RClusterPool.cxx

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/// \file RClusterPool.cxx
/// \author Jakob Blomer <jblomer@cern.ch>
/// \date 2020-03-11
/// \warning This is part of the ROOT 7 prototype! It will change without notice. It might trigger earthquakes. Feedback
/// is welcome!
 
/*************************************************************************
 * Copyright (C) 1995-2020, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/
 
#include <ROOT/RClusterPool.hxx>
#include <ROOT/RNTupleDescriptor.hxx>
#include <ROOT/RPageStorage.hxx>
 
#include <TError.h>
 
#include <algorithm>
#include <chrono>
#include <future>
#include <iostream>
#include <iterator>
#include <map>
#include <memory>
#include <mutex>
#include <set>
#include <utility>
 
bool ROOT::Internal::RClusterPool::RInFlightCluster::operator<(const RInFlightCluster &other) const
{
   if (fClusterKey.fClusterId == other.fClusterKey.fClusterId) {
      if (fClusterKey.fPhysicalColumnSet.size() == other.fClusterKey.fPhysicalColumnSet.size()) {
         for (auto itr1 = fClusterKey.fPhysicalColumnSet.begin(), itr2 = other.fClusterKey.fPhysicalColumnSet.begin();
              itr1 != fClusterKey.fPhysicalColumnSet.end(); ++itr1, ++itr2) {
            if (*itr1 == *itr2)
               continue;
            return *itr1 < *itr2;
         }
         // *this == other
         return false;
      }
      return fClusterKey.fPhysicalColumnSet.size() < other.fClusterKey.fPhysicalColumnSet.size();
   }
   return fClusterKey.fClusterId < other.fClusterKey.fClusterId;
}
 
ROOT::Internal::RClusterPool::RClusterPool(ROOT::Internal::RPageSource &pageSource, unsigned int clusterBunchSize)
   : fPageSource(pageSource), fClusterBunchSize(clusterBunchSize), fMetrics("RClusterPool")
{
   R__ASSERT(clusterBunchSize > 0);
 
   using ROOT::Experimental::Detail::RNTupleAtomicCounter;
   fCounters = std::make_unique<RCounters>(
      RCounters{*fMetrics.MakeCounter<RNTupleAtomicCounter *>("nCluster", "", "number of currently cached clusters")});
}
 
ROOT::Internal::RClusterPool::~RClusterPool()
{
   StopBackgroundThread();
}
 
void ROOT::Internal::RClusterPool::StartBackgroundThread()
{
   if (fThreadIo.joinable())
      return;
 
   fThreadIo = std::thread(&RClusterPool::ExecReadClusters, this);
}
 
void ROOT::Internal::RClusterPool::StopBackgroundThread()
{
   if (!fThreadIo.joinable())
      return;
 
   {
      // Controlled shutdown of the I/O thread
      std::unique_lock<std::mutex> lock(fLockWorkQueue);
      fReadQueue.emplace_back(RReadItem());
      fCvHasReadWork.notify_one();
   }
   fThreadIo.join();
}
 
void ROOT::Internal::RClusterPool::ExecReadClusters()
{
   std::deque<RReadItem> readItems;
   while (true) {
      {
         std::unique_lock<std::mutex> lock(fLockWorkQueue);
         fCvHasReadWork.wait(lock, [&]{ return !fReadQueue.empty(); });
         std::swap(readItems, fReadQueue);
      }
 
      while (!readItems.empty()) {
         std::vector<RCluster::RKey> clusterKeys;
         std::int64_t bunchId = -1;
         for (unsigned i = 0; i < readItems.size(); ++i) {
            const auto &item = readItems[i];
            // `kInvalidDescriptorId` is used as a marker for thread cancellation. Such item causes the
            // thread to terminate; thus, it must appear last in the queue.
            if (R__unlikely(item.fClusterKey.fClusterId == ROOT::kInvalidDescriptorId)) {
               R__ASSERT(i == (readItems.size() - 1));
               return;
            }
            if ((bunchId >= 0) && (item.fBunchId != bunchId))
               break;
            bunchId = item.fBunchId;
            clusterKeys.emplace_back(item.fClusterKey);
         }
 
         auto clusters = fPageSource.LoadClusters(clusterKeys);
         for (std::size_t i = 0; i < clusters.size(); ++i) {
            readItems[i].fPromise.set_value(std::move(clusters[i]));
         }
         readItems.erase(readItems.begin(), readItems.begin() + clusters.size());
      }
   } // while (true)
}
 
namespace {
 
/// Helper class for the (cluster, column list) pairs that should be loaded in the background
class RProvides {
   using DescriptorId_t = ROOT::DescriptorId_t;
   using ColumnSet_t = ROOT::Internal::RCluster::ColumnSet_t;
 
public:
   struct RInfo {
      std::int64_t fBunchId = -1;
      std::int64_t fFlags = 0;
      ColumnSet_t fPhysicalColumnSet;
   };
 
   static constexpr std::int64_t kFlagRequired = 0x01;
   static constexpr std::int64_t kFlagLast     = 0x02;
 
private:
   std::map<DescriptorId_t, RInfo> fMap;
 
