GC: ensure no leakage of evaluations for batch jobs. (#15097)

Prior to 2409f72 the code compared the modification index of a job to itself. Afterwards, the code compared the creation index of the job to itself. In either case there should never be a case of re-parenting of allocs causing the evaluation to trivially always result in false, which leads to unreclaimable memory.

Prior to this change allocations and evaluations for batch jobs were never garbage collected until the batch job was explicitly stopped. The new `batch_eval_gc_threshold` server configuration controls how often they are collected. The default threshold is `24h`.

.changelog/15097.txt

@@ -0,0 +1,3 @@
+```release-note:breaking-change
+core: Ensure no leakage of evaluations for batch jobs. Prior to this change allocations and evaluations for batch jobs were never garbage collected until the batch job was explicitly stopped. The new `batch_eval_gc_threshold` server configuration controls how often they are collected. The default threshold is `24h`.
+```

command/agent/agent.go

@@ -390,6 +390,13 @@ func convertServerConfig(agentConfig *Config) (*nomad.Config, error) {
 		}
 		conf.EvalGCThreshold = dur
 	}
+	if gcThreshold := agentConfig.Server.BatchEvalGCThreshold; gcThreshold != "" {
+		dur, err := time.ParseDuration(gcThreshold)
+		if err != nil {
+			return nil, err
+		}
+		conf.BatchEvalGCThreshold = dur
+	}
 	if gcThreshold := agentConfig.Server.DeploymentGCThreshold; gcThreshold != "" {
 		dur, err := time.ParseDuration(gcThreshold)
 		if err != nil {

command/agent/config.go

@@ -473,9 +473,14 @@ type ServerConfig struct {
 
 	// EvalGCThreshold controls how "old" an eval must be to be collected by GC.
 	// Age is not the only requirement for a eval to be GCed but the threshold
-	// can be used to filter by age.
+	// can be used to filter by age. Please note that batch job evaluations are
+	// controlled by 'BatchEvalGCThreshold' instead.
 	EvalGCThreshold string `hcl:"eval_gc_threshold"`
 
+	// BatchEvalGCThreshold controls how "old" an evaluation must be to be eligible
+	// for GC if the eval belongs to a batch job.
+	BatchEvalGCThreshold string `hcl:"batch_eval_gc_threshold"`
+
 	// DeploymentGCThreshold controls how "old" a deployment must be to be
 	// collected by GC. Age is not the only requirement for a deployment to be
 	// GCed but the threshold can be used to filter by age.
@@ -1861,6 +1866,9 @@ func (s *ServerConfig) Merge(b *ServerConfig) *ServerConfig {
 	if b.EvalGCThreshold != "" {
 		result.EvalGCThreshold = b.EvalGCThreshold
 	}
+	if b.BatchEvalGCThreshold != "" {
+		result.BatchEvalGCThreshold = b.BatchEvalGCThreshold
+	}
 	if b.DeploymentGCThreshold != "" {
 		result.DeploymentGCThreshold = b.DeploymentGCThreshold
 	}

command/agent/config_test.go

@@ -140,6 +140,7 @@ func TestConfig_Merge(t *testing.T) {
 			RaftMultiplier:         pointer.Of(5),
 			NumSchedulers:          pointer.Of(1),
 			NodeGCThreshold:        "1h",
+			BatchEvalGCThreshold:   "4h",
 			HeartbeatGrace:         30 * time.Second,
 			MinHeartbeatTTL:        30 * time.Second,
 			MaxHeartbeatsPerSecond: 30.0,
@@ -339,6 +340,7 @@ func TestConfig_Merge(t *testing.T) {
 			NumSchedulers:          pointer.Of(2),
 			EnabledSchedulers:      []string{structs.JobTypeBatch},
 			NodeGCThreshold:        "12h",
+			BatchEvalGCThreshold:   "4h",
 			HeartbeatGrace:         2 * time.Minute,
 			MinHeartbeatTTL:        2 * time.Minute,
 			MaxHeartbeatsPerSecond: 200.0,

nomad/config.go

@@ -153,8 +153,15 @@ type Config struct {
 
 	// EvalGCThreshold is how "old" an evaluation must be to be eligible
 	// for GC. This gives users some time to debug a failed evaluation.
+	//
+	// Please note that the rules for GC of evaluations which belong to a batch
+	// job are separate and controlled by `BatchEvalGCThreshold`
 	EvalGCThreshold time.Duration
 
