AliceO2Group · ktf · Jun 28, 2026
@@ -54,7 +54,7 @@ struct DataProcessingHelpers {
   /// starts the EoS timers and returns the new TransitionHandlingState in case as new state is requested
   static TransitionHandlingState updateStateTransition(ServiceRegistryRef const& ref, ProcessingPolicies const& policies);
   /// Helper to route messages for forwarding
-  static std::vector<fair::mq::Parts> routeForwardedMessageSet(FairMQDeviceProxy& proxy, std::vector<std::vector<fair::mq::MessagePtr>>& currentSetOfInputs,
+  static std::vector<fair::mq::Parts> routeForwardedMessageSet(FairMQDeviceProxy& proxy, std::vector<std::span<fair::mq::MessagePtr>>& currentSetOfInputs,
                                                                bool copy, bool consume);
   /// Helper to route messages for forwarding
   static void routeForwardedMessages(FairMQDeviceProxy& proxy, std::span<fair::mq::MessagePtr>& currentSetOfInputs, std::vector<fair::mq::Parts>& forwardedParts,

@@ -24,6 +24,7 @@
 
 #include <cstddef>
 #include <mutex>
+#include <span>
 #include <vector>
 #include <functional>
 
@@ -113,7 +114,7 @@ class DataRelayer
   ActivityStats processDanglingInputs(std::vector<ExpirationHandler> const&,
                                       ServiceRegistryRef context, bool createNew);
 
-  using OnDropCallback = std::function<void(TimesliceSlot, std::vector<std::vector<fair::mq::MessagePtr>>&, TimesliceIndex::OldestOutputInfo info)>;
+  using OnDropCallback = std::function<void(TimesliceSlot, std::vector<std::span<fair::mq::MessagePtr>>&, TimesliceIndex::OldestOutputInfo info)>;
 
   // Callback for when some messages are about to be owned by the the DataRelayer
   using OnInsertionCallback = std::function<void(ServiceRegistryRef&, std::span<fair::mq::MessagePtr>&)>;
@@ -153,11 +154,14 @@ class DataRelayer
   /// @returns the actions ready to be performed.
   void getReadyToProcess(std::vector<RecordAction>& completed);
 
-  /// Returns an input registry associated to the given timeslice and gives
-  /// ownership to the caller. This is because once the inputs are out of the
-  /// DataRelayer they need to be deleted once the processing is concluded.
-  std::vector<std::vector<fair::mq::MessagePtr>> consumeAllInputsForTimeslice(TimesliceSlot id);
+  /// Returns spans into the relayer's internal storage for the given timeslice.
+  /// The slot is marked invalid so new data can be relayed to it immediately,
+  /// but the underlying message vectors are NOT freed until releaseSlot() is
+  /// called. The caller must call releaseSlot() once it is done with the spans.
+  std::vector<std::span<fair::mq::MessagePtr>> consumeAllInputsForTimeslice(TimesliceSlot id);
   std::vector<std::vector<fair::mq::MessagePtr>> consumeExistingInputsForTimeslice(TimesliceSlot id);
+  /// Free the storage for a slot previously handed out by consumeAllInputsForTimeslice().
+  void releaseSlot(TimesliceSlot slot);
 
   /// Returns how many timeslices we can handle in parallel
   [[nodiscard]] size_t getParallelTimeslices() const;
@@ -204,6 +208,11 @@ class DataRelayer
   /// N is the maximum number of inflight timeslices, while
   /// M is the number of inputs which are requested.
   std::vector<std::vector<fair::mq::MessagePtr>> mCache;
+  /// Holding area for message vectors moved out of mCache by
+  /// consumeAllInputsForTimeslice(). Same NxM layout as mCache.
+  /// Spans returned to callers point here and remain valid until releaseSlot().
+  /// relay() never touches mConsumedCache, so no locking is needed in releaseSlot().
+  std::vector<std::vector<fair::mq::MessagePtr>> mConsumedCache;
 
   /// This is the index which maps a given timestamp to the associated
   /// cacheline.

