Merge branch 'fair-usage-of-shared-runners' into 'master'

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

CHANGELOG

@@ -22,6 +22,7 @@ v 8.9.0 (unreleased)
   - Added descriptions to notification settings dropdown
   - Improve note validation to prevent errors when creating invalid note via API
   - Reduce number of fog gem dependencies
+  - Implement a fair usage of shared runners
   - Remove project notification settings associated with deleted projects
   - Fix 404 page when viewing TODOs that contain milestones or labels in different projects
   - Add a metric for the number of new Redis connections created by a transaction

app/services/ci/register_build_service.rb

@@ -7,15 +7,19 @@ module Ci
 
       builds =
         if current_runner.shared?
-          # don't run projects which have not enables shared runners
-          builds.joins(:project).where(projects: { builds_enabled: true, shared_runners_enabled: true })
+          builds.
+            # don't run projects which have not enabled shared runners
+            joins(:project).where(projects: { builds_enabled: true, shared_runners_enabled: true }).
+
+            # this returns builds that are ordered by number of running builds
+            # we prefer projects that don't use shared runners at all
+            joins("LEFT JOIN (#{running_builds_for_shared_runners.to_sql}) AS project_builds ON ci_builds.gl_project_id=project_builds.gl_project_id").
+            order('COALESCE(project_builds.running_builds, 0) ASC', 'ci_builds.id ASC')
         else
-          # do run projects which are only assigned to this runner
-          builds.where(project: current_runner.projects.where(builds_enabled: true))
+          # do run projects which are only assigned to this runner (FIFO)
+          builds.where(project: current_runner.projects.where(builds_enabled: true)).order('created_at ASC')
         end
 
-      builds = builds.order('created_at ASC')
-
       build = builds.find do |build|
         build.can_be_served?(current_runner)
       end
@@ -35,5 +39,12 @@ module Ci
     rescue StateMachines::InvalidTransition
       nil
     end
+
+    private
+
+    def running_builds_for_shared_runners
+      Ci::Build.running.where(runner: Ci::Runner.shared).
+        group(:gl_project_id).select(:gl_project_id, 'count(*) AS running_builds')
+    end
   end
 end

spec/services/ci/register_build_service_spec.rb

@@ -45,11 +45,73 @@ module Ci
         end
       end
 
+      context 'deleted projects' do
+        before do
+          project.update(pending_delete: true)
+        end
+
+        context 'for shared runners' do
+          before do
+            project.update(shared_runners_enabled: true)
+          end
+
+          it 'does not pick a build' do
+            expect(service.execute(shared_runner)).to be_nil
+          end
+        end
+
+        context 'for specific runner' do
+          it 'does not pick a build' do
+            expect(service.execute(specific_runner)).to be_nil
+          end
+        end
+      end
+
       context 'allow shared runners' do
         before do
           project.update(shared_runners_enabled: true)
         end
 
+        context 'for multiple builds' do
+          let!(:project2) { create :empty_project, shared_runners_enabled: true }
+          let!(:pipeline2) { create :ci_pipeline, project: project2 }
+          let!(:project3) { create :empty_project, shared_runners_enabled: true }
+          let!(:pipeline3) { create :ci_pipeline, project: project3 }
+          let!(:build1_project1) { pending_build }
+          let!(:build2_project1) { FactoryGirl.create :ci_build, pipeline: pipeline }
+          let!(:build3_project1) { FactoryGirl.create :ci_build, pipeline: pipeline }
+          let!(:build1_project2) { FactoryGirl.create :ci_build, pipeline: pipeline2 }
+          let!(:build2_project2) { FactoryGirl.create :ci_build, pipeline: pipeline2 }
+          let!(:build1_project3) { FactoryGirl.create :ci_build, pipeline: pipeline3 }
+
+          it 'prefers projects without builds first' do
+            # it gets for one build from each of the projects
+            expect(service.execute(shared_runner)).to eq(build1_project1)
+            expect(service.execute(shared_runner)).to eq(build1_project2)
+            expect(service.execute(shared_runner)).to eq(build1_project3)
+
+            # then it gets a second build from each of the projects
+            expect(service.execute(shared_runner)).to eq(build2_project1)
+            expect(service.execute(shared_runner)).to eq(build2_project2)
+
+            # in the end the third build
+            expect(service.execute(shared_runner)).to eq(build3_project1)
+          end
+
+          it 'equalises number of running builds' do
+            # after finishing the first build for project 1, get a second build from the same project
+            expect(service.execute(shared_runner)).to eq(build1_project1)
+            build1_project1.success
+            expect(service.execute(shared_runner)).to eq(build2_project1)
+
+            expect(service.execute(shared_runner)).to eq(build1_project2)
+            build1_project2.success
+            expect(service.execute(shared_runner)).to eq(build2_project2)
+            expect(service.execute(shared_runner)).to eq(build1_project3)
+            expect(service.execute(shared_runner)).to eq(build3_project1)
+          end
+        end
+
         context 'shared runner' do
           let(:build) { service.execute(shared_runner) }