+	// BatchEvalGCThreshold is how "old" an evaluation must be to be eligible
+	// for GC if the eval belongs to a batch job.
+	BatchEvalGCThreshold time.Duration
+
 	// JobGCInterval is how often we dispatch a job to GC jobs that are
 	// available for garbage collection.
 	JobGCInterval time.Duration
@@ -460,6 +467,7 @@ func DefaultConfig() *Config {
 		ReconcileInterval:                60 * time.Second,
 		EvalGCInterval:                   5 * time.Minute,
 		EvalGCThreshold:                  1 * time.Hour,
+		BatchEvalGCThreshold:             24 * time.Hour,
 		JobGCInterval:                    5 * time.Minute,
 		JobGCThreshold:                   4 * time.Hour,
 		NodeGCInterval:                   5 * time.Minute,

nomad/core_sched.go

@@ -240,15 +240,20 @@ func (c *CoreScheduler) evalGC(eval *structs.Evaluation) error {
 
 	oldThreshold := c.getThreshold(eval, "eval",
 		"eval_gc_threshold", c.srv.config.EvalGCThreshold)
+	batchOldThreshold := c.getThreshold(eval, "eval",
+		"batch_eval_gc_threshold", c.srv.config.BatchEvalGCThreshold)
 
 	// Collect the allocations and evaluations to GC
 	var gcAlloc, gcEval []string
 	for raw := iter.Next(); raw != nil; raw = iter.Next() {
 		eval := raw.(*structs.Evaluation)
 
-		// The Evaluation GC should not handle batch jobs since those need to be
-		// garbage collected in one shot
-		gc, allocs, err := c.gcEval(eval, oldThreshold, false)
+		gcThreshold := oldThreshold
+		if eval.Type == structs.JobTypeBatch {
+			gcThreshold = batchOldThreshold
+		}
+
+		gc, allocs, err := c.gcEval(eval, gcThreshold, false)
 		if err != nil {
 			return err
 		}
@@ -299,33 +304,26 @@ func (c *CoreScheduler) gcEval(eval *structs.Evaluation, thresholdIndex uint64,
 	}
 
 	// If the eval is from a running "batch" job we don't want to garbage
-	// collect its allocations. If there is a long running batch job and its
-	// terminal allocations get GC'd the scheduler would re-run the
-	// allocations.
+	// collect its most current allocations. If there is a long running batch job and its
+	// terminal allocations get GC'd the scheduler would re-run the allocations. However,
+	// we do want to GC old Evals and Allocs if there are newer ones due to update.
+	//
+	// The age of the evaluation must also reach the threshold configured to be GCed so that
+	// one may debug old evaluations and referenced allocations.
 	if eval.Type == structs.JobTypeBatch {
 		// Check if the job is running
 
-		// Can collect if:
-		// Job doesn't exist
-		// Job is Stopped and dead
-		// allowBatch and the job is dead
-		collect := false
-		if job == nil {
-			collect = true
-		} else if job.Status != structs.JobStatusDead {
-			collect = false
-		} else if job.Stop {
-			collect = true
-		} else if allowBatch {
-			collect = true
-		}
-
-		// We don't want to gc anything related to a job which is not dead
-		// If the batch job doesn't exist we can GC it regardless of allowBatch
+		// Can collect if either holds:
+		//   - Job doesn't exist
+		//   - Job is Stopped and dead
+		//   - allowBatch and the job is dead
+		//
+		// If we cannot collect outright, check if a partial GC may occur
+		collect := job == nil || job.Status == structs.JobStatusDead && (job.Stop || allowBatch)
 		if !collect {
-			// Find allocs associated with older (based on createindex) and GC them if terminal
-			oldAllocs := olderVersionTerminalAllocs(allocs, job)
-			return false, oldAllocs, nil
+			oldAllocs := olderVersionTerminalAllocs(allocs, job, thresholdIndex)
+			gcEval := (len(oldAllocs) == len(allocs))
+			return gcEval, oldAllocs, nil
 		}
 	}
 
@@ -346,12 +344,12 @@ func (c *CoreScheduler) gcEval(eval *structs.Evaluation, thresholdIndex uint64,
 	return gcEval, gcAllocIDs, nil
 }
 
-// olderVersionTerminalAllocs returns terminal allocations whose job create index
-// is older than the job's create index
-func olderVersionTerminalAllocs(allocs []*structs.Allocation, job *structs.Job) []string {
+// olderVersionTerminalAllocs returns a list of terminal allocations that belong to the evaluation and may be
+// GCed.
+func olderVersionTerminalAllocs(allocs []*structs.Allocation, job *structs.Job, thresholdIndex uint64) []string {
 	var ret []string
 	for _, alloc := range allocs {
-		if alloc.Job != nil && alloc.Job.CreateIndex < job.CreateIndex && alloc.TerminalStatus() {
+		if alloc.CreateIndex < job.JobModifyIndex && alloc.ModifyIndex < thresholdIndex && alloc.TerminalStatus() {
 			ret = append(ret, alloc.ID)
 		}
 	}