Space reconciliation passes by ten times the last pass's time (closes #250)

The reconciliation pass reads a row and stats a file for every cached
file, and it ran on every eviction tick, so on a large cache a pass
longer than the tick was followed by the next one at once. It still runs
at startup and is considered on each tick, but it now starts no sooner
than ten times as long after the last pass ended as that pass took.
Eviction and the removal of expired negative cache rows still run on
every tick. The comments that bound the drift, and README.md, give the
new bound.

Model: opus-5-5

README.md

@@ -199,7 +199,11 @@ into place, so a crash leaves no file under its own name with part of its data.
 When pixa starts, and every five minutes after, it removes each cached source or
 transformed image whose file is empty, or not the size the database records for
 it, with a warning naming the file; the next request for it fetches the source
-or converts the image again.
+or converts the image again. Looking for those files reads the database once for
+every cached file, so pixa skips each five-minute look that would start less
+than ten times as long after the last look ended as that look took. After a look
+that took one minute, the next one starts 10 to 15 minutes after that look
+ended.
 
 pixa's target is 1-5k r/s, which has not been measured at that rate (see Load
 Test). Toward it, pixa keeps in memory the content types of the 10,000

TODO.md

@@ -30,6 +30,9 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-18 the next reconciliation pass waits at least ten times as long as
+  the last one took, so passes on a large cache no longer run back to back
+  (closes #250).
 - 2026-10-18 a `blocked_networks` entry in the IPv4-mapped form refused nothing;
   `::ffff:10.0.0.0/104` is now read as `10.0.0.0/8`, and such an entry shorter
   than `/96` aborts startup (closes #301).

internal/imgcache/eviction.go

@@ -18,6 +18,14 @@ import (
 // write-pressure wakeups triggered by stores.
 const DefaultEvictionInterval = 5 * time.Minute
 
+// reconciliationWaitFactor is how many times as long as a
+// reconciliation pass took the eviction loop waits at least, after that
+// pass ends, before it starts the next one. A pass reads a row and stats a
+// file for every cached file, so its time grows with the cache; waiting
+// ten times as long keeps reconciliation to at most a tenth of the time,
+// however large the cache is.
+const reconciliationWaitFactor = 10
+
 // evictionBatchSize is how many LRU candidates of each class (variants
 // and source blobs) one eviction pass fetches from the database.
 const evictionBatchSize = 100
@@ -438,10 +446,12 @@ func (c *Cache) notifyWritePressure() {
 }
 
 // StartEviction launches the background eviction goroutine, which
-// reconciles the database accounting with the cache directories and
 // removes the expired negative cache rows at startup and again on every
-// periodic tick thereafter, and evicts to the configured limit on the
+// periodic tick thereafter, reconciles the database accounting with the
-// given periodic interval and on write-pressure notifications. On a
+// cache directories at startup and again on the first periodic tick
+// that comes at least ten times as long after a reconciliation pass
+// ended as that pass took, and evicts to the configured limit on the given
+// periodic interval and on write-pressure notifications. On a
 // disabled cache it only removes the expired negative cache rows, as
 // the negative cache is used either way. It is a no-op when already
 // started. The goroutine outlives the caller, so it runs with its own
@@ -482,7 +492,7 @@ func (c *Cache) StopEviction(ctx context.Context) error {
 func (c *Cache) evictionLoop(ctx context.Context, interval time.Duration) {
 	defer close(c.evictionDone)
 
-	c.runReconciliationPass(ctx)
+	nextReconciliation := c.runReconciliationPass(ctx)
 	c.runCleanExpired(ctx)
 	c.runEvictionPass(ctx)
 
@@ -494,16 +504,20 @@ func (c *Cache) evictionLoop(ctx context.Context, interval time.Duration) {
 		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			// Reconciliation walks the cache directories, so it only
+			// Reconciliation walks the cache directories and reads a
-			// runs on the periodic ticker rather than on every
+			// row for every file, so it runs only on the periodic
-			// write-pressure wakeup, keeping it off the per-store hot
+			// ticker rather than on every write-pressure wakeup, and
-			// path. Reusing the eviction interval itself (rather than a
+			// only on the first tick at or after nextReconciliation,
-			// separate, longer one) is a deliberate choice: it is the
+			// which is ten times as long after the last pass ended as
-			// simplest option that still bounds how long a store's
+			// that pass took. Passes then never run back to back,
-			// best-effort accounting insert can stay silently
+			// however long one takes. A file whose best-effort
-			// unaccounted for to one interval, on a process that is
+			// accounting insert failed in a store stays unaccounted for
-			// already running this loop regardless.
+			// until the next pass, up to about one eviction interval
-			c.runReconciliationPass(ctx)
+			// plus ten times the last pass's time.
+			if !time.Now().Before(nextReconciliation) {
+				nextReconciliation = c.runReconciliationPass(ctx)
+			}
+
 			c.runCleanExpired(ctx)
 		case <-c.evictionPressure:
 		}
@@ -527,17 +541,25 @@ func (c *Cache) runEvictionPass(ctx context.Context) {
 }
 
 // runReconciliationPass runs one reconciliation pass, logging failures
-// instead of propagating them (the loop must keep running). It does
+// instead of propagating them (the loop must keep running), and returns
-// nothing once ctx is cancelled.
+// the earliest time the next pass may start: reconciliationWaitFactor
-func (c *Cache) runReconciliationPass(ctx context.Context) {
+// times as long after this pass ended as it took. It does nothing once
+// ctx is cancelled.
+func (c *Cache) runReconciliationPass(ctx context.Context) time.Time {
 	if ctx.Err() != nil {
-		return
+		return time.Now()
 	}
 
+	started := time.Now()
+
 	err := c.reconcileAccounting(ctx)
 	if err != nil {
 		c.log.Warn("cache accounting reconciliation failed", "error", err)
 	}
+
+	ended := time.Now()
+
+	return ended.Add(reconciliationWaitFactor * ended.Sub(started))
 }
 
 // runCleanExpired runs CleanExpired, logging a failure instead of
@@ -556,7 +578,8 @@ func (c *Cache) runCleanExpired(ctx context.Context) {
 
 // reconcileAccounting synchronizes the database size accounting with
 // the actual contents of the cache directories. It runs at startup and
-// again on every periodic eviction tick thereafter, off the request
+// then on the first periodic eviction tick that comes at least ten times
+// as long after the last pass ended as that pass took, off the request
 // hot path: it adopts variant files that predate the accounting table
 // (or whose accounting insert failed, e.g. StoreVariant's best-effort
 // insert under transient DB contention), drops accounting rows whose
@@ -566,9 +589,10 @@ func (c *Cache) runCleanExpired(ctx context.Context) {
 // files left behind by crashed writes, and last checks the total cache
 // usage against the tables. It reads the tables a page at a time.
 // Running it periodically, not just once, bounds how long such drift can
-// accumulate unaccounted for on a long-running process to one eviction
+// accumulate unaccounted for on a long-running process to about one
-// interval. Once ctx is cancelled, it stops at the next file or row and
+// eviction interval plus ten times the last pass's time.
-// returns ctx's error.
+// Once ctx is cancelled, it stops at the next file or row and returns
+// ctx's error.
 func (c *Cache) reconcileAccounting(ctx context.Context) error {
 	if c.disabled {
 		return nil