From e2acb8a7edde17977fe389744eac854c36dd3c98 Mon Sep 17 00:00:00 2001
From: Jerry Zhao <ch3nzhao@gmail.com>
Date: Mon, 12 Oct 2026 10:19:11 +0800
Subject: [PATCH] perf(storage): prefetch flush lookups and filter address tags
 on main

---
 include/lavik/storage/scan_hash_map.h | 38 ++++++++++++++++++++++-----
 src/storage/engine/flush.cpp          | 24 ++++++++++++++++-
 2 files changed, 54 insertions(+), 8 deletions(-)

diff --git a/include/lavik/storage/scan_hash_map.h b/include/lavik/storage/scan_hash_map.h
index 09433a698..373e3f146 100644
--- a/include/lavik/storage/scan_hash_map.h
+++ b/include/lavik/storage/scan_hash_map.h
@@ -1221,6 +1221,21 @@ class ScanHashMap {
     return FindAddressImpl<false>(address, hash, 0);
   }
 
+  // Owner-only hints for batches of independent address lookups. These do not
+  // resolve entries or establish membership; the eventual FindAddress must
+  // still validate the current table population and exact live address.
+  void PrefetchAddressMetadata() const noexcept {
+    __builtin_prefetch(&tables_[0], 0, 3);
+  }
+
+  void PrefetchAddress(std::uint32_t hash) const noexcept {
+    for (int t = 0, count = Rehashing() ? 2 : 1; t < count; ++t) {
+      const Table& table = tables_[t];
+      if (table.buckets_)
+        __builtin_prefetch(&table.buckets_[hash & BucketMask(table)], 0, 3);
+    }
+  }
+
   // Optional bucket fingerprint for FindAddress. Capture it from the same
   // immutable key digest as AddressHash while the entry is still live.
   static std::uint8_t AddressTag(const Digest& digest) noexcept {
@@ -2320,15 +2335,24 @@ class ScanHashMap {
       }
       const Bucket* bucket = &table.buckets_[hash & BucketMask(table)];
       while (bucket != nullptr) {
-        for (std::size_t slot = 0; slot < kEntriesPerBucket; ++slot) {
-          if (Occupied(*bucket, slot)) {
-            if constexpr (FilterTag) {
-              if (bucket->hashes_[slot] != tag) continue;
-            }
+        if constexpr (FilterTag) {
+          // Flush receipts already retain the key's tag. Use the same SIMD
+          // filter as key lookup, but still validate a live handle and exact
+          // address: tags collide and holes retain stale fingerprint bytes.
+          for (std::uint32_t matches = MatchingTags(*bucket, tag); matches != 0;
+               matches &= matches - 1) {
+            const std::size_t slot = std::countr_zero(matches);
+            if (!Occupied(*bucket, slot)) continue;
             const Entry* entry = Resolve(bucket->entries_[slot]);
-            if (reinterpret_cast<std::uintptr_t>(entry) == address) {
+            if (reinterpret_cast<std::uintptr_t>(entry) == address)
+              return entry;
+          }
+        } else {
+          for (std::size_t slot = 0; slot < kEntriesPerBucket; ++slot) {
+            if (!Occupied(*bucket, slot)) continue;
+            const Entry* entry = Resolve(bucket->entries_[slot]);
+            if (reinterpret_cast<std::uintptr_t>(entry) == address)
               return entry;
-            }
           }
         }
         bucket = Chained(*bucket) ? Child(table, bucket) : nullptr;
diff --git a/src/storage/engine/flush.cpp b/src/storage/engine/flush.cpp
index f7f19ce2f..eca10a772 100644
--- a/src/storage/engine/flush.cpp
+++ b/src/storage/engine/flush.cpp
@@ -487,7 +487,29 @@ Task<absl::Status> StorageEngine::Impl::FlushPendingBlocks(WorkerStore* store) {
     // header both fdatasync'd above), so the versions they superseded are no
     // longer anyone's durable copy and can leave their blocks' accounting.
     std::vector<RetiredRecord> retired_records;
-    for (const RecordIdentity& identity : pending->staged_records_) {
+    const auto prefetch_index = [&](std::size_t position, bool bucket) {
+      if (position >= pending->staged_records_.size()) return;
+      const auto& future = pending->staged_records_[position];
+      if (future.entry_address_ == 0 ||
+          future.index_generation_ != store->index_generations_[future.db_id_])
+        return;
+      const auto& index =
+          PartitionFor(*store, future.partition_id_).indexes_[future.db_id_];
+      if (bucket)
+        index.PrefetchAddress(future.entry_hash_);
+      else
+        index.PrefetchAddressMetadata();
+    };
+    for (std::size_t position = 0; position < pending->staged_records_.size();
+         ++position) {
+      // Batch receipts expose independent random lookups. Warm fixed map
+      // metadata first, then its bucket while processing
+      // earlier receipts. All hints stay on the owning worker and retain no
+      // entry pointer; generation, live membership and physical-boundary
+      // checks still follow.
+      prefetch_index(position + 16, false);
+      prefetch_index(position + 8, true);
+      const RecordIdentity& identity = pending->staged_records_[position];
       if (identity.retired_extents_ != nullptr) {
         SpawnExtentReclaim(*store, identity.retired_extents_);
       }