Async eviction implementation

TODO:
- support for chained items
- exposing per-tier statistics
- support for temporary mappings (CacheAllocator ctor without ShmSharedNew/Attach)

Author: victoria-mcgrath

cachelib/allocator/Cache.cpp

@@ -23,6 +23,12 @@
 namespace facebook {
 namespace cachelib {
 
+CacheBase::CacheBase(unsigned numTiers): numTiers_(numTiers) {}
+
+unsigned CacheBase::getNumTiers() const {
+  return numTiers_;
+}
+
 void CacheBase::setRebalanceStrategy(
     PoolId pid, std::shared_ptr<RebalanceStrategy> strategy) {
   std::unique_lock<std::mutex> l(lock_);

cachelib/allocator/Cache.h

@@ -56,7 +56,7 @@ enum class RemoveContext { kEviction, kNormal };
 // A base class of cache exposing members and status agnostic of template type.
 class CacheBase {
  public:
-  CacheBase() = default;
+  CacheBase(unsigned numTiers = 1);
   virtual ~CacheBase() = default;
 
   // Movable but not copyable
@@ -65,6 +65,9 @@ class CacheBase {
   CacheBase(CacheBase&&) = default;
   CacheBase& operator=(CacheBase&&) = default;
 
+  // TODO: come up with some reasonable number
+  static constexpr unsigned kMaxTiers = 8;
+
   // Get a string referring to the cache name for this cache
   virtual const std::string getCacheName() const = 0;
 
@@ -253,6 +256,10 @@ class CacheBase {
   // @return The number of slabs that were actually reclaimed (<= numSlabs)
   virtual unsigned int reclaimSlabs(PoolId id, size_t numSlabs) = 0;
 
+  unsigned getNumTiers() const;
+
+  unsigned numTiers_ = 1;
+
   // Protect 'poolRebalanceStragtegies_' and `poolResizeStrategies_`
   // and `poolOptimizeStrategy_`
   mutable std::mutex lock_;