gui: the sessions warm-up hands its garbage back to the system when it ends, so a magpie just started doesn't read ~470 MB at rest for its first two minutes (H20 on Discord)

The warm-up reads every session file for the Sessions page and Usage › Requests. On a 13.5k-session history it allocates ~830 MB (QueryPage ~570, StatsFor ~260) to keep ~80 MB of indexes, and Go's scavenger returns the freed pages only over the next minutes. warmUp now calls debug.FreeOSMemory once its reads are done: one forced GC and scavenge, once, at start.

RSS of a linux nogui `magpie web` in Docker on a copy of a real config with those sessions mounted read-only, sampled every 15 s from start (with the dsh round change before it): 15 s 472 -> 197 MB, 30 s 475 -> 285, 60 s 417 -> 281, 2 min 337 -> 281, 3.5 min 266 -> 218, 12 min ~250 -> ~204. TestWarmUpReturnsItsGarbage fails without the FreeOSMemory call (10 of 10: 46-334 MB of freed heap still held, or the heap not yet collected) and passes 10 of 10 with it. go test -tags nogui ./internal/gui ./internal/sessions (Docker golang:1.26.8, temp HOME), go vet -tags nogui ./internal/gui for linux, windows and darwin, and gofmt -l pass. Not measured on macOS: a fresh build there hangs at exec behind syspolicyd today.

internal/gui/sessions.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"os"
 	"runtime"
+	"runtime/debug"
 	"strconv"
 	"strings"
 	"sync"
@@ -320,12 +321,27 @@ func warmSessions() {
 	}
 	go func() {
 		time.Sleep(3 * time.Second)
-		statsFor(0)
-		statsFor(30)
-		usage.QueryPage(usage.All, usage.Filter{}, 0, 50)
+		warmUp(
+			func() { statsFor(0) },
+			func() { statsFor(30) },
+			func() { usage.QueryPage(usage.All, usage.Filter{}, 0, 50) },
+		)
 	}()
 }
 
+// warmUp runs the warm-up's reads, then hands what they threw away back to
+// the system at once. Reading a long history allocates many times what it
+// keeps (a 13k-session Codex and Claude Code history: ~830 MB allocated to
+// keep ~80 MB of indexes), and Go's scavenger returns those pages only over
+// the next minutes: a magpie that had just started read ~470 MB for its
+// first two minutes at rest.
+func warmUp(steps ...func()) {
+	for _, step := range steps {
+		step()
+	}
+	debug.FreeOSMemory()
+}
+
 // openTerminal runs a command in the chosen Mac terminal through a .command
 // file, as it would open from Finder: no Automation consent.
 // The shell is left open when the agent quits.

internal/gui/sessions_warm_test.go

@@ -0,0 +1,28 @@
+package gui
+
+import (
+	"runtime"
+	"testing"
+)
+
+// The warm-up's garbage goes back to the system when it ends, not over the
+// scavenger's next minutes: an idle magpie that just started read ~470 MB.
+func TestWarmUpReturnsItsGarbage(t *testing.T) {
+	var keep [][]byte
+	warmUp(func() {
+		// a read that allocates far more than it keeps
+		for range 256 {
+			keep = append(keep, make([]byte, 1<<20))
+		}
+		keep = [][]byte{keep[0]}
+	})
+	var m runtime.MemStats
+	runtime.ReadMemStats(&m)
+	if held := m.HeapIdle - m.HeapReleased; held > 32<<20 {
+		t.Fatalf("after the warm-up %d MB of freed heap is still held from the system", held>>20)
+	}
+	if m.HeapAlloc > 128<<20 {
+		t.Fatalf("after the warm-up %d MB of heap is still in use", m.HeapAlloc>>20)
+	}
+	runtime.KeepAlive(keep)
+}