Report the build's worker allocations when asked

rewatch-ocaml/README.md

@@ -97,6 +97,11 @@ to choose another count: a positive integer, lowered to the machine's limit of
 `max(12, min(32, CPUs))`. The main domain schedules dependencies;
 workers run compiler requests and publish what they produce.
 
+The runtime's allocation report at exit (`OCAMLRUNPARAM=v=0x400`) only counts
+the main domain. With `REWATCH_REPORT_WORKER_ALLOCATIONS` set, the build also
+prints `worker_allocated_words: <n>`, the words its workers allocated, which
+the compiler benchmarks (`scripts/compilerBenchmarks.js`) add to it.
+
 The driver gives each request fresh compiler state (flags, warnings, caches,
 predefined types, identifier counters) and restores it on every exit, so
 requests on different domains don't interfere and repeated requests produce

rewatch-ocaml/process.ml

@@ -226,6 +226,20 @@ let rec worker_loop pool =
    memory. 8M added memory for little more speed. *)
 let worker_minor_heap_words = 4 * 1024 * 1024
 
+(* The words that finished workers allocated. The runtime's own report at exit
+   (OCAMLRUNPARAM=v=0x400) counts only the main domain, so the compiler
+   benchmarks add these to it. Allocated words are counted as the runtime
+   counts them: minor plus major, without the promoted ones. *)
+let worker_allocated_words_total = Atomic.make 0
+
+let record_worker_allocations () =
+  let minor, promoted, major = Gc.counters () in
+  ignore
+    (Atomic.fetch_and_add worker_allocated_words_total
+       (int_of_float (minor +. major -. promoted)))
+
+let worker_allocated_words () = Atomic.get worker_allocated_words_total
+
 let create_worker_pool ~max_jobs notifier =
   let pool =
     {
@@ -251,10 +265,12 @@ let create_worker_pool ~max_jobs notifier =
      before re-raising the first error. *)
   let run_worker () =
     Gc.set {(Gc.get ()) with minor_heap_size = worker_minor_heap_words};
-    try worker_loop pool
-    with exn ->
-      Child.with_lock pool.mutex (fun () ->
-          if Option.is_none pool.worker_error then pool.worker_error <- Some exn)
+    (try worker_loop pool
+     with exn ->
+       Child.with_lock pool.mutex (fun () ->
+           if Option.is_none pool.worker_error then
+             pool.worker_error <- Some exn));
+    record_worker_allocations ()
   in
   let rec start_workers remaining =
     if remaining > 0 then (

rewatch-ocaml/process.mli

@@ -14,6 +14,10 @@ val succeeded : result -> bool
 val status_string : Unix.process_status -> string
 val default_max_jobs : int
 
+val worker_allocated_words : unit -> int
+(** The words that the worker domains of finished worker pools allocated,
+    which the runtime's report at exit leaves out. *)
+
 val task : ?env:Spawn.Env.t -> ?on_result:(result -> result) -> job -> task
 val concurrent_task :
   cancel:(unit -> unit) ->

rewatch-ocaml/rescript_ocaml.ml

@@ -64,8 +64,18 @@ let run = function
   | command ->
     with_termination_handlers (fun ~poll -> run_command ~poll command)
 
+(* For the compiler benchmarks (scripts/compilerBenchmarks.js): the runtime's
+   report at exit only counts the main domain's allocations, and the compiler
+   runs on worker domains. *)
+let report_worker_allocations () =
+  if Option.is_some (Sys.getenv_opt "REWATCH_REPORT_WORKER_ALLOCATIONS") then
+    at_exit (fun () ->
+        Printf.eprintf "worker_allocated_words: %d\n%!"
+          (Process.worker_allocated_words ()))
+
 let main () =
   Platform.configure_standard_streams ();
+  report_worker_allocations ();
   try
     match Cli.eval Sys.argv with
     | Cli.Run command -> run command

rewatch-ocaml/tests/run.sh

@@ -2398,3 +2398,19 @@ for attempt in first second; do
     exit 1
   fi
 done
+
+# The compiler benchmarks add the words the worker domains allocated, which
+# the runtime's report at exit leaves out, to that report.
+worker_allocations="$work/worker-allocations"
+mkdir -p "$worker_allocations/src"
+printf '%s\n' \
+  '{"name":"worker-allocations","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$worker_allocations/rescript.json"
+printf 'let x = 1\n' >"$worker_allocations/src/A.res"
+env REWATCH_REPORT_WORKER_ALLOCATIONS=1 REWATCH_COMPILER_DOMAINS=1 \
+  "$port" build "$worker_allocations" >"$worker_allocations/build.log" 2>&1
+worker_words=$(sed -n 's/^worker_allocated_words: \([0-9]*\)$/\1/p' "$worker_allocations/build.log")
+if ! test "${worker_words:-0}" -gt 0; then
+  cat "$worker_allocations/build.log" >&2
+  exit 1
+fi