libexpr: enable parallel marking in boehm-gc

Using parallel marking in GC speeds up evaluation a fair bit

```
Benchmark 1: ./lix-main/bin/nix search nixpkgs hello --no-eval-cache
  Time (mean ± σ):     20.740 s ±  0.046 s    [User: 17.583 s, System: 3.062 s]
  Range (min … max):   20.678 s … 20.794 s    10 runs

Benchmark 2: ./lix-parallel-marking/bin/nix search nixpkgs hello --no-eval-cache
  Time (mean ± σ):     15.037 s ±  0.080 s    [User: 19.602 s, System: 3.125 s]
  Range (min … max):   14.960 s … 15.227 s    10 runs

Summary
  ./lix-parallel-marking/bin/nix search nixpkgs hello --no-eval-cache ran
    1.38 ± 0.01 times faster than ./lix-main/bin/nix search nixpkgs hello --no-eval-cache
```

Based-on: https://github.com/NixOS/nix/pull/13708
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Change-Id: Ibc7625f21e0ee7c8ad66203eeb3aca5d83977731

doc/manual/change-authors.yml

@@ -86,6 +86,11 @@ ericson:
   display_name: John Ericson
   github: ericson2314
 
+getchoo:
+  display_name: Seth Flynn
+  forgejo: getchoo
+  github: getchoo
+
 gilice:
   forgejo: gilice
 

doc/manual/rl-next/parallel-marking.md

@@ -0,0 +1,9 @@
+---
+synopsis: Lix now enables parallel marking in boehm-gc
+issues: [fj#983]
+cls: [3880]
+category: Improvements
+credits: [edolstra, getchoo]
+---
+
+This brings a fairly modest performance improvement (~38% for `nixpkgs search hello`) to evaluation, especially in scenarios that necessitate larger heap sizes.

lix/libexpr/eval.cc

@@ -43,6 +43,21 @@
 #include <sys/resource.h>
 #include <boost/container/small_vector.hpp>
 
+// Ignore all internal signals boehm uses for parallel marking
+// FIXME: Find out how to do this with LLDB for macOS!
+#ifndef __APPLE__
+[[gnu::section(".debug_gdb_scripts"), gnu::used]]
+// TODO: We should use `SECTION_SCRIPT_ID_PYTHON_TEXT` from
+// `<gdb/section-scripts.h>` instead of hardcoding 4.
+// But why isn't this header exported by GDB?
+static const char printer_script[] =
+    "\4"
+    R"(lix-ignore-boehm-signals
+import gdb
+gdb.execute("handle SIGPWR SIGXCPU ignore")
+)";
+#endif
+
 #if HAVE_BOEHMGC
 
 #define GC_INCLUDE_NEW
@@ -161,6 +176,9 @@ void initLibExpr()
 
     GC_INIT();
 
+    // Enable parallel marking
+    GC_start_mark_threads();
+
     GC_set_oom_fn(oomHandler);
 
     /* Set the initial heap size to something fairly big (25% of

package.nix

@@ -95,7 +95,20 @@
 
     versionJson = builtins.fromJSON (builtins.readFile ./version.json);
 
-    boehmgc-nix = boehmgc.override { enableLargeConfig = true; };
+    boehmgc-nix = (boehmgc.override { enableLargeConfig = true; }).overrideAttrs (oldAttrs: {
+      /*
+        TODO: Use the `initialMarkStackSize` override when
+        https://github.com/NixOS/nixpkgs/pull/439943 hits a stable branch
+
+        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 = (oldAttrs.NIX_CFLAGS_COMPILE or "") + " -DINITIAL_MARK_STACK_SIZE=1048576";
+    });
 
     editline-lix = editline.overrideAttrs (prev: {
       configureFlags = (prev.configureFlags or [ ]) ++ [
@@ -469,7 +482,9 @@ stdenv.mkDerivation (finalAttrs: {
     runHook postInstallCheck
   '';
 
-  separateDebugInfo = !hostPlatform.isStatic && !finalAttrs.dontBuild;
+  # NOTE: This is disabled everywhere except Darwin due to Nixpkgs
+  # corrupting `.debug_gdb_scripts` segments (which currently aren't used on Darwin)
+  separateDebugInfo = !hostPlatform.isStatic && !finalAttrs.dontBuild && hostPlatform.isDarwin;
 
   strictDeps = true;