Refine spot instance startup

src/main/java/com/conveyal/analysis/components/broker/Broker.java

@@ -108,7 +108,8 @@ public interface Config {
      * Used when auto-starting spot instances. Set to a smaller value to increase the number of
      * workers requested automatically
      */
-    public final int TARGET_TASKS_PER_WORKER = 800;
+    public final int TARGET_TASKS_PER_WORKER_TRANSIT = 800;
+    public final int TARGET_TASKS_PER_WORKER_NONTRANSIT = 4_000;
 
     /**
      * We want to request spot instances to "boost" regional analyses after a few regional task
@@ -243,7 +244,8 @@ public void createOnDemandWorkerInCategory(WorkerCategory category, WorkerTags w
     /**
      * Create on-demand/spot workers for a given job, after certain checks
      * @param nOnDemand EC2 on-demand instances to request
-     * @param nSpot EC2 spot instances to request
+     * @param nSpot Target number of EC2 spot instances to request. The actual number requested may be lower if the
+     *              total number of workers running is approaching the maximum specified in the Broker config.
      */
     public void createWorkersInCategory (WorkerCategory category, WorkerTags workerTags, int nOnDemand, int nSpot) {
 
@@ -257,6 +259,13 @@ public void createWorkersInCategory (WorkerCategory category, WorkerTags workerT
             return;
         }
 
+        // Backoff: reduce the nSpot requested when the number of already running workers starts approaching the
+        // configured maximum
+        if (workerCatalog.totalWorkerCount() * 2 > config.maxWorkers()) {
+            nSpot = Math.min(nSpot, (config.maxWorkers() - workerCatalog.totalWorkerCount()) / 2);
+            LOG.info("Worker pool over half of maximum size. Number of new spot instances set to {}", nSpot);
+        }
+
         if (workerCatalog.totalWorkerCount() + nOnDemand + nSpot >= config.maxWorkers()) {
             String message = String.format(
                     "Maximum of %d workers already started, not starting more;" +
@@ -483,9 +492,27 @@ private void requestExtraWorkersIfAppropriate(Job job) {
         WorkerCategory workerCategory = job.workerCategory;
         int categoryWorkersAlreadyRunning = workerCatalog.countWorkersInCategory(workerCategory);
         if (categoryWorkersAlreadyRunning < MAX_WORKERS_PER_CATEGORY) {
-            // Start a number of workers that scales with the number of total tasks, up to a fixed number.
-            // TODO more refined determination of number of workers to start (e.g. using tasks per minute)
-            int targetWorkerTotal = Math.min(MAX_WORKERS_PER_CATEGORY, job.nTasksTotal / TARGET_TASKS_PER_WORKER);
+            // TODO more refined determination of number of workers to start (e.g. using observed tasks per minute
+            //  for recently completed tasks -- but what about when initial origins are in a desert/ocean?)
+            int targetWorkerTotal;
+            if (job.templateTask.hasTransit()) {
+                // Total computation for a task with transit depends on the number of stops and whether the
+                // network has frequency-based routes. The total computation for the job depends on these
+                // factors as well as the number of tasks (origins). Zoom levels add a complication: the number of
+                // origins becomes an even poorer proxy for the number of stops. We use a scale factor to compensate
+                // -- all else equal, high zoom levels imply fewer stops per origin (task) and a lower ideal target
+                // for number of workers. TODO reduce scale factor further when there are no frequency routes. But is
+                //  this worth adding a field to Job or RegionalTask?
+                float transitScaleFactor = (9f / job.templateTask.zoom);
+                targetWorkerTotal = (int) ((job.nTasksTotal / TARGET_TASKS_PER_WORKER_TRANSIT) * transitScaleFactor);
+            } else {
+                // Tasks without transit are simpler. They complete relatively quickly, and the total computation for
+                // the job increases roughly with linearly with the number of origins.
+                targetWorkerTotal = job.nTasksTotal / TARGET_TASKS_PER_WORKER_NONTRANSIT;
+            }
+
+            // Do not exceed the limit on workers per category TODO add similar limit per accessGroup or user
+            targetWorkerTotal = Math.min(targetWorkerTotal, MAX_WORKERS_PER_CATEGORY);
             // Guardrail until freeform pointsets are tested more thoroughly
             if (job.templateTask.originPointSet != null) targetWorkerTotal = Math.min(targetWorkerTotal, 5);
             int nSpot =  targetWorkerTotal - categoryWorkersAlreadyRunning;