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Reference Guide
ROOT::TTreeProcessorMT Class Reference

A class to process the entries of a TTree in parallel.

By means of its Process method, ROOT::TTreeProcessorMT provides a way to process the entries of a TTree in parallel. When invoking TTreeProcessor::Process, the user passes a function whose only parameter is a TTreeReader. The function iterates on a subrange of entries by using that TTreeReader.

The implementation of ROOT::TTreeProcessorMT parallelizes the processing of the subranges, each corresponding to a cluster in the TTree. This is possible thanks to the use of a ROOT::TThreadedObject, so that each thread works with its own TFile and TTree objects.

Definition at line 84 of file TTreeProcessorMT.hxx.

Public Member Functions

 TTreeProcessorMT (const std::vector< std::string_view > &filenames, std::string_view treename="", UInt_t nThreads=0u)
 Constructor based on a collection of file names. More...
 
 TTreeProcessorMT (std::string_view filename, std::string_view treename="", UInt_t nThreads=0u)
 Constructor based on a file name. More...
 
 TTreeProcessorMT (TTree &tree, const TEntryList &entries, UInt_t nThreads=0u)
 Constructor based on a TTree and a TEntryList. More...
 
 TTreeProcessorMT (TTree &tree, UInt_t nThreads=0u)
 Constructor based on a TTree. More...
 
void Process (std::function< void(TTreeReader &)> func)
 Process the entries of a TTree in parallel. More...
 

Static Public Member Functions

static unsigned int GetMaxTasksPerFilePerWorker ()
 Sets the maximum number of tasks created per file, per worker. More...
 
static void SetMaxTasksPerFilePerWorker (unsigned int m)
 Sets the maximum number of tasks created per file, per worker. More...
 

Private Member Functions

std::vector< std::string > FindTreeNames ()
 Retrieve the names of the TTrees in each of the input files, throw if a TTree cannot be found. More...
 
Internal::FriendInfo GetFriendInfo (TTree &tree)
 Get and store the names, aliases and file names of the friends of the tree. More...
 

Private Attributes

TEntryList fEntryList
 User-defined selection of entry numbers to be processed, empty if none was provided. More...
 
const std::vector< std::string > fFileNames
 Names of the files. More...
 
const Internal::FriendInfo fFriendInfo
 
ROOT::TThreadExecutor fPool
 ! Thread pool for processing. More...
 
const std::vector< std::string > fTreeNames
 TTree names (always same size and ordering as fFileNames) More...
 
ROOT::TThreadedObject< ROOT::Internal::TTreeViewfTreeView {TNumSlots{ROOT::GetThreadPoolSize()}}
 Thread-local TreeViews. More...
 

Static Private Attributes

static unsigned int fgMaxTasksPerFilePerWorker = 24U
 

#include <ROOT/TTreeProcessorMT.hxx>

Constructor & Destructor Documentation

◆ TTreeProcessorMT() [1/4]

TTreeProcessorMT::TTreeProcessorMT ( std::string_view  filename,
std::string_view  treename = "",
UInt_t  nThreads = 0u 
)

Constructor based on a file name.

Parameters
[in]filenameName of the file containing the tree to process.
[in]treenameName of the tree to process. If not provided, the implementation will search for a TTree key in the file and will use the first one it finds.
[in]nThreadsNumber of threads to create in the underlying thread-pool. The semantics of this argument are the same as for TThreadExecutor.

Definition at line 455 of file TTreeProcessorMT.cxx.

◆ TTreeProcessorMT() [2/4]

TTreeProcessorMT::TTreeProcessorMT ( const std::vector< std::string_view > &  filenames,
std::string_view  treename = "",
UInt_t  nThreads = 0u 
)

Constructor based on a collection of file names.

Parameters
[in]filenamesCollection of the names of the files containing the tree to process.
[in]treenameName of the tree to process. If not provided, the implementation will search filenames for a TTree key and will use the first one it finds in each file.
[in]nThreadsNumber of threads to create in the underlying thread-pool. The semantics of this argument are the same as for TThreadExecutor.

If different files contain TTrees with different names and automatic TTree name detection is not an option (for example, because some of the files contain multiple TTrees) please manually create a TChain and pass it to the appropriate TTreeProcessorMT constructor.

Definition at line 486 of file TTreeProcessorMT.cxx.

◆ TTreeProcessorMT() [3/4]

TTreeProcessorMT::TTreeProcessorMT ( TTree tree,
const TEntryList entries,
UInt_t  nThreads = 0u 
)

Constructor based on a TTree and a TEntryList.

Parameters
[in]treeTree or chain of files containing the tree to process.
[in]entriesList of entry numbers to process.
[in]nThreadsNumber of threads to create in the underlying thread-pool. The semantics of this argument are the same as for TThreadExecutor.

