first implem of trace-spall

dune-project

@@ -109,4 +109,22 @@
  (tags
   (trace tracing trace runtime-events)))
 
+(package
+ (name trace-spall)
+ (synopsis
+  "A fast binary backend for trace, emitting Spall profiler format (.spall)")
+ (depends
+  (ocaml
+   (>= 4.08))
+  (trace
+   (= :version))
+  (mtime
+   (>= 2.0))
+  (thread-local-storage
+   (>= 0.2))
+  base-unix
+  dune)
+ (tags
+  (trace tracing spall profiling)))
+
 ; See the complete stanza docs at https://dune.readthedocs.io/en/stable/dune-files.html#dune-project

src/spall/collector_spall.ml

@@ -0,0 +1,176 @@
+open struct
+  module TLS = Thread_local_storage
+  module A = Trace_core.Internal_.Atomic_
+
+  let with_lock mu f =
+    Mutex.lock mu;
+    match f () with
+    | v ->
+      Mutex.unlock mu;
+      v
+    | exception e ->
+      let bt = Printexc.get_raw_backtrace () in
+      Mutex.unlock mu;
+      Printexc.raise_with_backtrace e bt
+
+  let ( let@ ) = ( @@ )
+end
+
+open Trace_core
+
+type span += Span_spall
+
+type thread_state = {
+  tid: int;
+  pid: int;
+  mutable first_ts: int64;
+  buf: Buffer.t;
+  mutable prev: thread_state;
+  mutable next: thread_state;
+}
+
+type t = {
+  active: bool A.t;
+  pid: int;
+  mu: Mutex.t;  (** guards: [oc] writes, [all_states] updates *)
+  all_states: thread_state Lazy.t;
+      (** double linked list. first item is always the main thread. *)
+  oc: out_channel;
+  close_oc: bool;
+  high_water_mark: int;
+}
+
+(* One module-level TLS slot shared across all instances; stores a (collector,
+   thread_state) pair so we detect stale entries from a previous collector. *)
+let tls_key : (t * thread_state) TLS.t = TLS.create ()
+
+let flush_thread_ (self : t) (ts : thread_state) : unit =
+  let size = Buffer.length ts.buf in
+  if size > 0 then (
+    let hdr =
+      Writer.write_header ~size ~tid:ts.tid ~pid:ts.pid ~first_ts:ts.first_ts
+    in
+    output_string self.oc hdr;
+    Buffer.output_buffer self.oc ts.buf;
+    (* if we overallocated by a lot, free memory *)
+    Buffer.reset ts.buf;
+    ts.first_ts <- 0L
+  )
+
+open struct
+  let buf_size = 4_096
+  let default_high_water_mark = buf_size
+end
+
+let create_thread_state_ (self : t) : thread_state =
+  let tid = Trace_util.Mock_.get_tid () in
+  let buf = Buffer.create buf_size in
+  let rec ts =
+    { tid; pid = self.pid; first_ts = 0L; buf; prev = ts; next = ts }
+  in
+  TLS.set tls_key (self, ts);
+  ts
+
+(* Returns the thread_state for this thread+collector, creating it on first
+   call. Fast path (after initialization) is lock-free. *)
+let get_or_create_thread_ (self : t) : thread_state =
+  match TLS.get_opt tls_key with
+  | Some (c, ts) when c == self -> ts
+  | _ ->
+    let ts = create_thread_state_ self in
+    with_lock self.mu (fun () ->
+        let (lazy start) = self.all_states in
+        ts.prev <- start.prev;
