Implement Continus Batching.

(1) A function GenerateTWithContinuousBatching is added to use continuous batching when enabled.

(2) The ContinuousQBatch is added as a subclass of QBatch to manage prefill, insert, used-kv-cache-collection.

(3) Also expanded the unit test to more diverse cases.

PiperOrigin-RevId: 836090261

gemma/gemma.cc

@@ -18,6 +18,8 @@
 
 #include "gemma/gemma.h"
 
+#include <optional>
+
 #include "compression/types.h"  // GEMMA_DISABLED_TARGETS
 #include "util/zones.h"
 #ifndef HWY_DISABLED_TARGETS
@@ -35,7 +37,7 @@
 // After highway.h
 #include "gemma/attention.h"  // includes highway.h
 #include "gemma/gemma-inl.h"
-#include "gemma/vit.h"      // includes highway.h
+#include "gemma/vit.h"  // includes highway.h
 
 #ifndef GEMMA_CC_ONCE
 #define GEMMA_CC_ONCE
@@ -357,6 +359,10 @@ static HWY_NOINLINE void PrefillQBatch(const size_t max_prompt_size,
         (void)runtime_config.StreamToken(qbatch.QueryIdx(qi), pos_in_prompt,
                                          token, 0.0f);
         qbatch.MutablePos(qi) = pos_in_prompt;
+      } else {
+        // This prevents the kv cache of eos_id to be written to last prefilled
+        // token.
+        qbatch.MutablePos(qi) = qbatch.Prompt(qi).size();
       }
 
       qbatch.PrevToken(qi) = token;
@@ -589,6 +595,57 @@ static void GenerateT(const ModelConfig& config,
   timing_info.NotifyGenerateDone();
 }
 
+// Same as GenerateT, but uses ContinuousQBatch.
+static void GenerateTWithContinuousBatching(
+    const ModelConfig& config, const RuntimeConfig& runtime_config,
+    const AesCtrEngine& engine, const WeightsPtrs& weights,
+    Activations& activations, AllQueries& all_queries, MatMulEnv& env,
+    TimingInfo& timing_info) {
+  const size_t qbatch_size = runtime_config.decode_qbatch_size;
+
+  QBatch qbatch(0, qbatch_size, all_queries);
+  ContinuousQBatch prefill_batch(qbatch_size, all_queries);
+
+  hwy::BitSet4096<> non_eos;
+  const SampleFunc sample_token =
+      ChooseSampleFunc(runtime_config, engine, env.ctx);
+
+  int query_inserted = 0;
+  while (non_eos.Any() || query_inserted < all_queries.NumQueries()) {
+    for (size_t qi = 0; qi < qbatch.Size(); ++qi) {
+      // Continue if qi slot is still processing.
+      if (non_eos.Get(qi)) continue;
+      // Collect the kv_cache from the qi slot in the qbatch to the
+      // available_kv_caches_ in the prefill_batch.
+      prefill_batch.MaybeReleaseKV(qbatch.Single(qi));
+
+      // Prefill if no available prefilled queries to insert.
+      if (prefill_batch.ShouldPrefill()) {
+        prefill_batch.SetupNextBatchForPrefill();
+        PrefillTBatchOrQBatch(config, runtime_config, weights, activations,
+                              prefill_batch, env, timing_info);
+        activations.SetBatchSize(qbatch.Size());
+      }
+
+      // Get the next query to insert to the generate batch.
+      std::optional<size_t> qi_to_insert = prefill_batch.GetNextToInsert();
+      if (qi_to_insert) {
+        qbatch.Insert(qi_to_insert.value(), qi);
+        query_inserted++;
+
+        non_eos.Set(qi);
+        StreamAndUpdateEOSAfterPrefill(config, runtime_config, qbatch, non_eos,
+                                       qi);
+      }
+    }
+
+    Transformer(config, runtime_config, weights, activations, qbatch, env);
+    SampleAndStream(config, runtime_config, weights, sample_token, activations,
+                    qbatch, env, non_eos, timing_info);
+  }
+  timing_info.NotifyGenerateDone();
+}
+
 void GenerateSingleT(const PromptTokens& prompt, size_t pos, size_t prefix_end,
                      const ModelConfig& config,
                      const RuntimeConfig& runtime_config,
@@ -619,12 +676,17 @@ void GenerateBatchT(const ModelConfig& config,
                           all_queries[0].kv_cache.SeqLen(), env.ctx,
                           env.row_ptrs);
 
-  for (size_t start = 0; start < all_queries.NumQueries();
-       start += runtime_config.decode_qbatch_size) {
-    QBatch qbatch(start, runtime_config.decode_qbatch_size, all_queries);
-    // Generate a batch of one token for each of `qbatch.Size()` queries.
-    GenerateT(config, runtime_config, engine, weights, activations, qbatch, env,
-              timing_info);
+  if (runtime_config.use_continuous_batching) {
+    GenerateTWithContinuousBatching(config, runtime_config, engine, weights,
+                                    activations, all_queries, env, timing_info);
+  } else {
+    for (size_t start = 0; start < all_queries.NumQueries();
+         start += runtime_config.decode_qbatch_size) {
+      QBatch qbatch(start, runtime_config.decode_qbatch_size, all_queries);
+      // Generate a batch of one token for each of `qbatch.Size()` queries.
+      GenerateT(config, runtime_config, engine, weights, activations, qbatch,
+                env, timing_info);
+    }
   }
 }
 
