Fair usage of Shared Runners
## What does this MR do?
Introduces a fair usage scheduler for shared runners.
It tries to assign builds to shared runner from projects that have the lowest number of builds currently running on shared runners.
**Example 1**:
```
We have following builds in queue:
build 1 for project 1
build 2 for project 1
build 3 for project 1
build 4 for project 2
build 5 for project 2
build 6 for project 3
With the new algorithm we will assign builds in following order:
- We choose build 1, because project 1 doesn't run currently any builds and has the lowest build number from projects that doesn't run builds,
- We choose build 4, because project 2 doesn't run currently any builds and has the lowest build number from projects that doesn't run builds,
- We choose build 6, because project 3 doesn't run currently any builds and has the lowest build number from projects that doesn't run builds,
- We choose build 2, because project 1 as other it runs 1 build,
- We choose build 5, because project 2 runs 1 build, where project 1 runs 2 builds now,
- We choose build 3, because project 1 and runs 2 builds.
```
**Example 2**:
```
We have following builds in queue:
build 1 for project 1
build 2 for project 1
build 3 for project 1
build 4 for project 2
build 5 for project 2
build 6 for project 3
With the new algorithm we will assign builds in following order:
- We choose build 1, because project 1 doesn't run currently any builds and has the lowest build number from projects that doesn't run builds,
- We finish build 1,
- We choose build 2, because project 1 doesn't run currently any builds and has the lowest build number from projects that doesn't run builds,
- We choose build 4, because project 2 doesn't run currently any builds and has the lowest build number from projects that doesn't run builds,
- We finish build 4,
- We choose build 5, because project 2 doesn't run currently any builds and has the lowest build number from projects that doesn't run builds,
- We choose build 6, because project 3 doesn't run currently any builds,
- We choose build 3, because project 1, 2 and 3 runs exactly one build now,
```
## Why was this MR needed?
Currently, we are scheduling builds using FIFO. This is catastrophic if there are projects that create a 100-300 jobs, this basically eats most of available shared runners.
## Performance
All this logic is implemented with the help of SQL queries, because this is the fastest way to process 1k-2k pending builds in queue.
It's not the fastest SQL query, because it sorts based on number of running_builds, and this forces to calculate a number of running builds for all dependent projects. However, since we have one/two shared runners that asks every few seconds for builds this should have minimal impact on DB performance.
```
explain analyze SELECT "ci_builds".* FROM "ci_builds" JOIN (SELECT "ci_builds"."gl_project_id", count(case when status = 'running' AND runner_id = (SELECT "ci_runners"."id" FROM "ci_runners" WHERE "ci_runners"."is_shared" = 't') then 1 end) as running_builds FROM "ci_builds" INNER JOIN "projects" ON "projects"."id" = "ci_builds"."gl_project_id" AND "projects"."pending_delete" = 'f' WHERE "ci_builds"."type" IN ('Ci::Build') AND "ci_builds"."status" IN ('running', 'pending') AND "projects"."builds_enabled" = 't' AND "projects"."shared_runners_enabled" = 't' GROUP BY "ci_builds"."gl_project_id") AS projects ON ci_builds.gl_project_id=projects.gl_project_id WHERE "ci_builds"."type" IN ('Ci::Build') AND "ci_builds"."status" = 'pending' AND "ci_builds"."runner_id" IS NULL ORDER BY projects.running_builds ASC, ci_builds.id ASC;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------
Sort (cost=64777.28..64777.29 rows=1 width=1010) (actual time=301.794..302.535 rows=1537 loops=1)
Sort Key: (count(CASE WHEN (((public.ci_builds.status)::text = 'running'::text) AND (public.ci_builds.runner_id = $0)) THEN 1 ELSE NULL::integer END)), public.ci
_builds.id
Sort Method: quicksort Memory: 1423kB
-> Nested Loop (cost=63279.78..64777.27 rows=1 width=1010) (actual time=66.384..298.724 rows=1537 loops=1)
-> HashAggregate (cost=63177.15..63177.30 rows=15 width=15) (actual time=65.641..65.851 rows=187 loops=1)
InitPlan 1 (returns $0)
-> Seq Scan on ci_runners (cost=0.00..26963.66 rows=1 width=4) (actual time=1.145..34.381 rows=1 loops=1)
Filter: is_shared
Rows Removed by Filter: 6965
-> Nested Loop (cost=0.00..36186.34 rows=2715 width=15) (actual time=0.065..29.717 rows=1710 loops=1)
-> Index Scan using index_ci_builds_on_status on ci_builds (cost=0.00..8913.95 rows=3577 width=15) (actual time=0.051..12.012 rows=2583 loops
=1)
Index Cond: ((status)::text = ANY ('{running,pending}'::text[]))
Filter: ((type)::text = 'Ci::Build'::text)
Rows Removed by Filter: 1219
-> Index Scan using projects_pkey on projects (cost=0.00..7.61 rows=1 width=4) (actual time=0.003..0.004 rows=1 loops=2583)
Index Cond: (id = public.ci_builds.gl_project_id)
Filter: ((NOT pending_delete) AND builds_enabled AND shared_runners_enabled)
Rows Removed by Filter: 0
-> Bitmap Heap Scan on ci_builds (cost=102.63..106.64 rows=1 width=1002) (actual time=1.216..1.231 rows=8 loops=187)
Recheck Cond: ((gl_project_id = public.ci_builds.gl_project_id) AND ((status)::text = 'pending'::text))
Filter: ((runner_id IS NULL) AND ((type)::text = 'Ci::Build'::text))
-> BitmapAnd (cost=102.63..102.63 rows=1 width=0) (actual time=1.201..1.201 rows=0 loops=187)
-> Bitmap Index Scan on index_ci_builds_on_gl_project_id (cost=0.00..10.52 rows=241 width=0) (actual time=0.406..0.406 rows=1944 loops=187)
Index Cond: (gl_project_id = public.ci_builds.gl_project_id)
-> Bitmap Index Scan on index_ci_builds_on_status (cost=0.00..91.78 rows=3089 width=0) (actual time=0.652..0.652 rows=3362 loops=187)
Index Cond: ((status)::text = 'pending'::text)
Total runtime: 303.832 ms
```
## Specific runners
It doesn't affect the specific runners which still serve builds FIFO.
@stanhu @markpundsack @yorickpeterse What do you think?
See merge request !4634