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 <mutex>
#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;
 
   /// TODO(jblomer): should be an efficient index structure that allows
   ///   - random insert
   ///   - random delete
   ///   - searching by page
   std::vector<RPage> fPages;
   std::vector<std::int32_t> fReferences;
   std::mutex fLock;
 
   /// 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);
 
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);
   /// Like RegisterPage() but the reference counter is initialized to 0
   void PreloadPage(RPage page);
   /// 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(ColumnId_t columnId, NTupleSize_t globalIndex);
   RPageRef GetPage(ColumnId_t columnId, 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.fColumnId = page.fColumnId;
      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 physical column and cluster IDs to ther virtual counterparts
   void ChangeIds(DescriptorId_t columnId, DescriptorId_t clusterId)
   {
      fPage.fColumnId = columnId;
      fPage.fClusterInfo = RPage::RClusterInfo(clusterId, fPage.fClusterInfo.GetIndexOffset());
   }
 
   const RPage &Get() const { return fPage; }
};
 
} // namespace Internal
 
} // namespace Experimental
} // namespace ROOT
 
#endif