+        ts.next <- start;
+        start.prev.next <- ts;
+        start.prev <- ts);
+    ts
+
+let flush_all_ self =
+  let (lazy start) = self.all_states in
+  let cur = ref start in
+  let continue = ref true in
+  while !continue do
+    flush_thread_ self !cur;
+    cur := !cur.next;
+    if !cur == start then continue := false
+  done
+
+let close (self : t) : unit =
+  if A.exchange self.active false then (
+    let@ () = with_lock self.mu in
+    flush_all_ self;
+    flush self.oc;
+    if self.close_oc then close_out_noerr self.oc
+  )
+
+let create ~pid ~oc ?(close_oc = true)
+    ?(high_water_mark = default_high_water_mark) () : t =
+  let rec self =
+    {
+      active = A.make true;
+      pid;
+      mu = Mutex.create ();
+      all_states = lazy (create_thread_state_ self);
+      oc;
+      close_oc;
+      high_water_mark;
+    }
+  in
+  ignore (Lazy.force self.all_states : thread_state);
+  let hdr = Buffer.create 32 in
+  Writer.write_file_header hdr ~timestamp_unit:1e-3;
+  Buffer.output_buffer oc hdr;
+  flush oc;
+  self
+
+open struct
+  type st = t
+
+  let init _ = ()
+  let shutdown (self : st) = close self
+
+  let enter_span (self : st) ~__FUNCTION__:_ ~__FILE__:_ ~__LINE__:_ ~level:_
+      ~params:_ ~data:_ ~parent:_ name : span =
+    let ts = Trace_util.Mock_.now_ns () in
+    let tst = get_or_create_thread_ self in
+    if Buffer.length tst.buf = 0 then tst.first_ts <- ts;
+    Writer.write_begin tst.buf ~ts ~name;
+    (if Buffer.length tst.buf >= self.high_water_mark then
+       let@ () = with_lock self.mu in
+       flush_thread_ self tst);
+    Span_spall
+
+  let exit_span (self : st) sp =
+    match sp with
+    | Span_spall ->
+      let ts = Trace_util.Mock_.now_ns () in
+      let tst = get_or_create_thread_ self in
+      Writer.write_end tst.buf ~ts;
+      if Buffer.length tst.buf >= self.high_water_mark then
+        let@ () = with_lock self.mu in
+        flush_thread_ self tst
+    | _ -> ()
+
+  let add_data_to_span _st _sp _data = ()
+  let message _self ~level:_ ~params:_ ~data:_ ~span:_ _msg = ()
+  let metric _self ~level:_ ~params:_ ~data:_ _name _m = ()
+
+  let extension (self : st) ~level:_ ev =
+    match ev with
+    | Core_ext.Extension_set_thread_name name ->
+      let tst = get_or_create_thread_ self in
+      let@ () = with_lock self.mu in
+      Writer.write_name tst.buf ~kind:`Thread ~name
+    | Core_ext.Extension_set_process_name name ->
+      let tst = get_or_create_thread_ self in
+      let@ () = with_lock self.mu in
+      Writer.write_name tst.buf ~kind:`Process ~name
+    | _ -> ()
+end
+
+let callbacks : _ Collector.Callbacks.t =
+  Collector.Callbacks.make ~init ~shutdown ~enter_span ~exit_span
+    ~add_data_to_span ~message ~metric ~extension ()
+
+let collector (self : t) : Collector.t = Collector.C_some (self, callbacks)

