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prune.go
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prune.go
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// Copyright (c) 2015-2021 The Decred developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.
package blockchain
import (
"math"
"time"
)
// poissonConfidenceSecs returns the number of seconds it will take to produce
// an event at the provided confidence level given a Poisson distribution with 1
// event occurring in the given target interval.
func poissonConfidenceSecs(targetIntervalSecs int64, confidence float64) int64 {
// The waiting times between events in a Poisson distribution are
// exponentially distributed and the CDF for an exponential distribution is:
//
// x = 1 - e^-λt
//
// Since the goal rate is 1 event per target and time is in terms of
// targetIntervalSecs, let λ = 1, i = targetIntervalSecs, t = s/i, and x =
// confidence:
// => x = 1 - e^-(s/i)
//
// Solve for s:
// => e^-(s/i) = 1 - x
// => -(s/i) = ln(1 - x)
// => s = ln(1 - x) * -i
// => s = ln1p(-confidence) * -targetIntervalSecs
//
// Extra 0.5 to round up.
return int64(math.Log1p(-confidence)*-float64(targetIntervalSecs) + 0.5)
}
// chainPruner is used to occasionally prune the blockchain of old nodes that
// can be freed to the garbage collector.
type chainPruner struct {
chain *BlockChain
lastPruneTime time.Time
pruningInterval time.Duration
// prunedPerIntervalHint is the maximum expected number of nodes that will
// be pruned per pruning interval with a high degree of confidence.
prunedPerIntervalHint int64
}
// newChainPruner returns a new chain pruner.
func newChainPruner(chain *BlockChain) *chainPruner {
// Set the pruning interval to match the target time per block.
targetTimePerBlock := chain.chainParams.TargetTimePerBlock
pruningInterval := targetTimePerBlock
pruningIntervalSecs := int64(pruningInterval.Seconds())
// Calculate the maximum expected number of nodes that will be pruned per
// interval with a 99% confidence level to use as a hint for reducing the
// number of allocations.
//
// Note that as long as the pruning interval is the same as the target block
// interval, this will always result in the same value for all networks,
// but it's better to calculate it properly so it remains accurate if the
// pruning interval is changed.
const confidence = 0.99
targetTimePerBlockSecs := int64(targetTimePerBlock.Seconds())
confidenceSecs := poissonConfidenceSecs(targetTimePerBlockSecs, confidence)
pruneHintFloat := float64(confidenceSecs) / float64(pruningIntervalSecs)
pruneHint := int64(math.Round(pruneHintFloat))
return &chainPruner{
chain: chain,
lastPruneTime: time.Now(),
pruningInterval: pruningInterval,
prunedPerIntervalHint: pruneHint,
}
}
// pruneChainIfNeeded removes references to old information that should no
// longer be held in memory if the pruning interval has elapsed.
//
// This function MUST be called with the chain lock held (for writes).
func (c *chainPruner) pruneChainIfNeeded() {
now := time.Now()
duration := now.Sub(c.lastPruneTime)
if duration < c.pruningInterval {
return
}
c.lastPruneTime = now
c.chain.pruneStakeNodes()
}