Add the kernel module: transparent huge pages set per machine (hq issue 746)

With THP 'always' journald's mmapped journal files sit in 2 MiB folios, so each small journal write dirties and writes back 2 MiB; on the control node that saturated the root mirror holding the bus store and the store, and the controller lost its lease. The module sets the mode from a per-machine setting (default always, the kernel's own, so assigning it changes nothing until a machine sets madvise), verifies it took, rotates the journal once so new files are mapped under the new mode, gives back what it found when stopped, and serves kernel_huge_pages to measure file_thp per unit.
2026-10-14 23:31:13 +02:00
parent 0f76b6e232
commit 3a84044ece
10 changed files with 855 additions and 0 deletions
@@ -0,0 +1,50 @@
+// The kernel module's tools bundle (novox/hq to-be 42 Phase 1 row 11, issue 746).
+//
+// Served by the node's runtime over MCP on stdio through the Go SDK (ADR 0188, ADR 0193). It only reads:
+// the transparent huge pages the kernel runs with, and which units hold file pages in huge folios, which
+// is what a small write to a mapped file costs a whole huge page of writeback for (issue 746).
+package main
+
+import (
+	"fmt"
+	"os"
+
+	stdio "git.novox.be/novox/mesh-sdk/go"
+)
+
+// binaryName is what the build names this bundle's executable: the manifest's `binary`.
+const binaryName = "kernel-tools"
+
+func main() {
+	// An empty name serves as the module the runtime names (MESH_SERVED_MODULE): kernel.
+	if err := stdio.Serve("", tools(ThisMachine())); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+func tools(m *Machine) []stdio.Tool {
+	return []stdio.Tool{
+		{
+			Name: "kernel_huge_pages",
+			Description: "Transparent huge pages on this machine: the mode in force (always, madvise or never) and defrag, " +
+				"each per-size control with what inherit resolves to, the machine's huge pages by kind (anonymous, " +
+				"shared memory, file, file mapped at 2 MiB) from /proc/meminfo, and the units holding the most file " +
+				"pages in huge folios (memory.stat file_thp) with their dirty and writeback bytes and the bytes their " +
+				"cgroup has written (io.stat, the most any one device saw). A unit with file_thp that writes far more " +
+				"than its files grow is paying a whole huge page of writeback per small write (novox/hq issue 746). " +
+				"Replaces cat /sys/kernel/mm/transparent_hugepage/enabled, grep Huge /proc/meminfo and reading " +
+				"memory.stat by hand. (r)",
+			Input: schema(map[string]any{
+				"units": map[string]any{"type": "integer", "description": "how many units to name, the most file_thp first (default 10, 0 for none, at most 100)"},
+			}),
+			Run: func(args map[string]any) (any, error) {
+				n, err := whole(args, "units", 10, 0, 100)
+				if err != nil {
+					return nil, err
+				}
+				return m.HugePages(n)
+			},
+		},
+	}
+}