@@ -587,7 +587,7 @@ auto decongestionCallbackLate = [](AsyncTask& task, size_t aid) -> void {
 // the inputs which are shared between this device and others
 // to the next one in the daisy chain.
 // FIXME: do it in a smarter way than O(N^2)
-static auto forwardInputs = [](ServiceRegistryRef registry, TimesliceSlot slot, std::vector<std::vector<fair::mq::MessagePtr>>& currentSetOfInputs,
+static auto forwardInputs = [](ServiceRegistryRef registry, TimesliceSlot slot, std::vector<std::span<fair::mq::MessagePtr>>& currentSetOfInputs,
                                TimesliceIndex::OldestOutputInfo oldestTimeslice, bool copy, bool consume = true) {
   auto& proxy = registry.get<FairMQDeviceProxy>();
 
@@ -619,7 +619,7 @@ static auto forwardInputs = [](ServiceRegistryRef registry, TimesliceSlot slot,
   O2_SIGNPOST_END(forwarding, sid, "forwardInputs", "Forwarding done");
 };
 
-static auto cleanEarlyForward = [](ServiceRegistryRef registry, TimesliceSlot slot, std::vector<std::vector<fair::mq::MessagePtr>>& currentSetOfInputs,
+static auto cleanEarlyForward = [](ServiceRegistryRef registry, TimesliceSlot slot, std::vector<std::span<fair::mq::MessagePtr>>& currentSetOfInputs,
                                    TimesliceIndex::OldestOutputInfo oldestTimeslice, bool copy, bool consume = true) {
   auto& proxy = registry.get<FairMQDeviceProxy>();
 
@@ -629,8 +629,7 @@ static auto cleanEarlyForward = [](ServiceRegistryRef registry, TimesliceSlot sl
   // Always copy them, because we do not want to actually send them.
   // We merely need the side effect of the consume, if applicable.
   for (size_t ii = 0, ie = currentSetOfInputs.size(); ii < ie; ++ii) {
-    auto span = std::span<fair::mq::MessagePtr>(currentSetOfInputs[ii]);
-    DataProcessingHelpers::cleanForwardedMessages(span, consume);
+    DataProcessingHelpers::cleanForwardedMessages(currentSetOfInputs[ii], consume);
   }
 
   O2_SIGNPOST_END(forwarding, sid, "forwardInputs", "Cleaning done");
@@ -1278,7 +1277,7 @@ void DataProcessingDevice::Run()
       // - we can trigger further events from the queue
       // - we can guarantee this is the last thing we do in the loop (
       //   assuming no one else is adding to the queue before this point).
-      auto onDrop = [&registry = mServiceRegistry, lid](TimesliceSlot slot, std::vector<std::vector<fair::mq::MessagePtr>>& dropped, TimesliceIndex::OldestOutputInfo oldestOutputInfo) {
+      auto onDrop = [&registry = mServiceRegistry, lid](TimesliceSlot slot, std::vector<std::span<fair::mq::MessagePtr>>& dropped, TimesliceIndex::OldestOutputInfo oldestOutputInfo) {
         O2_SIGNPOST_START(device, lid, "run_loop", "Dropping message from slot %" PRIu64 ". Forwarding as needed.", (uint64_t)slot.index);
         ServiceRegistryRef ref{registry};
         ref.get<AsyncQueue>();
@@ -1985,7 +1984,7 @@ void DataProcessingDevice::handleData(ServiceRegistryRef ref, InputChannelInfo&
             nPayloadsPerHeader = 1;
             ii += (nMessages / 2) - 1;
           }
-          auto onDrop = [ref](TimesliceSlot slot, std::vector<std::vector<fair::mq::MessagePtr>>& dropped, TimesliceIndex::OldestOutputInfo oldestOutputInfo) {
+          auto onDrop = [ref](TimesliceSlot slot, std::vector<std::span<fair::mq::MessagePtr>>& dropped, TimesliceIndex::OldestOutputInfo oldestOutputInfo) {
             O2_SIGNPOST_ID_GENERATE(cid, async_queue);
             O2_SIGNPOST_EVENT_EMIT(async_queue, cid, "onDrop", "Dropping message from slot %zu. Forwarding as needed. Timeslice %zu",
                                    slot.index, oldestOutputInfo.timeslice.value);
@@ -2163,15 +2162,20 @@ bool DataProcessingDevice::tryDispatchComputation(ServiceRegistryRef ref, std::v
   // want to support multithreaded dispatching of operations, I can simply
   // move these to some thread local store and the rest of the lambdas
   // should work just fine.
-  std::vector<std::vector<fair::mq::MessagePtr>> currentSetOfInputs;
+  std::vector<std::span<fair::mq::MessagePtr>> currentSetOfInputs;
+  std::vector<std::vector<fair::mq::MessagePtr>> ownedInputs;
 