public:
   void Insert(DescriptorId_t clusterId, const RInfo &info)
   {
      fMap.emplace(clusterId, info);
   }
 
   bool Contains(DescriptorId_t clusterId) {
      return fMap.count(clusterId) > 0;
   }
 
   std::size_t GetSize() const { return fMap.size(); }
 
   void Erase(DescriptorId_t clusterId, const ColumnSet_t &physicalColumns)
   {
      auto itr = fMap.find(clusterId);
      if (itr == fMap.end())
         return;
      ColumnSet_t d;
      std::copy_if(itr->second.fPhysicalColumnSet.begin(), itr->second.fPhysicalColumnSet.end(),
                   std::inserter(d, d.end()),
                   [&physicalColumns](DescriptorId_t needle) { return physicalColumns.count(needle) == 0; });
      if (d.empty()) {
         fMap.erase(itr);
      } else {
         itr->second.fPhysicalColumnSet = d;
      }
   }
 
   decltype(fMap)::iterator begin() { return fMap.begin(); }
   decltype(fMap)::iterator end() { return fMap.end(); }
};
 
} // anonymous namespace
 
ROOT::Internal::RCluster *
ROOT::Internal::RClusterPool::GetCluster(ROOT::DescriptorId_t clusterId, const RCluster::ColumnSet_t &physicalColumns)
{
   StartBackgroundThread(); // ensure that the thread is started (no-op if it is already running)
 
   std::unordered_set<ROOT::DescriptorId_t> keep{fPageSource.GetPinnedClusters()};
   for (auto cid : fPageSource.GetPinnedClusters()) {
      for (ROOT::DescriptorId_t i = 1, next = cid; i < 2 * fClusterBunchSize; ++i) {
         const auto currentId = next;
         auto descriptorGuard = fPageSource.FindNextClusterId(currentId, next);
         if (next == ROOT::kInvalidDescriptorId ||
             !fPageSource.GetEntryRange().IntersectsWith(descriptorGuard->GetClusterDescriptor(next))) {
            break;
         }
 
         keep.insert(next);
      }
   }
 
   RProvides provide;
   // Determine following cluster ids and the column ids that we want to make available
   RProvides::RInfo provideInfo;
   provideInfo.fPhysicalColumnSet = physicalColumns;
   provideInfo.fBunchId = fBunchId;
   provideInfo.fFlags = RProvides::kFlagRequired;
   for (ROOT::DescriptorId_t i = 0, next = clusterId; i < 2 * fClusterBunchSize; ++i) {
      if (i == fClusterBunchSize)
         provideInfo.fBunchId = ++fBunchId;
 
      auto cid = next;
      auto descriptorGuard = fPageSource.FindNextClusterId(cid, next);
      if (next != ROOT::kInvalidNTupleIndex) {
         if (!fPageSource.GetEntryRange().IntersectsWith(descriptorGuard->GetClusterDescriptor(next)))
            next = ROOT::kInvalidDescriptorId;
      }
      if (next == ROOT::kInvalidDescriptorId)
         provideInfo.fFlags |= RProvides::kFlagLast;
 
      provide.Insert(cid, provideInfo);
 
      if (next == ROOT::kInvalidDescriptorId)
         break;
      provideInfo.fFlags = 0;
   }
 
   // Clear the cache from clusters not the in the look-ahead window or the set of pinned clusters
   for (auto itr = fPool.begin(); itr != fPool.end();) {
      if (provide.Contains(itr->first)) {
         ++itr;
         continue;
      }
      if (keep.count(itr->first) > 0) {
         ++itr;
         continue;
      }
      itr = fPool.erase(itr);
      fCounters->fNCluster.Dec();
   }
 
   // Move clusters that meanwhile arrived into cache pool
   {
      // This lock is held during iteration over several data structures: the collection of in-flight clusters,
      // the current pool of cached clusters, and the set of cluster ids to be preloaded.
      // All three collections are expected to be small (certainly < 100, more likely < 10).  All operations
      // are non-blocking and moving around small items (pointers, ids, etc).  Thus the overall locking time should
      // still be reasonably small and the lock is rarely taken (usually once per cluster).
      std::lock_guard<std::mutex> lockGuard(fLockWorkQueue);
 
      for (auto itr = fInFlightClusters.begin(); itr != fInFlightClusters.end(); ) {
         R__ASSERT(itr->fFuture.valid());
         if (itr->fFuture.wait_for(std::chrono::seconds(0)) != std::future_status::ready) {
            // Remove the set of columns that are already scheduled for being loaded
            provide.Erase(itr->fClusterKey.fClusterId, itr->fClusterKey.fPhysicalColumnSet);
            ++itr;
            continue;
         }
 
         auto cptr = itr->fFuture.get();
         R__ASSERT(cptr);
 
         const bool isExpired =
            !provide.Contains(itr->fClusterKey.fClusterId) && (keep.count(itr->fClusterKey.fClusterId) == 0);
         if (isExpired) {
            cptr.reset();
            itr = fInFlightClusters.erase(itr);
            continue;
         }
 