src/spall/collector_spall.mli

@@ -0,0 +1,12 @@
+type t
+
+val create :
+  pid:int ->
+  oc:out_channel ->
+  ?close_oc:bool ->
+  ?high_water_mark:int ->
+  unit ->
+  t
+
+val collector : t -> Trace_core.Collector.t
+val close : t -> unit

src/spall/dune

@@ -0,0 +1,6 @@
+(library
+ (name trace_spall)
+ (public_name trace-spall)
+ (synopsis
+  "A fast binary backend for trace, emitting Spall profiler format (.spall)")
+ (libraries trace.core trace.util unix threads thread-local-storage))

src/spall/spall.h

@@ -0,0 +1,361 @@
+// SPDX-FileCopyrightText: © 2023 Phillip Trudeau-Tavara <pmttavara@protonmail.com>
+// SPDX-License-Identifier: MIT
+
+/*
+
+TODO: Optional Helper APIs:
+
+  - Compression API: would require a mutexed lockable context (yuck...)
+      - Either using a ZIP library, a name cache + TIDPID cache, or both (but ZIP is likely more than enough!!!)
+      - begin()/end() writes compressed chunks to a caller-determined destination
+          - The destination can be the buffered-writing API or a custom user destination
+      - Ultimately need to take a lock with some granularity... can that be the caller's responsibility?
+
+  - Counter Event: should allow tracking arbitrary named values with a single event, for memory and frame profiling
+
+  - Ring-buffer API
+        spall_ring_init
+        spall_ring_emit_begin
+        spall_ring_emit_end
+        spall_ring_flush
+*/
+
+#ifndef SPALL_H
+#define SPALL_H
+
+#if !defined(_MSC_VER) || defined(__clang__)
+#define SPALL_NOINSTRUMENT __attribute__((no_instrument_function))
+#define SPALL_FORCEINLINE __attribute__((always_inline))
+#else
+#ifndef _CRT_SECURE_NO_WARNINGS
+#define _CRT_SECURE_NO_WARNINGS
+#endif
+#define SPALL_NOINSTRUMENT // Can't noinstrument on MSVC!
+#define SPALL_FORCEINLINE __forceinline
+#endif
+
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdbool.h>
+
+#define SPALL_FN static inline SPALL_NOINSTRUMENT
+#define SPALL_MIN(a, b) (((a) < (b)) ? (a) : (b))
+#define SPALL_MAX(a, b) (((a) > (b)) ? (a) : (b))
+
+#pragma pack(push, 1)
+
+typedef struct SpallHeader {
+    uint64_t magic_header; // = 0x0BADF00D
+    uint64_t version; // = 3
+    double   timestamp_unit;
+    uint64_t must_be_0;
+} SpallHeader;
+
+typedef enum {
+    SpallEventType_Invalid             = 0,
+    SpallEventType_Custom_Data         = 1, // Basic readers can skip this.
+    SpallEventType_StreamOver          = 2,
+
+    SpallEventType_Begin               = 3,
+    SpallEventType_End                 = 4,
+    SpallEventType_Instant             = 5,
+
+    SpallEventType_Overwrite_Timestamp = 6, // Retroactively change timestamp units - useful for incrementally improving RDTSC frequency.
+    SpallEventType_Pad_Skip            = 7,
+
+	SpallEventType_NameProcess         = 8,
+	SpallEventType_NameThread          = 9,
+} SpallEventType;
+
+typedef struct SpallBufferHeader {
+	uint32_t size;
+	uint32_t tid;
+	uint32_t pid;
+	uint64_t first_ts;
+} SpallBufferHeader;
+
+typedef struct SpallBeginEvent {
+    uint8_t type; // = SpallEventType_Begin
+    uint64_t when;
+
+    uint8_t name_length;
+    uint8_t args_length;
+} SpallBeginEvent;
+
+typedef struct SpallBeginEventMax {
+    SpallBeginEvent event;
+    char name_bytes[255];
+    char args_bytes[255];
+} SpallBeginEventMax;
+
+typedef struct SpallEndEvent {
+    uint8_t  type; // = SpallEventType_End
+    uint64_t when;
+} SpallEndEvent;
+
+typedef struct SpallPadSkipEvent {
+    uint8_t  type; // = SpallEventType_Pad_Skip
+    uint32_t size;
+} SpallPadSkipEvent;
+
+typedef struct SpallNameContainerEvent {
+	uint8_t type; // = SpallEventType_NameThread/Process
+	uint8_t name_length;
+} SpallNameContainerEvent;
+
+typedef struct SpallNameContainerEventMax {
