use heapless::HistoryBuffer instead of VecDeque

Cargo.lock

@@ -141,6 +141,15 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "atomic-polyfill"
+version = "0.1.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e3ff7eb3f316534d83a8a2c3d1674ace8a5a71198eba31e2e2b597833f699b28"
+dependencies = [
+ "critical-section",
+]
+
 [[package]]
 name = "atty"
 version = "0.2.14"
@@ -986,6 +995,12 @@ dependencies = [
  "itertools",
 ]
 
+[[package]]
+name = "critical-section"
+version = "1.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6548a0ad5d2549e111e1f6a11a6c2e2d00ce6a3dafe22948d67c2b443f775e52"
+
 [[package]]
 name = "crossbeam-channel"
 version = "0.5.6"
@@ -1551,6 +1566,15 @@ version = "1.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "eabb4a44450da02c90444cf74558da904edde8fb4e9035a9a6a4e15445af0bd7"
 
+[[package]]
+name = "hash32"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "47d60b12902ba28e2730cd37e95b8c9223af2808df9e902d4df49588d1470606"
+dependencies = [
+ "byteorder",
+]
+
 [[package]]
 name = "hashbrown"
 version = "0.12.3"
@@ -1566,6 +1590,18 @@ dependencies = [
  "ahash",
 ]
 
+[[package]]
+name = "heapless"
+version = "0.8.0"
+source = "git+https://github.com/japaric/heapless.git?rev=340e0cec99a73759a5704f726c2fa3eed8fc3af6#340e0cec99a73759a5704f726c2fa3eed8fc3af6"
+dependencies = [
+ "atomic-polyfill",
+ "hash32",
+ "rustc_version",
+ "spin 0.9.4",
+ "stable_deref_trait",
+]
+
 [[package]]
 name = "heck"
 version = "0.4.0"
@@ -2366,6 +2402,7 @@ dependencies = [
  "fail",
  "futures",
  "git-version",
+ "heapless",
  "hex",
  "hex-literal",
  "humantime",
@@ -3042,7 +3079,7 @@ dependencies = [
  "cc",
  "libc",
  "once_cell",
- "spin",
+ "spin 0.5.2",
  "untrusted",
  "web-sys",
  "winapi",
@@ -3589,6 +3626,21 @@ version = "0.5.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6e63cff320ae2c57904679ba7cb63280a3dc4613885beafb148ee7bf9aa9042d"
 
+[[package]]
+name = "spin"
+version = "0.9.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7f6002a767bff9e83f8eeecf883ecb8011875a21ae8da43bffb817a57e78cc09"
+dependencies = [
+ "lock_api",
+]
+
+[[package]]
+name = "stable_deref_trait"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a8f112729512f8e442d81f95a8a7ddf2b7c6b8a1a6f509a95864142b30cab2d3"
+
 [[package]]
 name = "static_assertions"
 version = "1.1.0"

Cargo.toml

@@ -46,6 +46,7 @@ futures-core = "0.3"
 futures-util = "0.3"
 git-version = "0.3"
 hashbrown = "0.13"
+heapless = { default-features=false, features=[], git = "https://github.com/japaric/heapless.git", rev = "340e0cec99a73759a5704f726c2fa3eed8fc3af6" } # upstream release pending
 hex = "0.4"
 hex-literal = "0.3"
 hmac = "0.12.1"

pageserver/Cargo.toml

@@ -71,6 +71,7 @@ enum-map.workspace = true
 enumset.workspace = true
 strum.workspace = true
 strum_macros.workspace = true
+heapless.workspace = true
 
 [dev-dependencies]
 criterion.workspace = true

pageserver/src/tenant/storage_layer.rs

@@ -13,9 +13,9 @@ use anyhow::Result;
 use bytes::Bytes;
 use enum_map::EnumMap;
 use enumset::EnumSet;
+use heapless::HistoryBuffer;
 use pageserver_api::models::LayerAccessKind;
 use pageserver_api::models::{HistoricLayerInfo, Key};
-use std::collections::VecDeque;
 use std::ops::Range;
 use std::path::PathBuf;
 use std::sync::atomic::{self, AtomicBool};
@@ -91,20 +91,20 @@ pub enum ValueReconstructResult {
 #[derive(Debug)]
 pub struct LayerAccessStats(Mutex<LayerAccessStatsInner>);
 
-#[derive(Debug, Clone)]
-struct LayerAccessStatFullDetails {
-    when: SystemTime,
-    task_kind: TaskKind,
-    access_kind: LayerAccessKind,
-}
-
 #[derive(Debug, Clone)]
 struct LayerAccessStatsInner {
     first_access: Option<LayerAccessStatFullDetails>,
     count_by_access_kind: EnumMap<LayerAccessKind, u64>,
     task_kind_flag: EnumSet<TaskKind>,
-    last_accesses: VecDeque<LayerAccessStatFullDetails>,
-    last_residence_changes: VecDeque<LayerResidenceStatus>,
+    last_accesses: HistoryBuffer<LayerAccessStatFullDetails, 16>,
+    last_residence_changes: HistoryBuffer<LayerResidenceStatus, 16>,
+}
+
+#[derive(Debug, Clone)]
+struct LayerAccessStatFullDetails {
+    when: SystemTime,
+    task_kind: TaskKind,
+    access_kind: LayerAccessKind,
 }
 
 #[derive(Debug, Clone)]
@@ -200,18 +200,13 @@ impl Default for LayerAccessStatsInner {
             first_access: None,
             count_by_access_kind: EnumMap::default(),
             task_kind_flag: EnumSet::default(),
-            last_accesses: VecDeque::with_capacity(LayerAccessStats::LAST_ACCESSES_MAX_LEN),
-            last_residence_changes: VecDeque::with_capacity(
-                LayerAccessStats::LAST_RESIDENCE_CHANGES_MAX_LEN,
-            ),
+            last_accesses: HistoryBuffer::default(),
+            last_residence_changes: HistoryBuffer::default(),
         }
     }
 }
 
 impl LayerAccessStats {
-    const LAST_ACCESSES_MAX_LEN: usize = 16;
-    const LAST_RESIDENCE_CHANGES_MAX_LEN: usize = 16;
-
     pub(crate) fn for_loading_layer(residence_status: LayerResidenceStatus) -> Self {
         let new = LayerAccessStats(Mutex::new(LayerAccessStatsInner::default()));
         new.record_residence_change(residence_status);
@@ -244,12 +239,7 @@ impl LayerAccessStats {
             return;
         }
         let mut inner = self.0.lock().unwrap();
-
-        // make room first to avoid reallocs
-        while inner.last_residence_changes.len() >= Self::LAST_RESIDENCE_CHANGES_MAX_LEN {
-            inner.last_residence_changes.pop_back();
-        }
-        inner.last_residence_changes.push_front(new_status);
+        inner.last_residence_changes.write(new_status);
     }
 
     fn record_access(&self, access_kind: LayerAccessKind, task_kind: TaskKind) {
@@ -267,11 +257,7 @@ impl LayerAccessStats {
             .get_or_insert_with(|| this_access.clone());
         inner.count_by_access_kind[access_kind] += 1;
         inner.task_kind_flag |= task_kind;
-        // make room first to avoid reallocs
-        while inner.last_accesses.len() >= Self::LAST_ACCESSES_MAX_LEN {
-            inner.last_accesses.pop_back();
-        }
-        inner.last_accesses.push_front(this_access);
+        inner.last_accesses.write(this_access);
     }
     fn reset(&self, what: LayerAccessStatsReset) {
         let mut inner = self.0.lock().unwrap();