RPagePool.hxx

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/// \file ROOT/RPagePool.hxx
/// \ingroup NTuple ROOT7
/// \author Jakob Blomer <jblomer@cern.ch>
/// \date 2018-10-09
/// \warning This is part of the ROOT 7 prototype! It will change without notice. It might trigger earthquakes. Feedback
/// is welcome!
 
/*************************************************************************
 * Copyright (C) 1995-2019, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/
 
#ifndef ROOT7_RPagePool
#define ROOT7_RPagePool
 
#include <ROOT/RPage.hxx>
#include <ROOT/RPageAllocator.hxx>
#include <ROOT/RNTupleUtil.hxx>
 
#include <cstddef>
#include <map>
#include <mutex>
#include <typeindex>
#include <typeinfo>
#include <unordered_map>
#include <unordered_set>
#include <vector>
 
namespace ROOT {
namespace Experimental {
 
namespace Internal {
 
// clang-format off
/**
\class ROOT::Experimental::Internal::RPagePool
\ingroup NTuple
\brief A thread-safe cache of pages loaded from the page source.
 
The page pool is used as a cache for pages loaded from a page source.
In this way, identical page needed at the same time, only need to be loaded once.
Page sources also use the page pool to stage (preload) pages unsealed by IMT tasks.
*/
// clang-format on
class RPagePool {
   friend class RPageRef;
 
public:
   // Search key for a set of pages covering the same column and in-memory target type.
   // Within the set of pages, one needs to find the page of a given index.
   struct RKey {
      DescriptorId_t fColumnId = kInvalidDescriptorId;
      std::type_index fInMemoryType = std::type_index(typeid(void));
 
      bool operator==(const RKey &other) const
      {
         return this->fColumnId == other.fColumnId && this->fInMemoryType == other.fInMemoryType;
      }
 
      bool operator!=(const RKey &other) const { return !(*this == other); }
   };
 
private:
   /// Hash function to be used in the unordered map fLookupByKey
   struct RKeyHasher {
      /// Like boost::hash_combine
      std::size_t operator()(const RKey &k) const
      {
         auto seed = std::hash<DescriptorId_t>()(k.fColumnId);
         return seed ^ (std::hash<std::type_index>()(k.fInMemoryType) + 0x9e3779b9 + (seed << 6) + (seed >> 2));
      }
   };
 
   /// Every page in the page pool is annotated with a search key and a reference counter.
   struct REntry {
      RPage fPage;
      RKey fKey;
      std::int64_t fRefCounter = 0;
   };
 
   /// Used in fLookupByKey to store both the absolute and the cluster-local page index of the referenced page.
   /// This allows to do binary search for one or the other. Note that elements in fLookupByKey must have
   /// _both_ values to be valid. If RPagePosition is used as a search key, only one of the two needs to be set.
   struct RPagePosition {
      NTupleSize_t fGlobalFirstElement = kInvalidNTupleIndex;
      RClusterIndex fClusterFirstElement;
 
      bool operator<(const RPagePosition &other) const
      {
         if ((fGlobalFirstElement != kInvalidNTupleIndex) && (other.fGlobalFirstElement != kInvalidNTupleIndex))
            return fGlobalFirstElement < other.fGlobalFirstElement;
 
         assert(fClusterFirstElement.GetClusterId() != kInvalidDescriptorId &&
                fClusterFirstElement.GetIndex() != kInvalidClusterIndex);
         assert(other.fClusterFirstElement.GetClusterId() != kInvalidDescriptorId &&
                other.fClusterFirstElement.GetIndex() != kInvalidClusterIndex);
         if (fClusterFirstElement.GetClusterId() == other.fClusterFirstElement.GetClusterId())
            return fClusterFirstElement.GetIndex() < other.fClusterFirstElement.GetIndex();
         return fClusterFirstElement.GetClusterId() < other.fClusterFirstElement.GetClusterId();
      }
 
      // Constructor used to store a page in fLookupByKey
      explicit RPagePosition(const RPage &page)
         : fGlobalFirstElement(page.GetGlobalRangeFirst()),
           fClusterFirstElement({page.GetClusterInfo().GetId(), page.GetClusterRangeFirst()})
      {
      }
 
