feat(config): add jemalloc_background_thread option

.schema/pgdog.schema.json

@@ -58,6 +58,7 @@
         "idle_healthcheck_delay": 5000,
         "idle_healthcheck_interval": 30000,
         "idle_timeout": 60000,
+        "jemalloc_background_thread": false,
         "load_balancing_strategy": "random",
         "load_schema": "auto",
         "log_connections": true,
@@ -811,6 +812,11 @@
           "default": 60000,
           "minimum": 0
         },
+        "jemalloc_background_thread": {
+          "description": "Enable jemalloc's background purge threads so freed memory is returned to the OS after allocation bursts. Equivalent to `_RJEM_MALLOC_CONF=background_thread:true`.\n\n_Default:_ `false`\n\n<https://docs.pgdog.dev/configuration/pgdog.toml/general/#jemalloc_background_thread>",
+          "type": "boolean",
+          "default": false
+        },
         "load_balancing_strategy": {
           "description": "Which strategy to use for load balancing read queries.\n\n_Default:_ `random`\n\n<https://docs.pgdog.dev/configuration/pgdog.toml/general/#load_balancing_strategy>",
           "$ref": "#/$defs/LoadBalancingStrategy",

pgdog-config/src/general.rs

@@ -806,6 +806,14 @@ pub struct General {
     /// <https://docs.pgdog.dev/configuration/pgdog.toml/general/#cutover_save_config>
     #[serde(default)]
     pub cutover_save_config: bool,
+
+    /// Enable jemalloc's background purge threads so freed memory is returned to the OS after allocation bursts. Equivalent to `_RJEM_MALLOC_CONF=background_thread:true`.
+    ///
+    /// _Default:_ `false`
+    ///
+    /// <https://docs.pgdog.dev/configuration/pgdog.toml/general/#jemalloc_background_thread>
+    #[serde(default = "General::jemalloc_background_thread")]
+    pub jemalloc_background_thread: bool,
 }
 
 impl Default for General {
@@ -911,6 +919,7 @@ impl Default for General {
             cutover_timeout: Self::cutover_timeout(),
             cutover_timeout_action: Self::cutover_timeout_action(),
             cutover_save_config: bool::default(),
+            jemalloc_background_thread: Self::jemalloc_background_thread(),
             unique_id_function: Self::unique_id_function(),
         }
     }
@@ -982,6 +991,10 @@ impl General {
         Self::env_bool_or_default("PGDOG_SCHEMA_RELOAD_ON_DDL", true)
     }
 
+    fn jemalloc_background_thread() -> bool {
+        Self::env_bool_or_default("PGDOG_JEMALLOC_BACKGROUND_THREAD", false)
+    }
+
     fn idle_healthcheck_interval() -> u64 {
         Self::env_or_default("PGDOG_IDLE_HEALTHCHECK_INTERVAL", 30_000)
     }
@@ -1737,6 +1750,15 @@ mod tests {
         assert!(!General::query_log_stdout());
     }
 
+    #[test]
+    fn test_jemalloc_background_thread_env() {
+        let _guard = set_env_var("PGDOG_JEMALLOC_BACKGROUND_THREAD", "true");
+        assert!(General::jemalloc_background_thread());
+
+        let _guard = remove_env_var("PGDOG_JEMALLOC_BACKGROUND_THREAD");
+        assert!(!General::jemalloc_background_thread());
+    }
+
     #[test]
     fn test_env_numeric_fields() {
         let _guard = set_env_var("PGDOG_BROADCAST_PORT", "7432");

pgdog/Cargo.toml

@@ -92,6 +92,7 @@ libc = "0.2"
 
 [target.'cfg(not(target_env = "msvc"))'.dependencies]
 tikv-jemallocator = "0.6"
+tikv-jemalloc-ctl = "0.6"
 
 
 [build-dependencies]

pgdog/src/lib.rs

@@ -52,6 +52,23 @@ static GLOBAL: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;
 #[global_allocator]
 static GLOBAL: &stats_alloc::StatsAlloc<System> = &stats_alloc::INSTRUMENTED_SYSTEM;
 
+/// Enable jemalloc's background purge threads so freed memory is returned to
+/// the OS after allocation bursts. No-op when disabled.
+#[cfg(all(not(test), not(target_env = "msvc")))]
+pub fn set_jemalloc_background_thread(enabled: bool) {
+    if !enabled {
+        return;
+    }
+    match tikv_jemalloc_ctl::background_thread::write(true) {
+        Ok(()) => tracing::info!("jemalloc background_thread enabled"),
+        Err(err) => tracing::warn!("could not enable jemalloc background_thread: {}", err),
+    }
+}
+
+/// No-op fallback where jemalloc is not the active allocator.
+#[cfg(any(test, target_env = "msvc"))]
+pub fn set_jemalloc_background_thread(_enabled: bool) {}
+
 /// Filter that dynamically installs or removes an inner
 /// [`TracingRateLimitLayer`] at runtime.
 ///

pgdog/src/main.rs

@@ -82,6 +82,8 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     config::overrides(overrides);
 
+    pgdog::set_jemalloc_background_thread(config.config.general.jemalloc_background_thread);
+
     plugin::load_from_config()?;
 
     let runtime = build_runtime(