cmd/btrfs-rec: Have all logging include live memory statistics

1 files changed, 132 insertions, 0 deletions

diff --git a/lib/textui/log_memstats.go b/lib/textui/log_memstats.go
new file mode 100644
index 0000000..39733c6
--- /dev/null
+++ b/lib/textui/log_memstats.go
@@ -0,0 +1,132 @@
+// Copyright (C) 2022  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package textui
+
+import (
+	"fmt"
+	"runtime"
+	"sync"
+	"time"
+)
+
+type LiveMemUse struct {
+	mu    sync.Mutex
+	stats runtime.MemStats
+	last  time.Time
+}
+
+var _ fmt.Stringer = (*LiveMemUse)(nil)
+
+const liveMemUseUpdateInterval = 1 * time.Second
+
+func (o *LiveMemUse) String() string {
+	o.mu.Lock()
+
+	// runtime.ReadMemStats() calls stopTheWorld(), so we want to
+	// rate-limit how often we call it.
+	if now := time.Now(); now.Sub(o.last) > liveMemUseUpdateInterval {
+		runtime.ReadMemStats(&o.stats)
+		o.last = now
+	}
+
+	// runtime.MemStats only knows about memory managed by the Go runtime;
+	// even for a pure Go program, there's also
+	//
+	//   - memory mapped to the executable itself
+	//   - vDSO and friends
+	//
+	// But those are pretty small, just a few MiB.
+	//
+	// OK, so: memory managed by the Go runtime.  runtime.MemStats is pretty
+	// obtuse, I think it was designed more for "debugging the Go runtime"
+	// than "monitoring the behavior of a Go program".  From the Go
+	// runtime's perspective, regions of the virtual address space are in
+	// one of 4 states (see `runtime/mem.go`):
+	//
+	//  - None : not mapped
+	//
+	//  - Reserved : mapped, but without r/w permissions (PROT_NONE); so
+	//    this region isn't actually backed by anything
+	//
+	//  - Prepared : mapped, but allowed to be collected by the OS
+	//    (MADV_FREE, or MADV_DONTNEED on systems without MADV_FREE); so
+	//    this region may or may not actually be backed by anything.
+	//
+	//  - Ready : mapped, ready to be used
+	//
+	// Normal tools count Reserved+Prepared+Ready toward the VSS (which is a
+	// little silly, when inspecting /proc/{pid}/maps to calculate the VSS,
+	// IMO they should exclude maps without r/w permissions, which would
+	// exclude Reserved), but we all know that VSS numbers are over
+	// inflated.  And RSS only useful if we fit in RAM and don't spill to
+	// swap (this is being written for btrfs-rec, which is quite likely to
+	// consume all RAM on a laptop).  Useful numbers are Ready and Prepared;
+	// as I said above, outside tools reporting Ready+Prepared would be easy
+	// and useful, but none do; but I don't think outside tools have a way
+	// to distinguish between Ready and Prepared (unless you can detect
+	// MADV_FREE/MADV_DONTNEED in /proc/{pid}/smaps?).
+	//
+	// Of the 3 mapped states, here's how we get them from runtime:
+	//
+	//  - Reserved : AFAICT, you can't :(
+	//
+	//  - Prepared : `runtime.MemStats.HeapReleased`
+	//
+	//  - Ready : `runtime.MemStats.Sys - runtime.MemStats.HeapReleased`
+	//    (that is, runtime.MemStats.Sys is Prepared+Ready)
+	//
+	// It's a bummer that we can't get Reserved from runtime, but as I've
+	// said, it's not super useful; it's only use would really be
+	// cross-referencing runtime's numbers against the VSS.
+	//
+	// The godocs for runtime.MemStats.Sys say "It's likely that not all of
+	// the virtual address space is backed by physical memory at any given
+	// moment, though in general it all was at some point."  That's both
+	// confusing and a lie.  It's confusing because it doesn't give you
+	// hooks to find out more; it could have said that this is
+	// Ready+Prepared and that Prepared is the portion of the space that
+	// might not be backed by physical memory, but instead it wants you to
+	// throw your hands up and say "this is too weird for me to understand".
+	// It's a lie because "in general it all was at some point" implies that
+	// all Prepared memory was previously Ready, which is false; it can go
+	// None->Reserved->Prepared (but it only does that Reserved->Prepared
+	// transition if it thinks it will need to transition it to Ready very
+	// soon, so maybe the doc author though that was negligible?).
+	//
+	// Now, those are still pretty opaque numbers; most of runtime.MemStats
+	// goes toward accounting for what's going on inside of Ready space.
+	//
+	// For our purposes, we don't care too much about specifics of how Ready
+	// space is being used; just how much is "actually storing data", vs
+	// "overhead from heap-fragmentation", vs "idle".
+
+	var (
+		// We're going to add up all of the `o.stats.{thing}Sys`
+		// variables and check that against `o.stats.Sys`, in order to
+		// make sure that we're not missing any {thing} when adding up
+		// `inuse`.
+		calcSys = o.stats.HeapSys + o.stats.StackSys + o.stats.MSpanSys + o.stats.MCacheSys + o.stats.BuckHashSys + o.stats.GCSys + o.stats.OtherSys
+		inuse   = o.stats.HeapInuse + o.stats.StackInuse + o.stats.MSpanInuse + o.stats.MCacheInuse + o.stats.BuckHashSys + o.stats.GCSys + o.stats.OtherSys
+	)
+	if calcSys != o.stats.Sys {
+		panic("should not happen")
+	}
+	prepared := o.stats.HeapReleased
+	ready := o.stats.Sys - prepared
+
+	readyFragOverhead := o.stats.HeapInuse - o.stats.HeapAlloc
+	readyData := inuse - readyFragOverhead
+	readyIdle := ready - inuse
+
+	o.mu.Unlock()
+
+	return Sprintf("Ready+Prepared=%.1f (Ready=%.1f (data:%.1f + fragOverhead:%.1f + idle:%.1f) ; Prepared=%.1f)",
+		IEC(ready+prepared, "B"),
+		IEC(ready, "B"),
+		IEC(readyData, "B"),
+		IEC(readyFragOverhead, "B"),
+		IEC(readyIdle, "B"),
+		IEC(prepared, "B"))
+}