+    SpallNameContainerEvent event;
+    char name_bytes[255];
+} SpallNameContainerEventMax;
+
+#pragma pack(pop)
+
+typedef struct SpallProfile SpallProfile;
+
+// Important!: If you define your own callbacks, mark them SPALL_NOINSTRUMENT!
+typedef bool (*SpallWriteCallback)(SpallProfile *self, const void *data, size_t length);
+typedef bool (*SpallFlushCallback)(SpallProfile *self);
+typedef void (*SpallCloseCallback)(SpallProfile *self);
+
+struct SpallProfile {
+    double timestamp_unit;
+    SpallWriteCallback write;
+    SpallFlushCallback flush;
+    SpallCloseCallback close;
+    void *data;
+};
+
+// Important!: If you are writing Begin/End events, then do NOT write
+//             events for the same PID + TID pair on different buffers!!!
+typedef struct SpallBuffer {
+    void *data;
+    size_t length;
+	uint32_t tid;
+	uint32_t pid;
+
+    // Internal data - don't assign this
+    size_t head;
+	uint64_t first_ts;
+} SpallBuffer;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+SPALL_FN SPALL_FORCEINLINE bool spall__file_write(SpallProfile *ctx, const void *p, size_t n) {
+    if (fwrite(p, n, 1, (FILE *)ctx->data) != 1) return false;
+    return true;
+}
+SPALL_FN bool spall__file_flush(SpallProfile *ctx) {
+    if (fflush((FILE *)ctx->data)) return false;
+    return true;
+}
+SPALL_FN void spall__file_close(SpallProfile *ctx) {
+    fclose((FILE *)ctx->data);
+    ctx->data = NULL;
+}
+
+SPALL_FN SPALL_FORCEINLINE bool spall__buffer_flush(SpallProfile *ctx, SpallBuffer *wb, uint64_t ts) {
+	wb->first_ts = SPALL_MAX(wb->first_ts, ts);
+
+	SpallBufferHeader hdr;
+	hdr.size = wb->head - sizeof(SpallBufferHeader);
+	hdr.pid = wb->pid;
+	hdr.tid = wb->tid;
+	hdr.first_ts = wb->first_ts;
+
+	memcpy(wb->data, &hdr, sizeof(hdr));
+
+	if (!ctx->write(ctx, wb->data, wb->head)) return false;
+	if (!ctx->flush(ctx)) return false;
+    wb->head = sizeof(SpallBufferHeader);
+    return true;
+}
+
+SPALL_FN bool spall_buffer_flush(SpallProfile *ctx, SpallBuffer *wb) {
+    if (!spall__buffer_flush(ctx, wb, 0)) return false;
+    return true;
+}
+
+SPALL_FN bool spall_buffer_quit(SpallProfile *ctx, SpallBuffer *wb) {
+    if (!spall_buffer_flush(ctx, wb)) return false;
+    return true;
+}
+
+SPALL_FN size_t spall_build_header(void *buffer, size_t rem_size, double timestamp_unit) {
+    size_t header_size = sizeof(SpallHeader);
+    if (header_size > rem_size) {
+        return 0;
+    }
+
+    SpallHeader *header = (SpallHeader *)buffer;
+    header->magic_header = 0x0BADF00D;
+    header->version = 3;
+    header->timestamp_unit = timestamp_unit;
+    header->must_be_0 = 0;
+    return header_size;
+}
+SPALL_FN SPALL_FORCEINLINE size_t spall_build_begin(void *buffer, size_t rem_size, const char *name, int32_t name_len, const char *args, int32_t args_len, uint64_t when) {
+    SpallBeginEventMax *ev = (SpallBeginEventMax *)buffer;
+    uint8_t trunc_name_len = (uint8_t)SPALL_MIN(name_len, 255); // will be interpreted as truncated in the app (?)
+    uint8_t trunc_args_len = (uint8_t)SPALL_MIN(args_len, 255); // will be interpreted as truncated in the app (?)
+
+    size_t ev_size = sizeof(SpallBeginEvent) + trunc_name_len + trunc_args_len;
+    if (ev_size > rem_size) {
+        return 0;
+    }
+
+    ev->event.type = SpallEventType_Begin;
+    ev->event.when = when;
+    ev->event.name_length = trunc_name_len;
+    ev->event.args_length = trunc_args_len;
+    memcpy(ev->name_bytes,                  name, trunc_name_len);
+    memcpy(ev->name_bytes + trunc_name_len, args, trunc_args_len);
+
+    return ev_size;
+}
+SPALL_FN SPALL_FORCEINLINE size_t spall_build_end(void *buffer, size_t rem_size, uint64_t when) {
+    size_t ev_size = sizeof(SpallEndEvent);
+    if (ev_size > rem_size) {