   //
-  auto getInputSpan = [ref, &currentSetOfInputs](TimesliceSlot slot, bool consume = true) {
+  auto getInputSpan = [ref, &currentSetOfInputs, &ownedInputs](TimesliceSlot slot, bool consume = true) {
     auto& relayer = ref.get<DataRelayer>();
     if (consume) {
       currentSetOfInputs = relayer.consumeAllInputsForTimeslice(slot);
     } else {
-      currentSetOfInputs = relayer.consumeExistingInputsForTimeslice(slot);
+      ownedInputs = relayer.consumeExistingInputsForTimeslice(slot);
+      currentSetOfInputs.resize(ownedInputs.size());
+      for (size_t i = 0; i < ownedInputs.size(); ++i) {
+        currentSetOfInputs[i] = std::span(ownedInputs[i]);
+      }
     }
     // Convert raw message indices directly to a DataRef in O(1).
     // Used both by the sequential PartIterator and as the fallback for positional access.
@@ -2245,7 +2249,7 @@ bool DataProcessingDevice::tryDispatchComputation(ServiceRegistryRef ref, std::v
   // to avoid double counting them.
   // This was actually the easiest solution we could find for
   // O2-646.
-  auto cleanTimers = [&currentSetOfInputs](TimesliceSlot slot, InputRecord& record) {
+  auto cleanTimers = [&currentSetOfInputs, &ownedInputs](TimesliceSlot slot, InputRecord& record) {
     assert(record.size() == currentSetOfInputs.size());
     for (size_t ii = 0, ie = record.size(); ii < ie; ++ii) {
       // assuming that for timer inputs we do have exactly one PartRef object
@@ -2258,8 +2262,10 @@ bool DataProcessingDevice::tryDispatchComputation(ServiceRegistryRef ref, std::v
       if (input.header == nullptr) {
         continue;
       }
-      // This will hopefully delete the message.
-      currentSetOfInputs[ii].clear();
+      // For the consume=false (Process) path, ownedInputs holds the actual
+      // message vectors and the span points into them.
+      ownedInputs[ii].clear();
+      currentSetOfInputs[ii] = {};
     }
   };
 
@@ -2412,9 +2418,11 @@ bool DataProcessingDevice::tryDispatchComputation(ServiceRegistryRef ref, std::v
       if (spec.forwards.empty() == false) {
         auto& timesliceIndex = ref.get<TimesliceIndex>();
         forwardInputs(ref, action.slot, currentSetOfInputs, timesliceIndex.getOldestPossibleOutput(), false);
+        ref.get<DataRelayer>().releaseSlot(action.slot);
         O2_SIGNPOST_END(device, aid, "device", "Forwarding inputs consume: %d.", false);
         continue;
       }
+      ref.get<DataRelayer>().releaseSlot(action.slot);
     }
     // If there is no optional inputs we canForwardEarly
     // the messages to that parallel processing can happen.
@@ -2567,6 +2575,9 @@ bool DataProcessingDevice::tryDispatchComputation(ServiceRegistryRef ref, std::v
     if (action.op == CompletionPolicy::CompletionOp::Process) {
       cleanTimers(action.slot, record);
     }
+    if (shouldConsume) {
+      ref.get<DataRelayer>().releaseSlot(action.slot);
+    }
     O2_SIGNPOST_END(device, aid, "device", "Done processing action on slot %lu for action %{public}s", action.slot.index, fmt::format("{}", action.op).c_str());
   }
   O2_SIGNPOST_END(device, sid, "device", "Start processing ready actions");