      // Search key constructors
      explicit RPagePosition(NTupleSize_t globalIndex) : fGlobalFirstElement(globalIndex) {}
      explicit RPagePosition(RClusterIndex clusterIndex) : fClusterFirstElement(clusterIndex) {}
   };
 
   std::vector<REntry> fEntries; ///< All cached pages in the page pool
   /// Used in ReleasePage() to find the page index in fPages
   std::unordered_map<void *, std::size_t> fLookupByBuffer;
   /// Used in GetPage() to find the right page in fEntries. Lookup for the key (pair of on-disk and in-memory type)
   /// takes place in O(1). The selected pages are identified by index into the fEntries vector (map's value)
   /// and sorted by the position of the page in the column (map's key). Thus, access to pages of the page set
   /// has logarithmic complexity.
   std::unordered_map<RKey, std::map<RPagePosition, std::size_t>, RKeyHasher> fLookupByKey;
   /// Remembers pages with reference counter 0, organized by the page's cluster id. The pages are identified
   /// by their page buffer address. The fLookupByBuffer map can be used to resolve the address to a page.
   /// Once a page gets used, it is removed from the unused pages list. Evict will remove all unused pages
   /// from a given cluster id.
   std::unordered_map<DescriptorId_t, std::unordered_set<void *>> fUnusedPages;
   std::mutex fLock; ///< The page pool is accessed concurrently due to parallel decompression
 
   /// Add a new page to the fLookupByBuffer and fLookupByKey data structures.
   REntry &AddPage(RPage page, const RKey &key, std::int64_t initialRefCounter);
 
   /// Give back a page to the pool and decrease the reference counter. There must not be any pointers anymore into
   /// this page. If the reference counter drops to zero, the page pool might decide to call the deleter given in
   /// during registration. Called by the RPageRef destructor.
   void ReleasePage(const RPage &page);
 
   /// Called by GetPage(), when the reference counter increases from zero to one
   void RemoveFromUnusedPages(const RPage &page);
 
   /// Called both by ReleasePage() and by Evict() to remove an unused page from the pool
   void ErasePage(std::size_t entryIdx, decltype(fLookupByBuffer)::iterator lookupByBufferItr);
 
public:
   RPagePool() = default;
   RPagePool(const RPagePool&) = delete;
   RPagePool& operator =(const RPagePool&) = delete;
   ~RPagePool() = default;
 
   /// Adds a new page to the pool. Upon registration, the page pool takes ownership of the page's memory.
   /// The new page has its reference counter set to 1.
   RPageRef RegisterPage(RPage page, RKey key);
   /// Like RegisterPage() but the reference counter is initialized to 0. In addition, the page is added
   /// to the set of unused pages of the page's cluster (see Evict()).
   void PreloadPage(RPage page, RKey key);
   /// Removes unused pages (pages with reference counter 0) from the page pool. Users of PreloadPage() should
   /// use Evict() appropriately to avoid accumulation of unused pages.
   void Evict(DescriptorId_t clusterId);
   /// Tries to find the page corresponding to column and index in the cache. If the page is found, its reference
   /// counter is increased
   RPageRef GetPage(RKey key, NTupleSize_t globalIndex);
   RPageRef GetPage(RKey key, RClusterIndex clusterIndex);
};
 
// clang-format off
/**
\class ROOT::Experimental::Internal::RPageRef
\ingroup NTuple
\brief Reference to a page stored in the page pool
 
The referenced page knows about its page pool and decreases the reference counter on destruction.
*/
// clang-format on
class RPageRef {
   friend class RPagePool;
 
   RPage fPage;
   RPagePool *fPagePool = nullptr;
 
   // Called as delegated constructor and directly by the page pool
   RPageRef(const RPage &page, RPagePool *pagePool) : fPagePool(pagePool)
   {
      // We leave the fPage::fPageAllocator member unset (nullptr), since fPage is a non-owning view on the page
      fPage.fBuffer = page.fBuffer;
      fPage.fElementSize = page.fElementSize;
      fPage.fNElements = page.fNElements;
      fPage.fMaxElements = page.fMaxElements;
      fPage.fRangeFirst = page.fRangeFirst;
      fPage.fClusterInfo = page.fClusterInfo;
   }
 
public:
   RPageRef() = default;
   RPageRef(const RPageRef &other) = delete;
   RPageRef &operator=(const RPageRef &other) = delete;
 
   RPageRef(RPageRef &&other) : RPageRef(other.fPage, other.fPagePool) { other.fPagePool = nullptr; }
 
   RPageRef &operator=(RPageRef &&other)
   {
      if (this != &other) {
         std::swap(fPage, other.fPage);
         std::swap(fPagePool, other.fPagePool);
      }
      return *this;
   }
 
   ~RPageRef()
   {
      if (fPagePool)
         fPagePool->ReleasePage(fPage);
   }
 
   /// Used by the friend virtual page source to map the cluster ID to its virtual counterpart
   void ChangeClusterId(DescriptorId_t clusterId)
   {
      fPage.fClusterInfo = RPage::RClusterInfo(clusterId, fPage.fClusterInfo.GetIndexOffset());
   }
 
   const RPage &Get() const { return fPage; }
};
 
} // namespace Internal
 
} // namespace Experimental
} // namespace ROOT
 
#endif