `"continuous":
+true` parameter. These jobs will try to run continuously until the user removes
+them. They may be temporarily paused to allow other jobs to make progress.
+
+`one-shot` replications : Replication jobs which are not `continuous`. If the
+`"continuous":true` parameter is not specified, by default, replication jobs
+will be `one-shot`. These jobs will try to run until they reach the end of the
+changes feed, then stop.
+
+`job priority` : A job attribute which indicates the likelihood of the job
+being executed before other jobs. Following the convention in the "Fair Share"
+paper, jobs with a lower priority value are at the front of the pending queue,
+and get executed first.
+
+`max_jobs` : Configuration parameter which specifies up to how many replication
+jobs to run on each `replication` node.
+
+`max_churn` : Configuration parameter which specifies a limit of how many new
+jobs to spawn during each rescheduling interval.
+
+---
+
+# Detailed Description
+
+The general idea behind the algorithm is to continuously monitor
+per-`_replicator` jobs statistics and update each job's priorities in
+proportion to the usage from all the jobs in the same `_replicator` db. To make
+sure all jobs eventually get a chance to run and do not starve, all the
+priorities are continuously boosted, such that jobs which haven't run for a
+while, and maybe be starved, will eventually get a chance to run.
+
+The algorithm has 3 basic components that can run mostly independently from
+each other:
+
+1) Keep track of `usage` for each `_replicator` db . In the paper this part is
+called "user-level scheduling". As jobs run, they send reports to this
+component. Those reports are accumulated for one period, then rolled up when
+the period ends. There is also a decay coefficient applied to account for
+recent historical usage (this is called `K1` in the paper). This ensures in
+absence of jobs running from a particular `_replicator` db, the usage would
+drops to 0 and the whole entry is removed from the table table altogether.
+
+ Every `UsageUpdateInterval` seconds (called `t1` in the paper):
+ For each `Db`:
+ ```
+ DecayCoeff = get_usage_decay_coefficient(0.5)
+ AccumulatedUsage = get_accumulated_usage(Db),
+ update_usage(Db, usage(Db) * DecayCoeff + AccumulatedUsage)
+ reset_accumulated_usage(Db)
+ ```
+
+2) Uniformly decay all process priorities. Periodically lower the priority
+values, and thus boost the priority, of all the pending and running jobs in the
+system. The paper in this step applies a per-process "nice" value, which is
+skipped in the initial proposal. It could be added later if needed.
+
+ Every `UniformPriorityBoostInterval` seconds (called `t2` in the paper):
+ For each `Job`:
+ ```
+ DecayCoeff = get_uniform_decay_coefficient(0.75),
+ Job#job.priority = Job#job.priority * DecayCoeff
+ ```
+
+[note]: If jobs were scheduled to run at an absolute future time (a deadline) this step could be avoided. Then, the effect of all the jobs needing to periodically move to the front of the queue would be accomplished instead by the current time (i.e. `now()`) moving head along the time-line.
+
+3) Adjust running process priority in proportion to the shares used by all the
+jobs in the same db:
+
+ Every `RunningPriorityReduceInterval` seconds (called `t3` in the paper):
+ For each `Job`:
+ ```
+ Db = Job#job.db,
+ SharesSq = shares(Db) * shares(Db),
+ Job#job.priority = Job#job.priority + (usage(Db) * pending(Db)) / SharesSq
+ ```
+
+### How Jobs Start and Stop
+
+During each rescheduling cycle, `max_churn` running jobs from the back of the
+queue are stopped and `max_churn` jobs from the front of the pending queue are
+started. This part is not modified from the existing scheduling algorithm,
+except now, the jobs would be ordered by their `priority` value before being
+ordered by their last start time.
+
+In addition, `one-shot` replication jobs would still be skipped when stopping
+and we'd let them run in order to maintain traditional replication semantics
+just like before.
+
+When picking the jobs to run exclude jobs which have been exponentially backed
+off due to repeated errors. This part is unmodified and from the original
+scheduler.
+
+### Configuration
+
+The decay coefficients and interval times for each of the 3 parts of the algorithm would be configurable in the `[replicator]` config section.
+
+Per-`_replicator` db shares would be configurable in the `[replicator.shares]` section as:
+
+```
+[replicator.shares]
+$prefix/_replicator = $numshares
+```
+
+By default each db is assigned 100 shares. Then higher number of shares should
+then indicated a larger proportion of scheduler resources allocated to that db.
+A lower number would get proportionally less shares.
+
+For example:
+
+```
+[replicator.shares]
+
+; This is the default
+; _replicator = 100
+
+high/_replicator = 200
+low/_replicator = 50
+```
+
+# Advantages and Disadvantages
+
+Advantages:
+
+ * Allow a fair share of resources between multiple `_replicator` db instances
+
+ * Can boost or lower the priority of some replication jobs by adjusting the
+ shares assigned to that database instance.
+
+Disadvantages:
+
+ * Adds more complexity to the scheduler
+
+# Key Changes
+
+ * Modifies replication scheduler
+
+## Applications and Modules affected
+
+ * `couch_replicator` application
+
+## HTTP API additions
+
+N/A
+
+## HTTP API deprecations
+
+N/A
+
+# Security Considerations
+
+None
+
+# References
+
+* [1]: https://lists.apache.org/thread.html/rebba9a43bfdf9696f2ce974b0fc7550a631c7b835e4c14e51cd27a87%40%3Cdev.couchdb.apache.org%3E "couchdb-dev"
+
+* [2]: https://proteusmaster.urcf.drexel.edu/urcfwiki/images/KayLauderFairShare.pdf "Fair Share Scheduler"
+
+# Co-authors
+
+ * Joan Touzet (@wohali)
+
+# Acknowledgments
+
+ * Joan Touzet (@wohali)