Enable parallel marking in boehm-gc

packaging/dependencies.nix

@@ -50,9 +50,13 @@ scope: {
         requiredSystemFeatures = [ ];
       };
 
-  boehmgc = pkgs.boehmgc.override {
-    enableLargeConfig = true;
-  };
+  boehmgc =
+    (pkgs.boehmgc.override {
+      enableLargeConfig = true;
+    }).overrideAttrs
+      (attrs: {
+        patches = attrs.patches or [ ] ++ [ ./patches/bdwgc-bigger-mark-stack.patch ];
+      });
 
   # TODO Hack until https://github.com/NixOS/nixpkgs/issues/45462 is fixed.
   boost =

packaging/patches/bdwgc-bigger-mark-stack.patch

@@ -0,0 +1,16 @@
+Increase the initial mark stack size to avoid stack overflows, since
+these inhibit parallel marking (see GC_mark_some()).
+
+diff --git a/mark.c b/mark.c
+index 50e6cfd7..907f7763 100644
+--- a/mark.c
++++ b/mark.c
+@@ -66,7 +66,7 @@ GC_INNER struct obj_kind GC_obj_kinds[MAXOBJKINDS] = {
+ };
+
+ # ifndef INITIAL_MARK_STACK_SIZE
+-#   define INITIAL_MARK_STACK_SIZE (1*HBLKSIZE)
++#   define INITIAL_MARK_STACK_SIZE (256*HBLKSIZE)
+                 /* INITIAL_MARK_STACK_SIZE * sizeof(mse) should be a    */
+                 /* multiple of HBLKSIZE.                                */
+                 /* The incremental collector actually likes a larger    */

src/libexpr/eval-gc.cc

@@ -15,8 +15,6 @@
 #    include <pthread_np.h>
 #  endif
 
-#  include <gc/gc.h>
-#  include <gc/gc_cpp.h>
 #  include <gc/gc_allocator.h>
 
 #  include <boost/coroutine2/coroutine.hpp>
@@ -53,6 +51,9 @@ static inline void initGCReal()
 
     GC_INIT();
 
+    /* Enable parallel marking. */
+    GC_allow_register_threads();
+
     /* Register valid displacements in case we are using alignment niches
        for storing the type information. This way tagged pointers are considered
        to be valid, even when they are not aligned. */

src/libexpr/include/nix/expr/eval-gc.hh

@@ -3,12 +3,13 @@
 
 #include <cstddef>
 
-// For `NIX_USE_BOEHMGC`, and if that's set, `GC_THREADS`
+// For `NIX_USE_BOEHMGC`
 #include "nix/expr/config.hh"
 
 #if NIX_USE_BOEHMGC
 
 #  define GC_INCLUDE_NEW
+#  define GC_THREADS 1
 
 #  include <gc/gc.h>
 #  include <gc/gc_cpp.h>

src/libexpr/include/nix/expr/eval-inline.hh

@@ -6,9 +6,6 @@
 #include "nix/expr/eval-error.hh"
 #include "nix/expr/eval-settings.hh"
 
-// For `NIX_USE_BOEHMGC`, and if that's set, `GC_THREADS`
-#include "nix/expr/config.hh"
-
 namespace nix {
 
 /**

src/libexpr/meson.build

@@ -60,8 +60,6 @@ if bdw_gc.found()
     define_value = cxx.has_function(funcspec).to_int()
     configdata_priv.set(define_name, define_value)
   endforeach
-  # Affects ABI, because it changes what bdw_gc itself does!
-  configdata_pub.set('GC_THREADS', 1)
 endif
 # Used in public header. Affects ABI!
 configdata_pub.set('NIX_USE_BOEHMGC', bdw_gc.found().to_int())