Increase the initial Boehm GC mark stack size

If the mark stack size is too small, it greatly inhibits parallel
marking, which is very bad for performance on multi-core systems.

packaging/dependencies.nix

@@ -50,9 +50,19 @@ scope: {
         requiredSystemFeatures = [ ];
       };
 
-  boehmgc = pkgs.boehmgc.override {
+  boehmgc =
-    enableLargeConfig = true;
+    (pkgs.boehmgc.override {
-  };
+      enableLargeConfig = true;
+    }).overrideAttrs
+      (attrs: {
+        # Increase the initial mark stack size to avoid stack
+        # overflows, since these inhibit parallel marking (see
+        # GC_mark_some()). To check whether the mark stack is too
+        # small, run Nix with GC_PRINT_STATS=1 and look for messages
+        # such as `Mark stack overflow`, `No room to copy back mark
+        # stack`, and `Grew mark stack to ... frames`.
+        NIX_CFLAGS_COMPILE = "-DINITIAL_MARK_STACK_SIZE=1048576";
+      });
 
   # TODO Hack until https://github.com/NixOS/nixpkgs/issues/45462 is fixed.
   boost =