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perf(prover): reduce synchronization allocs for FFT #668

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perf(prover): reduce synchronization allocs for FFT #668

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40 changes: 19 additions & 21 deletions prover/maths/fft/fft.go
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Original file line number Diff line number Diff line change
@@ -1,9 +1,11 @@
package fft

import (
"math/bits"
"sync"

"github.com/consensys/gnark-crypto/ecc"
"github.com/consensys/linea-monorepo/prover/utils/parallel"
"math/bits"

"github.com/consensys/linea-monorepo/prover/maths/field"
)
Expand All @@ -24,7 +26,6 @@ const butterflyThreshold = 16
// if decimation == DIF (decimation in frequency), the output will be in bit-reversed order
// if coset if set, the FFT(a) returns the evaluation of a on a coset.
func (domain *Domain) FFT(a []field.Element, decimation Decimation, opts ...Option) {

opt := fftOptions(opts...)

// find the stage where we should stop spawning go routines in our recursive calls
Expand All @@ -43,7 +44,6 @@ func (domain *Domain) FFT(a []field.Element, decimation Decimation, opts ...Opti
}
if decimation == DIT {
scale(domain.CosetTableReversed)

} else {
scale(domain.CosetTable)
}
Expand Down Expand Up @@ -108,9 +108,9 @@ func (domain *Domain) FFTInverse(a []field.Element, decimation Decimation, opts

}

func difFFT(a []field.Element, twiddles [][]field.Element, stage int, maxSplits int, chDone chan struct{}, nbTasks int) {
if chDone != nil {
defer close(chDone)
func difFFT(a []field.Element, twiddles [][]field.Element, stage int, maxSplits int, wg *sync.WaitGroup, nbTasks int) {
if wg != nil {
defer wg.Done()
}

n := len(a)
Expand All @@ -122,14 +122,13 @@ func difFFT(a []field.Element, twiddles [][]field.Element, stage int, maxSplits
}

m := n >> 1

parallelButterfly := (m > butterflyThreshold) && (stage < maxSplits)

// i == 0
if parallelButterfly {
parallel.Execute(m, func(start, end int) {
innerDIFWithTwiddles(a, twiddles[stage], start, end, m)
}, nbTasks/(1<<(stage)))
}, nbTasks/(1<<stage))
} else {
innerDIFWithTwiddles(a, twiddles[stage], 0, m, m)
}
Expand All @@ -140,19 +139,20 @@ func difFFT(a []field.Element, twiddles [][]field.Element, stage int, maxSplits

nextStage := stage + 1
if stage < maxSplits {
chDone := make(chan struct{}, 1)
go difFFT(a[m:n], twiddles, nextStage, maxSplits, chDone, nbTasks)
var wg sync.WaitGroup
wg.Add(1)
go difFFT(a[m:n], twiddles, nextStage, maxSplits, &wg, nbTasks)
difFFT(a[0:m], twiddles, nextStage, maxSplits, nil, nbTasks)
<-chDone
wg.Wait()
} else {
difFFT(a[0:m], twiddles, nextStage, maxSplits, nil, nbTasks)
difFFT(a[m:n], twiddles, nextStage, maxSplits, nil, nbTasks)
}
}

func ditFFT(a []field.Element, twiddles [][]field.Element, stage int, maxSplits int, chDone chan struct{}, nbTasks int) {
if chDone != nil {
defer close(chDone)
func ditFFT(a []field.Element, twiddles [][]field.Element, stage int, maxSplits int, wg *sync.WaitGroup, nbTasks int) {
if wg != nil {
defer wg.Done()
}

n := len(a)
Expand All @@ -164,17 +164,15 @@ func ditFFT(a []field.Element, twiddles [][]field.Element, stage int, maxSplits
}

m := n >> 1

parallelButterfly := (m > butterflyThreshold) && (stage < maxSplits)

nextStage := stage + 1

if stage < maxSplits {
// that's the only time we fire go routines
chDone := make(chan struct{}, 1)
go ditFFT(a[m:n], twiddles, nextStage, maxSplits, chDone, nbTasks)
var wg sync.WaitGroup
wg.Add(1)
go ditFFT(a[m:n], twiddles, nextStage, maxSplits, &wg, nbTasks)
ditFFT(a[0:m], twiddles, nextStage, maxSplits, nil, nbTasks)
<-chDone
wg.Wait()
} else {
ditFFT(a[0:m], twiddles, nextStage, maxSplits, nil, nbTasks)
ditFFT(a[m:n], twiddles, nextStage, maxSplits, nil, nbTasks)
Expand All @@ -183,7 +181,7 @@ func ditFFT(a []field.Element, twiddles [][]field.Element, stage int, maxSplits
if parallelButterfly {
parallel.Execute(m, func(start, end int) {
innerDITWithTwiddles(a, twiddles[stage], start, end, m)
}, nbTasks/(1<<(stage)))
}, nbTasks/(1<<stage))
} else {
innerDITWithTwiddles(a, twiddles[stage], 0, m, m)
}
Expand Down