From 5ec026b659effcc9fb17aefca9adea2ef358c1ac Mon Sep 17 00:00:00 2001
From: Dan Larkin <dan@arangodb.com>
Date: Wed, 24 May 2017 13:45:53 -0400
Subject: [PATCH] Added sizeHint method for caches to improve warmup
 procedures.

---
 arangod/Cache/Cache.cpp | 9 +++++++++
 arangod/Cache/Cache.h   | 8 ++++++++
 arangod/Cache/Table.h   | 5 ++---
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arangod/Cache/Cache.cpp b/arangod/Cache/Cache.cpp
index d659e1bc54..d649187b2c 100644
--- a/arangod/Cache/Cache.cpp
+++ b/arangod/Cache/Cache.cpp
@@ -115,6 +115,15 @@ uint64_t Cache::usage() {
   return usage;
 }
 
+void Cache::sizeHint(uint64_t numElements) {
+  uint64_t numBuckets = static_cast<uint64_t>(static_cast<double>(numElements)
+    / (static_cast<double>(_slotsPerBucket) * Table::idealUpperRatio));
+  uint32_t requestedLogSize = 0;
+  for (; (static_cast<uint64_t>(1) << requestedLogSize) < numBuckets;
+    requestedLogSize++) {}
+  requestMigrate(requestedLogSize);
+}
+
 std::pair<double, double> Cache::hitRates() {
   double lifetimeRate = std::nan("");
   double windowedRate = std::nan("");
diff --git a/arangod/Cache/Cache.h b/arangod/Cache/Cache.h
index 075273eca6..bb75527f63 100644
--- a/arangod/Cache/Cache.h
+++ b/arangod/Cache/Cache.h
@@ -97,6 +97,14 @@ class Cache : public std::enable_shared_from_this<Cache> {
   //////////////////////////////////////////////////////////////////////////////
   uint64_t usage();
 
+  //////////////////////////////////////////////////////////////////////////////
+  /// @brief Gives hint to attempt to preallocate space for an incoming load.
+  ///
+  /// The parameter specifies an expected number of elements to be inserted.
+  /// This allows for migration to an appropriately-sized table.
+  //////////////////////////////////////////////////////////////////////////////
+  void sizeHint(uint64_t numElements);
+
   //////////////////////////////////////////////////////////////////////////////
   /// @brief Returns the cache hit-rates.
   ///
diff --git a/arangod/Cache/Table.h b/arangod/Cache/Table.h
index c4779689a3..83d747ca09 100644
--- a/arangod/Cache/Table.h
+++ b/arangod/Cache/Table.h
@@ -39,6 +39,8 @@ namespace cache {
 ////////////////////////////////////////////////////////////////////////////////
 class Table : public std::enable_shared_from_this<Table> {
  public:
+   static constexpr double idealLowerRatio = 0.05;
+   static constexpr double idealUpperRatio = 0.33;
   static const uint32_t minLogSize;
   static const uint32_t maxLogSize;
   static constexpr uint32_t standardLogSizeAdjustment = 6;
@@ -186,9 +188,6 @@ class Table : public std::enable_shared_from_this<Table> {
   uint32_t idealSize();
 
  private:
-  static constexpr double idealLowerRatio = 0.05;
-  static constexpr double idealUpperRatio = 0.33;
-
   State _state;
 
   uint32_t _logSize;