@@ -393,15 +393,14 @@ void DataProcessingHelpers::cleanForwardedMessages(std::span<fair::mq::MessagePt
 }
 
 auto DataProcessingHelpers::routeForwardedMessageSet(FairMQDeviceProxy& proxy,
-                                                     std::vector<std::vector<fair::mq::MessagePtr>>& currentSetOfInputs,
+                                                     std::vector<std::span<fair::mq::MessagePtr>>& currentSetOfInputs,
                                                      const bool copyByDefault, bool consume) -> std::vector<fair::mq::Parts>
 {
   // we collect all messages per forward in a map and send them together
   std::vector<fair::mq::Parts> forwardedParts(proxy.getNumForwardChannels());
 
   for (size_t ii = 0, ie = currentSetOfInputs.size(); ii < ie; ++ii) {
-    auto span = std::span<fair::mq::MessagePtr>(currentSetOfInputs[ii]);
-    routeForwardedMessages(proxy, span, forwardedParts, copyByDefault, consume);
+    routeForwardedMessages(proxy, currentSetOfInputs[ii], forwardedParts, copyByDefault, consume);
   }
   return forwardedParts;
 };

@@ -412,22 +412,20 @@ void DataRelayer::pruneCache(TimesliceSlot slot, OnDropCallback onDrop)
                      ref = mContext](TimesliceSlot slot) {
     if (onDrop) {
       auto oldestPossibleTimeslice = index.getOldestPossibleOutput();
-      // State of the computation
-      std::vector<std::vector<fair::mq::MessagePtr>> dropped(numInputTypes);
+      // Build spans over the cache entries (no copy or move of message ownership).
+      // The spans are valid for the duration of the onDrop call; entries are
+      // cleared below after the callback returns.
+      std::vector<std::span<fair::mq::MessagePtr>> droppedSpans(numInputTypes);
       for (size_t ai = 0, ae = numInputTypes; ai != ae; ++ai) {
         auto cacheId = slot.index * numInputTypes + ai;
         cachedStateMetrics[cacheId] = CacheEntryStatus::RUNNING;
-        // TODO: in the original implementation of the cache, there have been only two messages per entry,
-        // check if the 2 above corresponds to the number of messages.
-        if (!cache[cacheId].empty()) {
-          dropped[ai] = std::move(cache[cacheId]);
-        }
+        droppedSpans[ai] = cache[cacheId];
       }
-      bool anyDropped = std::any_of(dropped.begin(), dropped.end(), [](auto& m) { return !m.empty(); });
+      bool anyDropped = std::any_of(droppedSpans.begin(), droppedSpans.end(), [](auto& s) { return !s.empty(); });
       if (anyDropped) {
         O2_SIGNPOST_ID_GENERATE(aid, data_relayer);
         O2_SIGNPOST_EVENT_EMIT(data_relayer, aid, "pruneCache", "Dropping stuff from slot %zu with timeslice %zu", slot.index, oldestPossibleTimeslice.timeslice.value);
-        onDrop(slot, dropped, oldestPossibleTimeslice);
+        onDrop(slot, droppedSpans, oldestPossibleTimeslice);
       }
     }
     assert(cache.empty() == false);
@@ -886,58 +884,33 @@ void DataRelayer::updateCacheStatus(TimesliceSlot slot, CacheEntryStatus oldStat
   }
 }
 
