From 305fe61ac1abd0542c7a1dbe37055ab3bf338701 Mon Sep 17 00:00:00 2001
From: "Alex Chi Z." <4198311+skyzh@users.noreply.github.com>
Date: Thu, 3 Jul 2025 09:37:11 -0700
Subject: [PATCH] fix(pageserver): also print open layer size in backpressure
 (#12440)

## Problem

Better investigate memory usage during backpressure

## Summary of changes

Print open layer size if backpressure is activated

Signed-off-by: Alex Chi Z <chi@neon.tech>
---
 pageserver/src/tenant/timeline.rs | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/pageserver/src/tenant/timeline.rs b/pageserver/src/tenant/timeline.rs
index 84fb9a96af..165ae61379 100644
--- a/pageserver/src/tenant/timeline.rs
+++ b/pageserver/src/tenant/timeline.rs
@@ -4729,7 +4729,7 @@ impl Timeline {
                 }
 
                 // Fetch the next layer to flush, if any.
-                let (layer, l0_count, frozen_count, frozen_size) = {
+                let (layer, l0_count, frozen_count, frozen_size, open_layer_size) = {
                     let layers = self.layers.read(LayerManagerLockHolder::FlushLoop).await;
                     let Ok(lm) = layers.layer_map() else {
                         info!("dropping out of flush loop for timeline shutdown");
@@ -4742,8 +4742,13 @@ impl Timeline {
                         .iter()
                         .map(|l| l.estimated_in_mem_size())
                         .sum();
+                    let open_layer_size: u64 = lm
+                        .open_layer
+                        .as_ref()
+                        .map(|l| l.estimated_in_mem_size())
+                        .unwrap_or(0);
                     let layer = lm.frozen_layers.front().cloned();
-                    (layer, l0_count, frozen_count, frozen_size)
+                    (layer, l0_count, frozen_count, frozen_size, open_layer_size)
                     // drop 'layers' lock
                 };
                 let Some(layer) = layer else {
@@ -4756,7 +4761,7 @@ impl Timeline {
                     if l0_count >= stall_threshold {
                         warn!(
                             "stalling layer flushes for compaction backpressure at {l0_count} \
-                            L0 layers ({frozen_count} frozen layers with {frozen_size} bytes)"
+                            L0 layers ({frozen_count} frozen layers with {frozen_size} bytes, {open_layer_size} bytes in open layer)"
                         );
                         let stall_timer = self
                             .metrics
@@ -4809,7 +4814,7 @@ impl Timeline {
                         let delay = flush_duration.as_secs_f64();
                         info!(
                             "delaying layer flush by {delay:.3}s for compaction backpressure at \
-                            {l0_count} L0 layers ({frozen_count} frozen layers with {frozen_size} bytes)"
+                            {l0_count} L0 layers ({frozen_count} frozen layers with {frozen_size} bytes, {open_layer_size} bytes in open layer)"
                         );
                         let _delay_timer = self
                             .metrics