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Reuse copy buffers in OCaml rewatch #8869

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Reuse copy buffers in OCaml rewatch #8869

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1 change: 1 addition & 0 deletions changes/ocaml-rewatch-copy-buffers.internal.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
- OCaml rewatch: reuse one copy buffer per worker instead of allocating one per published file, which makes clean builds faster. https://github.com/rescript-lang/rescript/pull/8869
14 changes: 10 additions & 4 deletions rewatch-ocaml/file_util.ml
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Expand Up @@ -109,6 +109,13 @@ let write_file_atomic ?(ensure_parent = true) path contents =
Sys.rename candidate path;
published := true)

(* Copy and comparison buffers, one set per domain: allocating fresh 64 KiB
buffers for every copied or compared file was the largest single source of
allocation in a build. Neither function calls the other while it holds the
buffers. *)
let copy_buffer = Domain.DLS.new_key (fun () -> Bytes.create 65_536)
let compare_buffer = Domain.DLS.new_key (fun () -> Bytes.create 65_536)

let copy_existing_file source destination =
let input_descriptor =
Unix.openfile source [Unix.O_RDONLY; Unix.O_CLOEXEC] 0
Expand All @@ -125,7 +132,7 @@ let copy_existing_file source destination =
let output_channel = open_out_bin destination in
try
with_output_channel output_channel (fun output_channel ->
let buffer = Bytes.create 65_536 in
let buffer = Domain.DLS.get copy_buffer in
let rec copy () =
let count = input input_channel buffer 0 (Bytes.length buffer) in
if count > 0 then (
Expand Down Expand Up @@ -173,9 +180,8 @@ let files_equal first second =
Fun.protect
~finally:(fun () -> close_in_noerr second_channel)
(fun () ->
let buffer_size = 65_536 in
let first_buffer = Bytes.create buffer_size in
let second_buffer = Bytes.create buffer_size in
let first_buffer = Domain.DLS.get copy_buffer in
let second_buffer = Domain.DLS.get compare_buffer in
let rec read_chunk channel buffer offset =
if offset = Bytes.length buffer then offset
else
Expand Down
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