         // Noop unless the page source has a task scheduler
         fPageSource.UnzipCluster(cptr.get());
 
         // We either put a fresh cluster into a free slot or we merge the cluster with an existing one
         auto existingCluster = fPool.find(cptr->GetId());
         if (existingCluster != fPool.end()) {
            existingCluster->second->Adopt(std::move(*cptr));
         } else {
            const auto cid = cptr->GetId();
            fPool.emplace(cid, std::move(cptr));
            fCounters->fNCluster.Inc();
         }
         itr = fInFlightClusters.erase(itr);
      }
 
      // Determine clusters which get triggered for background loading
      for (const auto &[_, cptr] : fPool) {
         provide.Erase(cptr->GetId(), cptr->GetAvailPhysicalColumns());
      }
 
      // Figure out if enough work accumulated to justify I/O calls
      bool skipPrefetch = false;
      if (provide.GetSize() < fClusterBunchSize) {
         skipPrefetch = true;
         for (const auto &kv : provide) {
            if ((kv.second.fFlags & (RProvides::kFlagRequired | RProvides::kFlagLast)) == 0)
               continue;
            skipPrefetch = false;
            break;
         }
      }
 
      // Update the work queue and the in-flight cluster list with new requests. We already hold the work queue
      // mutex
      // TODO(jblomer): we should ensure that clusterId is given first to the I/O thread.  That is usually the
      // case but it's not ensured by the code
      if (!skipPrefetch) {
         for (const auto &kv : provide) {
            R__ASSERT(!kv.second.fPhysicalColumnSet.empty());
 
            RReadItem readItem;
            readItem.fClusterKey.fClusterId = kv.first;
            readItem.fBunchId = kv.second.fBunchId;
            readItem.fClusterKey.fPhysicalColumnSet = kv.second.fPhysicalColumnSet;
 
            RInFlightCluster inFlightCluster;
            inFlightCluster.fClusterKey.fClusterId = kv.first;
            inFlightCluster.fClusterKey.fPhysicalColumnSet = kv.second.fPhysicalColumnSet;
            inFlightCluster.fFuture = readItem.fPromise.get_future();
            fInFlightClusters.emplace_back(std::move(inFlightCluster));
 
            fReadQueue.emplace_back(std::move(readItem));
         }
         if (!fReadQueue.empty())
            fCvHasReadWork.notify_one();
      }
   } // work queue lock guard
 
   return WaitFor(clusterId, physicalColumns);
}
 
ROOT::Internal::RCluster *
ROOT::Internal::RClusterPool::WaitFor(ROOT::DescriptorId_t clusterId, const RCluster::ColumnSet_t &physicalColumns)
{
   while (true) {
      // Fast exit: the cluster happens to be already present in the cache pool
      auto result = fPool.find(clusterId);
      if (result != fPool.end()) {
         bool hasMissingColumn = false;
         for (auto cid : physicalColumns) {
            if (result->second->ContainsColumn(cid))
               continue;
 
            hasMissingColumn = true;
            break;
         }
         if (!hasMissingColumn)
            return result->second.get();
      }
 
      // Otherwise the missing data must have been triggered for loading by now, so block and wait
      decltype(fInFlightClusters)::iterator itr;
      {
         std::lock_guard<std::mutex> lockGuardInFlightClusters(fLockWorkQueue);
         itr = fInFlightClusters.begin();
         for (; itr != fInFlightClusters.end(); ++itr) {
            if (itr->fClusterKey.fClusterId == clusterId)
               break;
         }
         R__ASSERT(itr != fInFlightClusters.end());
         // Note that the fInFlightClusters is accessed concurrently only by the I/O thread.  The I/O thread
         // never changes the structure of the in-flight clusters array (it does not add, remove, or swap elements).
         // Therefore, it is safe to access the element pointed to by itr here even after fLockWorkQueue
         // is released.  We need to release the lock before potentially blocking on the cluster future.
      }
 
      auto cptr = itr->fFuture.get();
      // We were blocked waiting for the cluster, so assume that nobody discarded it.
      R__ASSERT(cptr != nullptr);
 
      // Noop unless the page source has a task scheduler
      fPageSource.UnzipCluster(cptr.get());
 
      if (result != fPool.end()) {
         result->second->Adopt(std::move(*cptr));
      } else {
         const auto cid = cptr->GetId();
         fPool.emplace(cid, std::move(cptr));
         fCounters->fNCluster.Inc();
      }
 
      std::lock_guard<std::mutex> lockGuardInFlightClusters(fLockWorkQueue);
      fInFlightClusters.erase(itr);
   }
}
 
void ROOT::Internal::RClusterPool::WaitForInFlightClusters()
{
   while (true) {
      decltype(fInFlightClusters)::iterator itr;
      {
         std::lock_guard<std::mutex> lockGuardInFlightClusters(fLockWorkQueue);
         itr = fInFlightClusters.begin();
         while (itr != fInFlightClusters.end() &&
                itr->fFuture.wait_for(std::chrono::seconds(0)) == std::future_status::ready) {
            ++itr;
         }
         if (itr == fInFlightClusters.end())
            break;
      }
 
      itr->fFuture.wait();
   }
}