Fix increased memory usage in sysinfo since Router 1.59.0 (backport #6634)

.changesets/fix_simon_rayonless_sysinfo.md

@@ -0,0 +1,12 @@
+### Fix increased memory usage in `sysinfo` since Router 1.59.0 ([PR #6634](https://github.com/apollographql/router/pull/6634))
+
+In version 1.59.0, Apollo Router started using the `sysinfo` crate to gather metrics about available CPUs and RAM. By default, that crate uses `rayon` internally to parallelize its handling of system processes. In turn, rayon creates a pool of long-lived threads.
+
+In a particular benchmark on a 32-core Linux server, this caused resident memory use to increase by about 150 MB. This is likely a combination of stack space (which only gets freed when the thread terminates) and per-thread space reserved by the heap allocator to reduce cross-thread synchronization cost.
+
+This regression is now fixed by:
+
+* Disabling `sysinfo`’s use of `rayon`, so the thread pool is not created and system processes information is gathered in a sequential loop.
+* Making `sysinfo` not gather that information in the first place since Router does not use it.
+
+By [@SimonSapin](https://github.com/SimonSapin) in https://github.com/apollographql/router/pull/6634

Cargo.lock

@@ -6603,7 +6603,6 @@ dependencies = [
  "libc",
  "memchr",
  "ntapi",
- "rayon",
  "windows 0.57.0",
 ]
 

apollo-router/Cargo.toml

@@ -212,7 +212,7 @@ serde_yaml = "0.8.26"
 static_assertions = "1.1.0"
 strum_macros = "0.26.0"
 sys-info = "0.9.1"
-sysinfo = { version = "0.32.0", features = ["windows"] }
+sysinfo = { version = "0.32.0", features = ["system", "windows"], default_features = false }
 thiserror = "1.0.61"
 tokio.workspace = true
 tokio-stream = { version = "0.1.15", features = ["sync", "net"] }

apollo-router/src/plugins/fleet_detector.rs

@@ -44,7 +44,8 @@ struct SystemGetter {
 impl SystemGetter {
     fn new() -> Self {
         let mut system = System::new();
-        system.refresh_all();
+        system.refresh_cpu_all();
+        system.refresh_memory();
         Self {
             system,
             start: Instant::now(),