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275 lines
8.1 KiB
275 lines
8.1 KiB
//
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// Written by Maxim Khitrov (November 2012)
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//
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// Package flowrate provides the tools for monitoring and limiting the flow rate
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// of an arbitrary data stream.
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package flowrate
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import (
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"math"
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"sync"
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"time"
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)
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// Monitor monitors and limits the transfer rate of a data stream.
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type Monitor struct {
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mu sync.Mutex // Mutex guarding access to all internal fields
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active bool // Flag indicating an active transfer
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start time.Duration // Transfer start time (clock() value)
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bytes int64 // Total number of bytes transferred
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samples int64 // Total number of samples taken
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rSample float64 // Most recent transfer rate sample (bytes per second)
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rEMA float64 // Exponential moving average of rSample
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rPeak float64 // Peak transfer rate (max of all rSamples)
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rWindow float64 // rEMA window (seconds)
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sBytes int64 // Number of bytes transferred since sLast
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sLast time.Duration // Most recent sample time (stop time when inactive)
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sRate time.Duration // Sampling rate
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tBytes int64 // Number of bytes expected in the current transfer
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tLast time.Duration // Time of the most recent transfer of at least 1 byte
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}
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// New creates a new flow control monitor. Instantaneous transfer rate is
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// measured and updated for each sampleRate interval. windowSize determines the
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// weight of each sample in the exponential moving average (EMA) calculation.
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// The exact formulas are:
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//
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// sampleTime = currentTime - prevSampleTime
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// sampleRate = byteCount / sampleTime
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// weight = 1 - exp(-sampleTime/windowSize)
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// newRate = weight*sampleRate + (1-weight)*oldRate
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//
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// The default values for sampleRate and windowSize (if <= 0) are 100ms and 1s,
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// respectively.
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func New(sampleRate, windowSize time.Duration) *Monitor {
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if sampleRate = clockRound(sampleRate); sampleRate <= 0 {
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sampleRate = 5 * clockRate
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}
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if windowSize <= 0 {
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windowSize = 1 * time.Second
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}
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now := clock()
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return &Monitor{
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active: true,
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start: now,
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rWindow: windowSize.Seconds(),
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sLast: now,
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sRate: sampleRate,
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tLast: now,
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}
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}
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// Update records the transfer of n bytes and returns n. It should be called
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// after each Read/Write operation, even if n is 0.
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func (m *Monitor) Update(n int) int {
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m.mu.Lock()
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m.update(n)
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m.mu.Unlock()
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return n
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}
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// Hack to set the current rEMA.
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func (m *Monitor) SetREMA(rEMA float64) {
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m.mu.Lock()
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m.rEMA = rEMA
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m.samples++
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m.mu.Unlock()
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}
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// IO is a convenience method intended to wrap io.Reader and io.Writer method
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// execution. It calls m.Update(n) and then returns (n, err) unmodified.
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func (m *Monitor) IO(n int, err error) (int, error) {
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return m.Update(n), err
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}
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// Done marks the transfer as finished and prevents any further updates or
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// limiting. Instantaneous and current transfer rates drop to 0. Update, IO, and
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// Limit methods become NOOPs. It returns the total number of bytes transferred.
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func (m *Monitor) Done() int64 {
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m.mu.Lock()
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if now := m.update(0); m.sBytes > 0 {
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m.reset(now)
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}
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m.active = false
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m.tLast = 0
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n := m.bytes
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m.mu.Unlock()
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return n
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}
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// timeRemLimit is the maximum Status.TimeRem value.
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const timeRemLimit = 999*time.Hour + 59*time.Minute + 59*time.Second
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// Status represents the current Monitor status. All transfer rates are in bytes
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// per second rounded to the nearest byte.
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type Status struct {
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Active bool // Flag indicating an active transfer
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Bytes int64 // Total number of bytes transferred
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Samples int64 // Total number of samples taken
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InstRate int64 // Instantaneous transfer rate
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CurRate int64 // Current transfer rate (EMA of InstRate)
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AvgRate int64 // Average transfer rate (Bytes / Duration)
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PeakRate int64 // Maximum instantaneous transfer rate
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BytesRem int64 // Number of bytes remaining in the transfer
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Start time.Time // Transfer start time
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Duration time.Duration // Time period covered by the statistics
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Idle time.Duration // Time since the last transfer of at least 1 byte
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TimeRem time.Duration // Estimated time to completion
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Progress Percent // Overall transfer progress
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}
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// Status returns current transfer status information. The returned value
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// becomes static after a call to Done.