-std::vector<std::vector<fair::mq::MessagePtr>> DataRelayer::consumeAllInputsForTimeslice(TimesliceSlot slot)
+std::vector<std::span<fair::mq::MessagePtr>> DataRelayer::consumeAllInputsForTimeslice(TimesliceSlot slot)
 {
   std::scoped_lock<O2_LOCKABLE(std::recursive_mutex)> lock(mMutex);
-
   const auto numInputTypes = mDistinctRoutesIndex.size();
-  // State of the computation
-  std::vector<std::vector<fair::mq::MessagePtr>> messages(numInputTypes);
-  auto& cache = mCache;
-  auto& index = mTimesliceIndex;
-
-  // Nothing to see here, this is just to make the outer loop more understandable.
-  auto jumpToCacheEntryAssociatedWith = [](TimesliceSlot) {
-    return;
-  };
-
-  // We move ownership so that the cache can be reused once the computation is
-  // finished. We mark the given cache slot invalid, so that it can be reused
-  // This means we can still handle old messages if there is still space in the
-  // cache where to put them.
-  auto moveHeaderPayloadToOutput = [&messages,
-                                    &cachedStateMetrics = mCachedStateMetrics,
-                                    &cache, &index, &numInputTypes](TimesliceSlot s, size_t arg) {
-    auto cacheId = s.index * numInputTypes + arg;
-    cachedStateMetrics[cacheId] = CacheEntryStatus::RUNNING;
-    // TODO: in the original implementation of the cache, there have been only two messages per entry,
-    // check if the 2 above corresponds to the number of messages.
-    if (!cache[cacheId].empty()) {
-      messages[arg] = std::move(cache[cacheId]);
-    }
-    index.markAsInvalid(s);
-  };
-
-  // An invalid set of arguments is a set of arguments associated to an invalid
-  // timeslice, so I can simply do that. I keep the assertion there because in principle
-  // we should have dispatched the timeslice already!
-  // FIXME: what happens when we have enough timeslices to hit the invalid one?
-  auto invalidateCacheFor = [&numInputTypes, &index, &cache](TimesliceSlot s) {
-    for (size_t ai = s.index * numInputTypes, ae = ai + numInputTypes; ai != ae; ++ai) {
-      assert(std::accumulate(cache[ai].begin(), cache[ai].end(), true, [](bool result, auto const& element) { return result && element.get() == nullptr; }));
-      cache[ai].clear();
-    }
-    index.markAsInvalid(s);
-  };
-
-  // Outer loop here.
-  jumpToCacheEntryAssociatedWith(slot);
-  for (size_t ai = 0, ae = numInputTypes; ai != ae; ++ai) {
-    moveHeaderPayloadToOutput(slot, ai);
+  // Move message vectors from mCache into mConsumedCache (same NxM layout) so that:
+  //   - mCache entries are immediately empty and the slot can be reused by relay()
+  //   - the messages remain alive in mConsumedCache until releaseSlot() is called
+  // relay() never touches mConsumedCache, so the returned spans are safe to use
+  // concurrently with new relay() calls into this (now-invalid) slot.
+  std::vector<std::span<fair::mq::MessagePtr>> spans(numInputTypes);
+  for (size_t ai = 0; ai < numInputTypes; ++ai) {
+    auto cacheId = slot.index * numInputTypes + ai;
+    mCachedStateMetrics[cacheId] = CacheEntryStatus::RUNNING;
+    mConsumedCache[cacheId] = std::move(mCache[cacheId]);
+    spans[ai] = mConsumedCache[cacheId];
   }
-  invalidateCacheFor(slot);
+  mTimesliceIndex.markAsInvalid(slot);
+  return spans;
+}
 
-  return messages;
+void DataRelayer::releaseSlot(TimesliceSlot slot)
+{
+  // No lock needed: relay() only touches mCache, never mConsumedCache.
+  const auto numInputTypes = mDistinctRoutesIndex.size();
+  for (size_t ai = slot.index * numInputTypes, ae = ai + numInputTypes; ai != ae; ++ai) {
+    mConsumedCache[ai].clear();
+  }
 }
 
 std::vector<std::vector<fair::mq::MessagePtr>> DataRelayer::consumeExistingInputsForTimeslice(TimesliceSlot slot)
@@ -991,6 +964,9 @@ void DataRelayer::clear()
   for (auto& cache : mCache) {
     cache.clear();
   }
+  for (auto& consumed : mConsumedCache) {
+    consumed.clear();
+  }
   for (size_t s = 0; s < mTimesliceIndex.size(); ++s) {
     mTimesliceIndex.markAsInvalid(TimesliceSlot{s});
   }
@@ -1024,6 +1000,7 @@ void DataRelayer::publishMetrics()
   // maybe misleading to have the allocation in a function primarily for
   // metrics publishing, do better in setPipelineLength?
   mCache.resize(numInputTypes * mTimesliceIndex.size());
+  mConsumedCache.resize(mCache.size());
   auto& states = mContext.get<DataProcessingStates>();
 
   mCachedStateMetrics.resize(mCache.size());