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Chris LuandGitHub
8d388acc0e
* mount: tolerance-window write pattern detection for concurrent writeback WriterPattern decides whether each dirty chunk is buffered in RAM (NewMemChunk) or spilled to an on-disk swap file (NewSwapFileChunk), so a sequential stream misclassified as random is pushed through disk-backed swap. The old detector compared each write's start to the previous write's exact stop offset (atomic.Swap of lastWriteStopOffset, then lastOffset == offset). That is brittle under concurrent FUSE writeback: MonitorWriteAt runs before the per-handle write lock, so parallel Write upcalls interleave the swap, and writeback flushes dirty pages slightly out of offset order. A genuinely sequential stream then repeatedly reads as random and spills to swap. Replace the exact match with the same frontier + tolerance approach used on the read side: track a never-regressing max frontier (offset+size) via a CAS loop and treat a write whose start is within SeqTolerance (8 MiB) of that frontier as sequential. The existing +/-ModeChangeLimit hysteresis is kept, so a single outlier write can't flip the mode. Adds page_writer_pattern_test.go covering sequentiality, hysteresis, reorder tolerance, the inclusive tolerance boundary, frontier non-regression (the CAS invariant), and recovery back to sequential mode. * mount: read write frontier inside the CAS loop Capture the frontier from the CAS loop's own load rather than a separate up-front snapshot, so the sequentiality diff is judged against the freshest pre-image even if a concurrent writeback upcall advances the frontier while we loop. Also drops the now-redundant load. Behavior is unchanged for the single-threaded tests; addresses review feedback on #9984.
69 lines
2.3 KiB
Go
69 lines
2.3 KiB
Go
package mount
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import "sync/atomic"
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type WriterPattern struct {
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isSequentialCounter int64
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writeFrontier int64 // highest (offset+size) observed across writes
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chunkSize int64
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}
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const ModeChangeLimit = 3
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// SeqTolerance: a write whose start is within this many bytes of the current
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// write frontier still counts as sequential. Using a tolerance window rather
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// than strict contiguity absorbs reordered/concurrent FUSE writeback — multiple
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// Write upcalls for one handle interleave (MonitorWriteAt runs before the
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// per-handle write lock) and writeback flushes dirty pages slightly out of
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// offset order — while still rejecting far random seeks.
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const SeqTolerance = 8 << 20 // 8 MiB
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// For streaming write: keep dirty chunks in memory (NewMemChunk).
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// For random write: fall back to the on-disk swap file (NewSwapFileChunk).
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func NewWriterPattern(chunkSize int64) *WriterPattern {
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return &WriterPattern{
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isSequentialCounter: 0,
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writeFrontier: 0,
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chunkSize: chunkSize,
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}
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}
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func (rp *WriterPattern) MonitorWriteAt(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 write 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 writeback upcall advances
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// the frontier while we loop. Lock-free, consistent with the atomic counter.
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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.writeFrontier)
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if end <= frontier || atomic.CompareAndSwapInt64(&rp.writeFrontier, frontier, end) {
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break
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}
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}
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// near = this write starts within SeqTolerance of where writes had reached.
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// Hysteresis (the ±ModeChangeLimit counter) keeps a single outlier write
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// from flipping the mode and spilling a sequential stream to the swap file.
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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 {
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if counter > -ModeChangeLimit {
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atomic.AddInt64(&rp.isSequentialCounter, -1)
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}
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}
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}
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func (rp *WriterPattern) IsSequentialMode() bool {
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return atomic.LoadInt64(&rp.isSequentialCounter) >= 0
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}
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