@@ -721,5 +783,64 @@ void Gemma::GenerateImageTokens(const RuntimeConfig& runtime_config,
   env.ctx.pools.MaybeStopSpinning(runtime_config.use_spinning);
 }
 
+ContinuousQBatch::ContinuousQBatch(size_t max_size, AllQueries& queries)
+    : QBatch(0, max_size, queries) {
+  for (size_t i = start_; i < queries_.NumQueries(); ++i) {
+    if (!queries_[i].kv_cache.IsEmpty()) {
+      // Put the kv_cache to the available_kv_caches_ instead; leaving the
+      // kv_cache in the queries_ is very confusing. This simplifies the logic
+      // of kv_cache management.
+      available_kv_caches_.push_back(queries_[i].kv_cache);
+      queries_[i].kv_cache = KVCachePtr();
+    }
+  }
+}
+
+bool ContinuousQBatch::ShouldPrefill() const {
+  const bool no_available_to_insert = next_to_insert_ == next_to_prefill_;
+  const int more_queries_to_prefill = next_to_prefill_ < queries_.NumQueries();
+  return no_available_to_insert && more_queries_to_prefill;
+}
+
+void ContinuousQBatch::SetupNextBatchForPrefill() {
+  start_ = next_to_prefill_;
+  size_ = HWY_MIN(max_size_, queries_.NumQueries() - start_);
+  HWY_DASSERT(size_ != 0);
+  HWY_DASSERT(start_ + size_ <= queries_.NumQueries());
+  query_idx_.clear();
+  query_idx_.reserve(size_);
+  for (size_t i = 0; i < size_; ++i) {
+    const size_t next_query_idx = start_ + i;
+    query_idx_.push_back(next_query_idx);
+    HWY_ASSERT(queries_[next_query_idx].kv_cache.IsEmpty());
+    queries_[next_query_idx].kv_cache = available_kv_caches_.back();
+    available_kv_caches_.pop_back();
+  }
+  next_to_prefill_ += size_;
+}
+
+std::optional<size_t> ContinuousQBatch::GetNextToInsert() {
+  if (next_to_insert_ == next_to_prefill_) {
+    return std::nullopt;
+  }
+  next_to_insert_++;
+  return next_to_insert_ - 1;
+}
+
+void ContinuousQBatch::MaybeReleaseKV(const QBatch& from) {
+  const int query_to_collect = from.QueryIdx(0);
+  // Only collect if the query to collect is not the same as the next query to
+  // insert. This happens at the beginning of each Generate call.
+  if (query_to_collect != next_to_insert_) {
+    // Only clear the KV cache if there are more queries to insert; Otherwise
+    // we get a crash because Transformer will still access that KV cache.
+    if (next_to_insert_ < queries_.NumQueries()) {
+      available_kv_caches_.push_back(from.KV(0));
+      ZeroInit(from.KV(0).kv_cache);
+      from.KV(0) = KVCachePtr();
+    }
+  }
+}
+
 }  // namespace gcpp
 #endif  // HWY_ONCE

gemma/gemma.h

@@ -18,6 +18,7 @@
 
 #include <stdio.h>
 
+#include <optional>
 #include <vector>
 
 // IWYU pragma: begin_exports
@@ -89,17 +90,17 @@ struct AllQueries {
       const hwy::Span<const PromptTokens>& prompts,
       const hwy::Span<KVCachePtr>& kv_caches,
       const hwy::Span<const size_t>& prefix_end = hwy::Span<const size_t>()) {
-    HWY_ASSERT(prompts.size() == kv_caches.size());
     HWY_ASSERT(prompts.size() == prefix_end.size() || prefix_end.size() == 0);
-    per_query_.reserve(kv_caches.size());
-    for (size_t i = 0; i < kv_caches.size(); ++i) {
-      HWY_ASSERT(kv_caches[i].SeqLen() == kv_caches[0].SeqLen());
+    per_query_.reserve(prompts.size());
+    for (size_t i = 0; i < prompts.size(); ++i) {
+      HWY_ASSERT(kv_caches.size() == 0 ||
+                 kv_caches[i].SeqLen() == kv_caches[0].SeqLen());
       per_query_.push_back(PerQuery{
           .prompt = prompts[i],
           .mutable_pos = 0,
           .initial_pos = 0,
           .prefix_end = prefix_end.size() == 0 ? 0 : prefix_end[i],
-          .kv_cache = kv_caches[i],
+          .kv_cache = kv_caches.size() == 0 ? KVCachePtr() : kv_caches[i],
       });
     }
   }
@@ -142,10 +143,13 @@ class QBatch {
     HWY_ASSERT(max_size_ <= kMaxBatchSize);
     HWY_DASSERT(size_ != 0);
     HWY_DASSERT(start_ + size_ <= queries_.NumQueries());
+    for (int i = 0; i < size_; ++i) {
+      query_idx_.push_back(start_ + i);
+    }
   }
 
