diff --git a/build.mill b/build.mill
index 58350b72..0e22d492 100644
--- a/build.mill
+++ b/build.mill
@@ -278,6 +278,12 @@ object sjsonnet extends VersionFileModule {
     def releaseMode = ReleaseMode.ReleaseFull
     def nativeLTO = LTO.Full
     def nativeMultithreading = None
+    // Concurrent GC: collects on background threads, overlapping collection with
+    // evaluation. On allocation-heavy configs (jrsonnet realworld suite) this is
+    // ~13% faster than the default immix and collapses immix's stop-the-world
+    // latency variance (kube-prometheus ±55ms -> ±5ms), while keeping the same
+    // RSS (it still frees -- bounded, safe on small machines). Output is identical.
+    def nativeGC = "commix"
 
     // Build aklomp/base64 as a static library for SIMD-accelerated base64.
     // We pin to a specific upstream commit (no git submodule) and clone on