pageserver: shut down idle walredo processes (#5877)

The longer a pageserver runs, the more walredo processes it accumulates
from tenants that are touched intermittently (e.g. by availability
checks). This can lead to getting OOM killed.

Changes:
- Add an Instant recording the last use of the walredo process for a
tenant
- After compaction iteration in the background task, check for idleness
and stop the walredo process if idle for more than 10x compaction
period.

Cc: #3620

Co-authored-by: Joonas Koivunen <joonas@neon.tech>
Co-authored-by: Shany Pozin <shany@neon.tech>

pageserver/src/tenant.rs

@@ -291,6 +291,16 @@ impl From<harness::TestRedoManager> for WalRedoManager {
 }
 
 impl WalRedoManager {
+    pub(crate) fn maybe_quiesce(&self, idle_timeout: Duration) {
+        match self {
+            Self::Prod(mgr) => mgr.maybe_quiesce(idle_timeout),
+            #[cfg(test)]
+            Self::Test(_) => {
+                // Not applicable to test redo manager
+            }
+        }
+    }
+
     pub async fn request_redo(
         &self,
         key: crate::repository::Key,

pageserver/src/tenant/tasks.rs

@@ -198,6 +198,10 @@ async fn compaction_loop(tenant: Arc<Tenant>, cancel: CancellationToken) {
 
             warn_when_period_overrun(started_at.elapsed(), period, BackgroundLoopKind::Compaction);
 
+            // Perhaps we did no work and the walredo process has been idle for some time:
+            // give it a chance to shut down to avoid leaving walredo process running indefinitely.
+            tenant.walredo_mgr.maybe_quiesce(period * 10);
+
             // Sleep
             if tokio::time::timeout(sleep_duration, cancel.cancelled())
                 .await

pageserver/src/walredo.rs

@@ -91,6 +91,7 @@ struct ProcessOutput {
 pub struct PostgresRedoManager {
     tenant_id: TenantId,
     conf: &'static PageServerConf,
+    last_redo_at: std::sync::Mutex<Option<Instant>>,
     redo_process: RwLock<Option<Arc<WalRedoProcess>>>,
 }
 
@@ -187,10 +188,26 @@ impl PostgresRedoManager {
         PostgresRedoManager {
             tenant_id,
             conf,
+            last_redo_at: std::sync::Mutex::default(),
             redo_process: RwLock::new(None),
         }
     }
 
+    /// This type doesn't have its own background task to check for idleness: we
+    /// rely on our owner calling this function periodically in its own housekeeping
+    /// loops.
+    pub(crate) fn maybe_quiesce(&self, idle_timeout: Duration) {
+        if let Ok(g) = self.last_redo_at.try_lock() {
+            if let Some(last_redo_at) = *g {
+                if last_redo_at.elapsed() >= idle_timeout {
+                    drop(g);
+                    let mut guard = self.redo_process.write().unwrap();
+                    *guard = None;
+                }
+            }
+        }
+    }
+
     ///
     /// Process one request for WAL redo using wal-redo postgres
     ///
@@ -205,6 +222,8 @@ impl PostgresRedoManager {
         wal_redo_timeout: Duration,
         pg_version: u32,
     ) -> anyhow::Result<Bytes> {
+        *(self.last_redo_at.lock().unwrap()) = Some(Instant::now());
+
         let (rel, blknum) = key_to_rel_block(key).context("invalid record")?;
         const MAX_RETRY_ATTEMPTS: u32 = 1;
         let mut n_attempts = 0u32;