Definition at line 528 of file TTreeProcessorMT.cxx.

◆ TTreeProcessorMT() [4/4]

TTreeProcessorMT::TTreeProcessorMT ( TTree tree,
UInt_t  nThreads = 0u 
)

Constructor based on a TTree.

Parameters
[in]treeTree or chain of files containing the tree to process.
[in]nThreadsNumber of threads to create in the underlying thread-pool. The semantics of this argument are the same as for TThreadExecutor.

Definition at line 540 of file TTreeProcessorMT.cxx.

Member Function Documentation

◆ FindTreeNames()

std::vector< std::string > TTreeProcessorMT::FindTreeNames ( )
private

Retrieve the names of the TTrees in each of the input files, throw if a TTree cannot be found.

Definition at line 421 of file TTreeProcessorMT.cxx.

◆ GetFriendInfo()

Internal::FriendInfo TTreeProcessorMT::GetFriendInfo ( TTree tree)
private

Get and store the names, aliases and file names of the friends of the tree.

Parameters
[in]treeThe main tree whose friends to

Note that "friends of friends" and circular references in the lists of friends are not supported.

Definition at line 367 of file TTreeProcessorMT.cxx.

◆ GetMaxTasksPerFilePerWorker()

unsigned int TTreeProcessorMT::GetMaxTasksPerFilePerWorker ( )
static

Sets the maximum number of tasks created per file, per worker.

Returns
The maximum number of tasks created per file, per worker

Definition at line 625 of file TTreeProcessorMT.cxx.

◆ Process()

void TTreeProcessorMT::Process ( std::function< void(TTreeReader &)>  func)

Process the entries of a TTree in parallel.

The user-provided function receives a TTreeReader which can be used to iterate on a subrange of entries

// Select branches to read
while (readerSubRange.Next()) {
// Use content of current entry
}
});

The user needs to be aware that each of the subranges can potentially be processed in parallel. This means that the code of the user function should be thread safe.

Parameters
[in]funcUser-defined function that processes a subrange of entries

Definition at line 561 of file TTreeProcessorMT.cxx.

◆ SetMaxTasksPerFilePerWorker()

void TTreeProcessorMT::SetMaxTasksPerFilePerWorker ( unsigned int  maxTasksPerFile)
static

Sets the maximum number of tasks created per file, per worker.

Parameters
[in]maxTasksPerFileName of the file containing the tree to process.

This allows to create a reasonable number of tasks even if any of the processed files features a bad clustering, for example with a lot of entries and just a few entries per cluster.

Definition at line 637 of file TTreeProcessorMT.cxx.

Member Data Documentation

◆ fEntryList

TEntryList ROOT::TTreeProcessorMT::fEntryList
private

User-defined selection of entry numbers to be processed, empty if none was provided.

Definition at line 89 of file TTreeProcessorMT.hxx.

◆ fFileNames

const std::vector<std::string> ROOT::TTreeProcessorMT::fFileNames
private

Names of the files.

Definition at line 86 of file TTreeProcessorMT.hxx.

◆ fFriendInfo

const Internal::FriendInfo ROOT::TTreeProcessorMT::fFriendInfo
private

Definition at line 90 of file TTreeProcessorMT.hxx.

◆ fgMaxTasksPerFilePerWorker

unsigned int ROOT::TTreeProcessorMT::fgMaxTasksPerFilePerWorker = 24U
staticprivate

Definition at line 99 of file TTreeProcessorMT.hxx.

◆ fPool

ROOT::TThreadExecutor ROOT::TTreeProcessorMT::fPool
private

! Thread pool for processing.

Definition at line 91 of file TTreeProcessorMT.hxx.

◆ fTreeNames

const std::vector<std::string> ROOT::TTreeProcessorMT::fTreeNames
private

TTree names (always same size and ordering as fFileNames)

Definition at line 87 of file TTreeProcessorMT.hxx.

◆ fTreeView

ROOT::TThreadedObject<ROOT::Internal::TTreeView> ROOT::TTreeProcessorMT::fTreeView {TNumSlots{ROOT::GetThreadPoolSize()}}
private

Thread-local TreeViews.

Definition at line 95 of file TTreeProcessorMT.hxx.

Libraries for ROOT::TTreeProcessorMT:
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The documentation for this class was generated from the following files:
TTreeReader
A simple, robust and fast interface to read values from ROOT columnar datasets such as TTree,...
Definition: TTreeReader.h:44
ROOT::TTreeProcessorMT::Process
void Process(std::function< void(TTreeReader &)> func)
Process the entries of a TTree in parallel.
Definition: TTreeProcessorMT.cxx:561
TTreeReader::Next
Bool_t Next()
Move to the next entry (or index of the TEntryList if that is set).
Definition: TTreeReader.h:182