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https://github.com/seaweedfs/seaweedfs.git
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Chris LuandDevin
8227ae3f49
A far ReadAt on a fresh ReaderPattern left the sequential counter at -1, so the next buffer of the same ranged request landed on the frontier and flipped the verdict straight back to sequential — readChunkSliceAt then paid a whole-chunk fetch for the remainder of the range. Drop the counter to -ModeChangeLimit when random mode is entered so the verdict needs sustained sequential evidence to undo, matching the hysteresis an established sequential stream already gets. Generated with [Devin](https://devin.ai) Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
70 lines
2.3 KiB
Go
70 lines
2.3 KiB
Go
package filer
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import (
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"sync/atomic"
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)
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type ReaderPattern struct {
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isSequentialCounter int64
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readFrontier int64 // highest (offset+size) observed across reads
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}
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const ModeChangeLimit = 3
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// SeqTolerance: a read whose start is within this many bytes of the current read
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// frontier still counts as sequential. Using a tolerance window rather than
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// strict contiguity absorbs reordered/concurrent readahead (multiple ReadAt can
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// be in flight at once) while still rejecting far random jumps.
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const SeqTolerance = 8 << 20 // 8 MiB
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// For streaming read: only cache the first chunk
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// For random read: only fetch the requested range, instead of the whole chunk
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func NewReaderPattern() *ReaderPattern {
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return &ReaderPattern{
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isSequentialCounter: 0,
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readFrontier: 0,
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}
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}
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func (rp *ReaderPattern) MonitorReadAt(offset int64, size int) {
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// Advance the frontier to max(frontier, offset+size) and capture, in the same
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// CAS loop, the pre-image this read is judged against. Reading the frontier
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// inside the loop (rather than once up front) keeps `diff` below comparing
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// against the freshest value even if a concurrent readahead advances the
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// frontier while we loop. Lock-free, consistent with the rest of this type.
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end := offset + int64(size)
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var frontier int64
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for {
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frontier = atomic.LoadInt64(&rp.readFrontier)
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if end <= frontier || atomic.CompareAndSwapInt64(&rp.readFrontier, frontier, end) {
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break
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}
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}
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// near = this read starts within SeqTolerance of where reads had reached.
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// Hysteresis (the ±ModeChangeLimit counter) keeps a single outlier read from
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// flipping the mode.
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diff := offset - frontier
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if diff < 0 {
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diff = -diff
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}
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counter := atomic.LoadInt64(&rp.isSequentialCounter)
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if diff <= SeqTolerance {
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if counter < ModeChangeLimit {
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atomic.AddInt64(&rp.isSequentialCounter, 1)
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}
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} else if counter <= 0 {
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// Entering random mode is a strong verdict: drop to the bottom of
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// the window so the contiguous tail of one ranged request cannot
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// flip it back on the next buffer read and pay a whole-chunk fetch.
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atomic.StoreInt64(&rp.isSequentialCounter, -ModeChangeLimit)
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} else {
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atomic.AddInt64(&rp.isSequentialCounter, -1)
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}
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}
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func (rp *ReaderPattern) IsRandomMode() bool {
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return atomic.LoadInt64(&rp.isSequentialCounter) < 0
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}
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