Simple weak array benchmark that mirrors original hugebench structure. Used for testing Transparent Huge Pages and OCaml GC compaction.

hugebench_weak_simple.ml

(* Simple weak array benchmark that mirrors original hugebench structure *)

let enable_hugepages () =
  let ch = open_in "/proc/self/maps" in
  let module Syscall = struct
    external madvise : (int64[@unboxed]) -> (int64[@unboxed]) -> (int[@untagged]) -> (int[@untagged]) =
      "unimplemented" "madvise"
  end in
  try
    while true do
      Scanf.sscanf (input_line ch) "%Lx-%Lx %s %Lx %x:%x %Ld %s"
        (fun start_addr end_addr perms offset devmaj devmin inode path ->
          let len = Int64.sub end_addr start_addr in
          if perms = "rw-p" && inode = 0L && path <> "[heap]" && path <> "[stack]" && len >= 0x200000L then
            ignore (Syscall.madvise start_addr len 25))
    done
  with End_of_file -> close_in ch

type t = {
  n : int;
  mutable opt : int option
}

(* Benchmark accessing through weak array *)
let bench weak_arr =
  let before = Sys.time () in
  let iters = 50_000 in
  for i = 1 to iters do
    let sum = ref 0 in
    for n = 0 to Weak.length weak_arr - 1 do
      match Weak.get weak_arr n with
      | Some obj -> sum := !sum + obj.n
      | None -> ()  (* Shouldn't happen in this benchmark *)
    done;
    ignore (Sys.opaque_identity !sum);
  done;
  let after = Sys.time () in
  Printf.printf "  %.3f s (%.1f ns/elem)\n%!"
    (after -. before)
    (1e9 *. ((after -. before) /. float_of_int (iters * Weak.length weak_arr)))

let () =
  (* Create objects - same as original hugebench *)
  let opts = Array.init 10_000_000 (fun n -> {n; opt=Some n}) in
  
  (* Create weak array holding subset of objects *)
  let weak_subset = Weak.create 10_000 in
  for i = 0 to 9_999 do
    Weak.set weak_subset i (Some opts.(i * 1000))
  done;
  
  Gc.minor ();
  
  Printf.printf "After initialisation and minor GC:\n%!";
  for _ = 1 to 3 do bench weak_subset done;
  
  enable_hugepages ();
  Printf.printf "After MADV_COLLAPSE to enable hugepages:\n%!";
  for _ = 1 to 3 do bench weak_subset done;
  
  (* Clear the options, making space in the size-1 pool for compaction *)
  opts |> Array.iter (fun t -> t.opt <- None);
  Gc.compact ();
  Printf.printf "After compaction:\n%!";
  for _ = 1 to 3 do bench weak_subset done;
  
  ignore (Sys.opaque_identity (opts, weak_subset))