M12.8: the app no longer freezes every 10 s on Windows: the system memory is read in the background

Every 10 seconds the UI thread re-read the system's memory for the cache budgets, and on
Windows that starts PowerShell (`Get-CimInstance Win32_OperatingSystem`) and waits for it:
about a second, sometimes two, during which the whole app froze. The reading now runs on a
background thread (`sysinfo::MemoryWatch`); `Session::memory_tick` takes the last reading and
starts the next one, never waiting. Console programs started for it no longer open a console
window (`CREATE_NO_WINDOW`).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/engine/src/lib.rs

@@ -373,6 +373,8 @@ pub struct Session {
     /// The last physical-memory reading (Settings ▸ Memory & CPU budgets); `None` = unknown
     /// (headless sessions don't query it, see [`Session::memory_tick`]).
     pub sys_memory: Option<sysinfo::SysMemory>,
+    /// Reads the system's memory in the background for [`Session::memory_tick`].
+    pub memory_watch: sysinfo::MemoryWatch,
     /// Switches Affect Nested Comps as last applied (a change empties the layer cache).
     pub applied_nested_switches: Option<bool>,
     /// Runs renders and analyses off the UI thread where there are no threads (the web app's
@@ -465,6 +467,7 @@ impl Default for Session {
             plugin_loader: None,
             script_ui: scriptui::ScriptUi::default(),
             sys_memory: None,
+            memory_watch: Default::default(),
             applied_nested_switches: None,
             offload: None,
             offloaded: vec![],

crates/engine/src/session_settings.rs

@@ -75,11 +75,13 @@ impl Session {
         self.footage.set_cache_budget(b.media);
     }
 
-    /// Re-read the system's memory (desktop frontends call this every few seconds) and re-apply
-    /// the cache budgets when the reading changes them. Returns the effective budgets.
+    /// Take the latest reading of the system's memory and start the next one in the background
+    /// (desktop frontends call this every few seconds; it never waits for the system), then
+    /// re-apply the cache budgets when the reading changes them. Returns the effective budgets.
     pub fn memory_tick(&mut self) -> crate::prefs::CacheBudgets {
         let before = self.prefs.cache_budgets(self.sys_memory);
-        self.sys_memory = crate::sysinfo::memory().or(self.sys_memory);
+        self.sys_memory = self.memory_watch.latest().or(self.sys_memory);
+        self.memory_watch.poll();
         let after = self.prefs.cache_budgets(self.sys_memory);
         if after != before {
             self.apply_cache_budgets();

crates/engine/src/sysinfo.rs

@@ -4,7 +4,11 @@
 //! Pure Rust and dependency-free: the values come from the operating system's own reporting
 //! (`/proc/meminfo` on Linux, `sysctl`/`vm_stat` on macOS, `wmic`/PowerShell on Windows). Every
 //! query may fail (sandboxes, the web): callers treat `None` as "unknown" and keep their
-//! configured budgets.
+//! configured budgets. The periodic reading behind the cache budgets runs on a background
+//! thread ([`MemoryWatch`]): on Windows it starts PowerShell, which takes about a second.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::{Arc, Mutex, PoisonError};
 
 use serde::Serialize;
 
@@ -23,10 +27,51 @@ impl SysMemory {
     }
 }
 
+/// The system's memory, read in the background: [`MemoryWatch::poll`] starts a reading (unless
+/// one is running) and [`MemoryWatch::latest`] returns the last one without waiting.
+#[derive(Clone, Default)]
+pub struct MemoryWatch {
+    latest: Arc<Mutex<Option<SysMemory>>>,
+    busy: Arc<AtomicBool>,
+}
+
+impl MemoryWatch {
+    /// Start a new reading in the background (no threads on the web: read inline, it is
+    /// unknown there anyway).
+    pub fn poll(&self) {
+        if self.busy.swap(true, Ordering::AcqRel) {
+            return;
+        }
+        let (latest, busy) = (self.latest.clone(), self.busy.clone());
+        let read = move || {
+            if let Some(m) = memory() {
+                *latest.lock().unwrap_or_else(PoisonError::into_inner) = Some(m);
+            }
+            busy.store(false, Ordering::Release);
+        };
+        #[cfg(not(target_arch = "wasm32"))]
+        if std::thread::Builder::new().name("ec-memory".into()).spawn(read).is_err() {
+            self.busy.store(false, Ordering::Release);
+        }
+        #[cfg(target_arch = "wasm32")]
+        read();
+    }
+
+    /// The last reading (`None` until one finished).
+    pub fn latest(&self) -> Option<SysMemory> {
+        *self.latest.lock().unwrap_or_else(PoisonError::into_inner)
+    }
+}
+
 #[cfg(not(target_arch = "wasm32"))]
 #[allow(dead_code)]
 fn run(cmd: &str, args: &[&str]) -> Option<String> {
-    let out = std::process::Command::new(cmd).args(args).output().ok()?;
+    let mut c = std::process::Command::new(cmd);
+    c.args(args);
+    // A console program started from the windowed app would flash a console window.
+    #[cfg(target_os = "windows")]
+    std::os::windows::process::CommandExt::creation_flags(&mut c, 0x0800_0000); // CREATE_NO_WINDOW
+    let out = c.output().ok()?;
     out.status.success().then(|| String::from_utf8_lossy(&out.stdout).to_string())
 }
 

crates/engine/src/tests_settings.rs

@@ -464,3 +464,25 @@ fn open_recent_increment_and_revert() {
     assert_eq!(autosave::increment_path("/x/v1.ecproj", |p| p.ends_with("v1 2.ecproj")), "/x/v1 3.ecproj");
     let _ = std::fs::remove_dir_all(d);
 }
+
+#[test]
+fn memory_tick_never_waits_for_the_system() {
+    let mut s = Session::default();
+    let t0 = std::time::Instant::now();
+    s.memory_tick();
+    s.memory_tick();
+    // Reading the system's memory starts PowerShell on Windows (about a second): the UI thread
+    // only takes the last reading.
+    assert!(t0.elapsed() < std::time::Duration::from_millis(200), "{:?}", t0.elapsed());
+    if crate::sysinfo::memory().is_some() {
+        let mut got = None;
+        for _ in 0..200 {
+            got = s.memory_watch.latest();
+            if got.is_some() {
+                break;
+            }
+            std::thread::sleep(std::time::Duration::from_millis(50));
+        }
+        assert!(got.is_some_and(|m| m.total > 0), "the reading arrives in the background");
+    }
+}