snapshot of https://github.com/seaweedfs/seaweedfs.git (master, 2026-09-16; blobs > 131072B omitted)

weed/filer/reader_pattern.go

@@ -0,0 +1,66 @@
+package filer
+
+import (
+	"sync/atomic"
+)
+
+type ReaderPattern struct {
+	isSequentialCounter int64
+	readFrontier        int64 // highest (offset+size) observed across reads
+}
+
+const ModeChangeLimit = 3
+
+// SeqTolerance: a read whose start is within this many bytes of the current read
+// frontier still counts as sequential. Using a tolerance window rather than
+// strict contiguity absorbs reordered/concurrent readahead (multiple ReadAt can
+// be in flight at once) while still rejecting far random jumps.
+const SeqTolerance = 8 << 20 // 8 MiB
+
+// For streaming read: only cache the first chunk
+// For random read: only fetch the requested range, instead of the whole chunk
+
+func NewReaderPattern() *ReaderPattern {
+	return &ReaderPattern{
+		isSequentialCounter: 0,
+		readFrontier:        0,
+	}
+}
+
+func (rp *ReaderPattern) MonitorReadAt(offset int64, size int) {
+	// Advance the frontier to max(frontier, offset+size) and capture, in the same
+	// CAS loop, the pre-image this read is judged against. Reading the frontier
+	// inside the loop (rather than once up front) keeps `diff` below comparing
+	// against the freshest value even if a concurrent readahead advances the
+	// frontier while we loop. Lock-free, consistent with the rest of this type.
+	end := offset + int64(size)
+	var frontier int64
+	for {
+		frontier = atomic.LoadInt64(&rp.readFrontier)
+		if end <= frontier || atomic.CompareAndSwapInt64(&rp.readFrontier, frontier, end) {
+			break
+		}
+	}
+
+	// near = this read starts within SeqTolerance of where reads had reached.
+	// Hysteresis (the ±ModeChangeLimit counter) keeps a single outlier read from
+	// flipping the mode.
+	diff := offset - frontier
+	if diff < 0 {
+		diff = -diff
+	}
+	counter := atomic.LoadInt64(&rp.isSequentialCounter)
+	if diff <= SeqTolerance {
+		if counter < ModeChangeLimit {
+			atomic.AddInt64(&rp.isSequentialCounter, 1)
+		}
+	} else {
+		if counter > -ModeChangeLimit {
+			atomic.AddInt64(&rp.isSequentialCounter, -1)
+		}
+	}
+}
+
+func (rp *ReaderPattern) IsRandomMode() bool {
+	return atomic.LoadInt64(&rp.isSequentialCounter) < 0
+}