Add Scheduling Topology Hints

include/faabric/scheduler/Scheduler.h

@@ -102,7 +102,8 @@ class Scheduler
 
     faabric::util::SchedulingDecision callFunctions(
       std::shared_ptr<faabric::BatchExecuteRequest> req,
-      bool forceLocal = false);
+      faabric::util::SchedulingTopologyHint =
+        faabric::util::SchedulingTopologyHint::NORMAL);
 
     faabric::util::SchedulingDecision callFunctions(
       std::shared_ptr<faabric::BatchExecuteRequest> req,
@@ -233,7 +234,7 @@ class Scheduler
 
     faabric::util::SchedulingDecision makeSchedulingDecision(
       std::shared_ptr<faabric::BatchExecuteRequest> req,
-      bool forceLocal);
+      faabric::util::SchedulingTopologyHint topologyHint);
 
     faabric::util::SchedulingDecision doCallFunctions(
       std::shared_ptr<faabric::BatchExecuteRequest> req,

include/faabric/util/scheduling.h

@@ -41,4 +41,17 @@ class SchedulingDecision
                     int32_t appIdx,
                     int32_t groupIdx);
 };
+
+// Scheduling topology hints help the scheduler decide which host to assign new
+// requests in a batch.
+//  - NORMAL: bin-packs requests to slots in hosts starting from the master
+//            host, and overloadds the master if it runs out of resources.
+//  - NEVER_ALONE: never allocates a single (non-master) request to a host
+//                 without other requests of the batch.
+enum SchedulingTopologyHint
+{
+    NORMAL,
+    FORCE_LOCAL,
+    NEVER_ALONE
+};
 }

src/scheduler/FunctionCallServer.cpp

@@ -77,7 +77,7 @@ void FunctionCallServer::recvExecuteFunctions(const uint8_t* buffer,
     // This host has now been told to execute these functions no matter what
     // TODO - avoid this copy
     scheduler.callFunctions(std::make_shared<faabric::BatchExecuteRequest>(msg),
-                            true);
+                            faabric::util::SchedulingTopologyHint::FORCE_LOCAL);
 }
 
 void FunctionCallServer::recvUnregister(const uint8_t* buffer,

src/scheduler/Scheduler.cpp

@@ -210,7 +210,7 @@ void Scheduler::notifyExecutorShutdown(Executor* exec,
 
 faabric::util::SchedulingDecision Scheduler::callFunctions(
   std::shared_ptr<faabric::BatchExecuteRequest> req,
-  bool forceLocal)
+  faabric::util::SchedulingTopologyHint topologyHint)
 {
     // Note, we assume all the messages are for the same function and have the
     // same master host
@@ -224,7 +224,8 @@ faabric::util::SchedulingDecision Scheduler::callFunctions(
 
     // If we're not the master host, we need to forward the request back to the
     // master host. This will only happen if a nested batch execution happens.
-    if (!forceLocal && masterHost != thisHost) {
+    if (topologyHint != faabric::util::SchedulingTopologyHint::FORCE_LOCAL &&
+        masterHost != thisHost) {
         std::string funcStr = faabric::util::funcToString(firstMsg, false);
         SPDLOG_DEBUG("Forwarding {} back to master {}", funcStr, masterHost);
 
@@ -236,12 +237,13 @@ faabric::util::SchedulingDecision Scheduler::callFunctions(
 
     faabric::util::FullLock lock(mx);
 
-    SchedulingDecision decision = makeSchedulingDecision(req, forceLocal);
+    SchedulingDecision decision = makeSchedulingDecision(req, topologyHint);
 
     // Send out point-to-point mappings if necessary (unless being forced to
     // execute locally, in which case they will be transmitted from the
     // master)
-    if (!forceLocal && (firstMsg.groupid() > 0)) {
+    if (topologyHint != faabric::util::SchedulingTopologyHint::FORCE_LOCAL &&
+        (firstMsg.groupid() > 0)) {
         broker.setAndSendMappingsFromSchedulingDecision(decision);
     }
 
@@ -251,14 +253,14 @@ faabric::util::SchedulingDecision Scheduler::callFunctions(
 
 faabric::util::SchedulingDecision Scheduler::makeSchedulingDecision(
   std::shared_ptr<faabric::BatchExecuteRequest> req,
-  bool forceLocal)
+  faabric::util::SchedulingTopologyHint topologyHint)
 {
     int nMessages = req->messages_size();
     faabric::Message& firstMsg = req->mutable_messages()->at(0);
     std::string funcStr = faabric::util::funcToString(firstMsg, false);
 
     std::vector<std::string> hosts;
-    if (forceLocal) {
+    if (topologyHint == faabric::util::SchedulingTopologyHint::FORCE_LOCAL) {
         // We're forced to execute locally here so we do all the messages
         for (int i = 0; i < nMessages; i++) {
             hosts.push_back(thisHost);
@@ -296,6 +298,14 @@ faabric::util::SchedulingDecision Scheduler::makeSchedulingDecision(
                 int available = r.slots() - r.usedslots();
                 int nOnThisHost = std::min(available, remainder);
 
+                // Under the NEVER_ALONE topology hint, we never choose a host
+                // unless we can schedule at least two requests in it.
+                if (topologyHint ==
+                      faabric::util::SchedulingTopologyHint::NEVER_ALONE &&
+                    nOnThisHost < 2) {
+                    continue;
+                }
+
                 for (int i = 0; i < nOnThisHost; i++) {
                     hosts.push_back(h);
                 }
@@ -323,6 +333,12 @@ faabric::util::SchedulingDecision Scheduler::makeSchedulingDecision(
                 int available = r.slots() - r.usedslots();
                 int nOnThisHost = std::min(available, remainder);
 
+                if (topologyHint ==
+                      faabric::util::SchedulingTopologyHint::NEVER_ALONE &&
+                    nOnThisHost < 2) {
+                    continue;
+                }
+
                 // Register the host if it's exected a function
                 if (nOnThisHost > 0) {
                     registeredHosts[funcStr].insert(h);
@@ -342,11 +358,26 @@ faabric::util::SchedulingDecision Scheduler::makeSchedulingDecision(
         // At this point there's no more capacity in the system, so we
         // just need to overload locally
         if (remainder > 0) {
-            SPDLOG_DEBUG(
-              "Overloading {}/{} {} locally", remainder, nMessages, funcStr);
+            std::string overloadedHost = thisHost;
+
+            // Under the NEVER_ALONE scheduling topology hint we want to
+            // overload the last host we assigned requests to.
+            if (topologyHint ==
+                  faabric::util::SchedulingTopologyHint::NEVER_ALONE &&
+                !hosts.empty()) {
+                overloadedHost = hosts.back();
+            }
+
+            SPDLOG_DEBUG("Overloading {}/{} {} {}",
+                         remainder,
+                         nMessages,
+                         funcStr,
+                         overloadedHost == thisHost
+                           ? "locally"
+                           : "to host " + overloadedHost);
 
             for (int i = 0; i < remainder; i++) {
-                hosts.push_back(thisHost);
+                hosts.push_back(overloadedHost);
             }
         }
     }
@@ -636,7 +667,11 @@ void Scheduler::callFunction(faabric::Message& msg, bool forceLocal)
     req->set_type(req->FUNCTIONS);
 
     // Make the call
-    callFunctions(req, forceLocal);
+    if (forceLocal) {
+        callFunctions(req, faabric::util::SchedulingTopologyHint::FORCE_LOCAL);
+    } else {
+        callFunctions(req);
+    }
 }
 
 void Scheduler::clearRecordedMessages()