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func (m *Monitor) Status() Status {
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m.mu.Lock()
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now := m.update(0)
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s := Status{
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Active: m.active,
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Start: clockToTime(m.start),
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Duration: m.sLast - m.start,
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Idle: now - m.tLast,
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Bytes: m.bytes,
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Samples: m.samples,
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PeakRate: round(m.rPeak),
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BytesRem: m.tBytes - m.bytes,
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Progress: percentOf(float64(m.bytes), float64(m.tBytes)),
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}
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if s.BytesRem < 0 {
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s.BytesRem = 0
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}
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if s.Duration > 0 {
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rAvg := float64(s.Bytes) / s.Duration.Seconds()
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s.AvgRate = round(rAvg)
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if s.Active {
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s.InstRate = round(m.rSample)
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s.CurRate = round(m.rEMA)
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if s.BytesRem > 0 {
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if tRate := 0.8*m.rEMA + 0.2*rAvg; tRate > 0 {
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ns := float64(s.BytesRem) / tRate * 1e9
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if ns > float64(timeRemLimit) {
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ns = float64(timeRemLimit)
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}
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s.TimeRem = clockRound(time.Duration(ns))
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}
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}
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}
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}
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m.mu.Unlock()
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return s
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}
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// Limit restricts the instantaneous (per-sample) data flow to rate bytes per
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// second. It returns the maximum number of bytes (0 <= n <= want) that may be
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// transferred immediately without exceeding the limit. If block == true, the
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// call blocks until n > 0. want is returned unmodified if want < 1, rate < 1,
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// or the transfer is inactive (after a call to Done).
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//
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// At least one byte is always allowed to be transferred in any given sampling
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// period. Thus, if the sampling rate is 100ms, the lowest achievable flow rate
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// is 10 bytes per second.
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//
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// For usage examples, see the implementation of Reader and Writer in io.go.
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func (m *Monitor) Limit(want int, rate int64, block bool) (n int) {
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if want < 1 || rate < 1 {
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return want
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}
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m.mu.Lock()
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// Determine the maximum number of bytes that can be sent in one sample
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limit := round(float64(rate) * m.sRate.Seconds())
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if limit <= 0 {
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limit = 1
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}
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// If block == true, wait until m.sBytes < limit
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if now := m.update(0); block {
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for m.sBytes >= limit && m.active {
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now = m.waitNextSample(now)
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}
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}
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// Make limit <= want (unlimited if the transfer is no longer active)
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if limit -= m.sBytes; limit > int64(want) || !m.active {
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limit = int64(want)
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}
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m.mu.Unlock()
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if limit < 0 {
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limit = 0
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}
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return int(limit)
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}
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// SetTransferSize specifies the total size of the data transfer, which allows
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// the Monitor to calculate the overall progress and time to completion.
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func (m *Monitor) SetTransferSize(bytes int64) {
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if bytes < 0 {
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bytes = 0
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}
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m.mu.Lock()
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m.tBytes = bytes
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m.mu.Unlock()
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}
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// update accumulates the transferred byte count for the current sample until
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// clock() - m.sLast >= m.sRate. The monitor status is updated once the current
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// sample is done.
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func (m *Monitor) update(n int) (now time.Duration) {
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if !m.active {
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return
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}
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if now = clock(); n > 0 {
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m.tLast = now
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}
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m.sBytes += int64(n)
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if sTime := now - m.sLast; sTime >= m.sRate {
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t := sTime.Seconds()
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if m.rSample = float64(m.sBytes) / t; m.rSample > m.rPeak {
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m.rPeak = m.rSample
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}
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// Exponential moving average using a method similar to *nix load
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// average calculation. Longer sampling periods carry greater weight.
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if m.samples > 0 {
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w := math.Exp(-t / m.rWindow)
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m.rEMA = m.rSample + w*(m.rEMA-m.rSample)
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} else {
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m.rEMA = m.rSample
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}
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m.reset(now)
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}
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return
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}
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// reset clears the current sample state in preparation for the next sample.
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func (m *Monitor) reset(sampleTime time.Duration) {
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m.bytes += m.sBytes
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m.samples++
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m.sBytes = 0
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m.sLast = sampleTime
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}
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// waitNextSample sleeps for the remainder of the current sample. The lock is
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// released and reacquired during the actual sleep period, so it's possible for
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// the transfer to be inactive when this method returns.
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func (m *Monitor) waitNextSample(now time.Duration) time.Duration {
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const minWait = 5 * time.Millisecond
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current := m.sLast
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// sleep until the last sample time changes (ideally, just one iteration)
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for m.sLast == current && m.active {
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d := current + m.sRate - now
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m.mu.Unlock()
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if d < minWait {
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d = minWait
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}
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time.Sleep(d)
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m.mu.Lock()
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now = m.update(0)
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}
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return now
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}
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