   // Returns a single-query view starting at `qi` relative to this batch.
-  QBatch Single(size_t qi) const { return QBatch(start_ + qi, 1, queries_); }
+  QBatch Single(size_t qi) const { return QBatch(QueryIdx(qi), 1, queries_); }
 
   // How many queries in this batch, <= `queries_.NumQueries()` and `max_size_`.
   size_t Size() const { return size_; }
@@ -153,7 +157,7 @@ class QBatch {
   // Returns index for use with `AllQueries` and `BatchStreamToken`.
   size_t QueryIdx(size_t qi) const {
     HWY_DASSERT(qi < size_);
-    return start_ + qi;
+    return query_idx_[qi];
   }
 
   // Accessor functions to bridge the previous SoA and current AoS layout.
@@ -171,13 +175,48 @@ class QBatch {
   KVCachePtr& KV(size_t qi) const { return queries_[QueryIdx(qi)].kv_cache; }
   int& PrevToken(size_t qi) { return queries_[QueryIdx(qi)].prev_token; }
 
- private:
+  // let query_idx_[to] point to the from in the queries_; this is only used if
+  // the slot in the QBatch is less than the number of queries.
+  void Insert(size_t from, size_t to) {
+    if (from == to) return;
+    HWY_ASSERT(!queries_[from].kv_cache.IsEmpty());
+    HWY_ASSERT(queries_[to].kv_cache.IsEmpty());
+    // Conceptually, insert from.query to location to.
+    query_idx_[to] = from;
+  }
+
+ protected:
   size_t start_;
   size_t max_size_;
   AllQueries& queries_;
+  std::vector<size_t> query_idx_;
   size_t size_;
 };
 
+// Used for continuous batching.
+class ContinuousQBatch : public QBatch {
+ public:
+  ContinuousQBatch(size_t max_size, AllQueries& queries);
+
+  // Whether we should prefill the next batch, i.e. next_to_insert_ ==
+  // next_to_prefill_.
+  bool ShouldPrefill() const;
+
+  // Setup the query_idx_ to point to the next group of queries to prefill.
+  void SetupNextBatchForPrefill();
+
+  // Get the next query to insert to the generate batch.
+  std::optional<size_t> GetNextToInsert();
+
+  // Collect the kv_cache from QBatch to available_kv_caches_.
+  void MaybeReleaseKV(const QBatch& from);
+
+ public:
+  int next_to_prefill_ = 0;
+  int next_to_insert_ = 0;
+  std::vector<KVCachePtr> available_kv_caches_;
+};
+
 struct TimingInfo {
   // be sure to populate prefill_start before calling NotifyPrefill.
   void NotifyPrefill(size_t tokens) {

gemma/gemma_args.h

@@ -163,6 +163,9 @@ struct RuntimeConfig {
   // default decision is likely sufficient because it is based on whether
   // threads are successfully pinned.
   mutable Tristate use_spinning = Tristate::kDefault;
+
+  // Whether to use continuous batching.
+  bool use_continuous_batching = false;
 };
 
 struct InferenceArgs : public ArgsBase<InferenceArgs> {

gemma/kv_cache.h

@@ -28,6 +28,13 @@ namespace gcpp {
 
 using KV_t = float;
 
+// A non-owning view of a KVCache.
+struct KVCachePtr {
+  bool IsEmpty() const { return kv_cache.Rows() == 0; }
+  size_t SeqLen() const { return kv_cache.Rows(); }
+  MatPtrT<KV_t> kv_cache;
+};
+
 struct KVCache {
   KVCache(const ModelConfig& config, const InferenceArgs& inference_args,
           const Allocator& allocator);
@@ -40,6 +47,8 @@ struct KVCache {
 
   MatStorageT<KV_t> kv_cache;  // [seq_len, layers * kv_heads * qkv_dim * 2]
 
+  KVCachePtr ToPtr() { return KVCachePtr{.kv_cache = kv_cache}; }
+
  private:
   const Allocator& allocator_;
 
@@ -47,12 +56,6 @@ struct KVCache {
   KVCache(const Extents2D& kv_extents, const Allocator& allocator);
 };
 
-// A non-owning view of a KVCache.
-struct KVCachePtr {
-  size_t SeqLen() const { return kv_cache.Rows(); }
-  MatPtrT<KV_t> kv_cache;
-};
-
 // Convenience function to create views into KVCaches.
 std::vector<KVCachePtr> ToKVCachePtrs(const hwy::Span<KVCache>& kv_caches);