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Fixing an O(N^2) scaling problem caused by TTree::Draw() #650

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Fixing an O(N^2) scaling problem caused by TTree::Draw() #650

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Fixing an O(N^2) scaling problem caused by TTree::Draw() 
 * TTreeFormala::GetRealInstance repeatedly went trough a while loop
   that was trasversed again and again for each new query
   (with lineary increasing instances).
   This led to a real problem when drawing on a branch that consistent
   of many data objects (themselves containing small variable sized containers).

 * The problem is fixed by caching the state of a previous call.
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@sawenzel
sawenzel committed Jun 21, 2017
commit c172e229906f34bf8544555c36f9cd20bef3d733
10 changes: 10 additions & 0 deletions tree/treeplayer/inc/TTreeFormula.h
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Original file line number Diff line number Diff line change
Expand Up @@ -84,6 +84,14 @@ friend class TTreeFormulaManager;
kMaxIf = 205
};

// Helper struct to hold a cache
// that can accelerate calculation of the RealIndex.
struct RealInstanceCache {
Int_t fInstanceCache = 0;
Int_t fLocalIndexCache = 0;
Int_t fVirtAccumCache = 0;
};

TTree *fTree; //! pointer to Tree
Int_t fCodes[kMAXCODES]; // List of leaf numbers referenced in formula
Int_t fNdata[kMAXCODES]; //! This caches the physical number of element in the leaf or datamember.
Expand Down Expand Up @@ -121,6 +129,8 @@ friend class TTreeFormulaManager;

LongDouble_t* fConstLD; //! local version of fConsts able to store bigger numbers

RealInstanceCache fRealInstanceCache; //! Cache accelerating the GetRealInstance function

TTreeFormula(const char *name, const char *formula, TTree *tree, const std::vector<std::string>& aliases);
void Init(const char *name, const char *formula);
Bool_t BranchHasMethod(TLeaf* leaf, TBranch* branch, const char* method,const char* params, Long64_t readentry) const;
Expand Down
19 changes: 14 additions & 5 deletions tree/treeplayer/src/TTreeFormula.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -3412,8 +3412,14 @@ Int_t TTreeFormula::GetRealInstance(Int_t instance, Int_t codeindex) {
}
break;
case -1: {
local_index = 0;
Int_t virt_accum = 0;
if (instance <= fRealInstanceCache.fInstanceCache) {
fRealInstanceCache.fLocalIndexCache = 0;
fRealInstanceCache.fVirtAccumCache = 0;
}
fRealInstanceCache.fInstanceCache = instance;
local_index = fRealInstanceCache.fLocalIndexCache;
Int_t virt_accum = fRealInstanceCache.fVirtAccumCache;

Int_t maxloop = fManager->fCumulUsedVarDims->GetSize();
if (maxloop == 0) {
local_index--;
Expand All @@ -3424,12 +3430,15 @@ Int_t TTreeFormula::GetRealInstance(Int_t instance, Int_t codeindex) {
virt_accum += fManager->fCumulUsedVarDims->GetArray()[local_index];
local_index++;
} while( instance >= virt_accum && local_index<maxloop);
if (local_index==maxloop && (instance >= virt_accum)) {
local_index--;
local_index--;
// update the cache
fRealInstanceCache.fVirtAccumCache = virt_accum - fManager->fCumulUsedVarDims->GetArray()[local_index];
fRealInstanceCache.fLocalIndexCache = local_index;

if (local_index==(maxloop-1) && (instance >= virt_accum)) {
instance = fNdata[0]+1; // out of bounds.
if (check) return fNdata[0]+1;
} else {
local_index--;
if (fManager->fCumulUsedVarDims->At(local_index)) {
instance -= (virt_accum - fManager->fCumulUsedVarDims->At(local_index));
} else {
Expand Down