+        return 0;
+    }
+
+    SpallEndEvent *ev = (SpallEndEvent *)buffer;
+    ev->type = SpallEventType_End;
+    ev->when = when;
+
+    return ev_size;
+}
+SPALL_FN SPALL_FORCEINLINE size_t spall_build_name(void *buffer, size_t rem_size, const char *name, int32_t name_len, SpallEventType type) {
+    SpallNameContainerEventMax *ev = (SpallNameContainerEventMax *)buffer;
+    uint8_t trunc_name_len = (uint8_t)SPALL_MIN(name_len, 255); // will be interpreted as truncated in the app (?)
+
+    size_t ev_size = sizeof(SpallNameContainerEvent) + trunc_name_len;
+    if (ev_size > rem_size) {
+        return 0;
+    }
+
+    ev->event.type = type;
+    ev->event.name_length = trunc_name_len;
+    memcpy(ev->name_bytes, name, trunc_name_len);
+
+    return ev_size;
+}
+
+SPALL_FN void spall_quit(SpallProfile *ctx) {
+    if (!ctx) return;
+    if (ctx->close) ctx->close(ctx);
+
+    memset(ctx, 0, sizeof(*ctx));
+}
+
+SPALL_FN bool spall_init_callbacks(double timestamp_unit,
+								   SpallWriteCallback write,
+								   SpallFlushCallback flush,
+								   SpallCloseCallback close,
+								   void *userdata,
+								   SpallProfile *ctx) {
+
+    if (timestamp_unit < 0) return false;
+
+    memset(ctx, 0, sizeof(*ctx));
+    ctx->timestamp_unit = timestamp_unit;
+    ctx->data = userdata;
+    ctx->write = write;
+    ctx->flush = flush;
+    ctx->close = close;
+
+	SpallHeader header;
+	size_t len = spall_build_header(&header, sizeof(header), timestamp_unit);
+	if (!ctx->write(ctx, &header, len)) {
+		spall_quit(ctx);
+		return false;
+	}
+
+    return true;
+}
+
+SPALL_FN bool spall_init_file(const char* filename, double timestamp_unit, SpallProfile *ctx) {
+    if (!filename) return false;
+
+    FILE *f = fopen(filename, "wb"); // TODO: handle utf8 and long paths on windows
+    if (f) { // basically freopen() but we don't want to force users to lug along another macro define
+        fclose(f);
+        f = fopen(filename, "ab");
+    }
+	if (!f) { return false; }
+
+    return spall_init_callbacks(timestamp_unit, spall__file_write, spall__file_flush, spall__file_close, (void *)f, ctx);
+}
+
+SPALL_FN bool spall_flush(SpallProfile *ctx) {
+    if (!ctx->flush(ctx)) return false;
+    return true;
+}
+
+SPALL_FN bool spall_buffer_init(SpallProfile *ctx, SpallBuffer *wb) {
+	// Fails if buffer is not big enough to contain at least one event!
+	if (wb->length < sizeof(SpallBufferHeader) + sizeof(SpallBeginEventMax)) {
+		return false;
+	}
+
+	wb->head = sizeof(SpallBufferHeader);
+	return true;
+}
+
+
+SPALL_FN SPALL_FORCEINLINE bool spall_buffer_begin_args(SpallProfile *ctx, SpallBuffer *wb, const char *name, int32_t name_len, const char *args, int32_t args_len, uint64_t when) {
+	if ((wb->head + sizeof(SpallBeginEventMax)) > wb->length) {
+		if (!spall__buffer_flush(ctx, wb, when)) {
+			return false;
+		}
+	}
+
+	wb->head += spall_build_begin((char *)wb->data + wb->head, wb->length - wb->head, name, name_len, args, args_len, when);
+
+    return true;
+}
+
+SPALL_FN bool spall_buffer_begin(SpallProfile *ctx, SpallBuffer *wb, const char *name, int32_t name_len, uint64_t when) {
+    return spall_buffer_begin_args(ctx, wb, name, name_len, "", 0, when);
+}
+
+SPALL_FN bool spall_buffer_end(SpallProfile *ctx, SpallBuffer *wb, uint64_t when) {
+	if ((wb->head + sizeof(SpallEndEvent)) > wb->length) {
+		if (!spall__buffer_flush(ctx, wb, when)) {
+			return false;
+		}
+	}
+
+	wb->head += spall_build_end((char *)wb->data + wb->head, wb->length - wb->head, when);
+	return true;
+}
+
+SPALL_FN bool spall_buffer_name_thread(SpallProfile *ctx, SpallBuffer *wb, const char *name, int32_t name_len) {
+	if ((wb->head + sizeof(SpallNameContainerEvent)) > wb->length) {
+		if (!spall__buffer_flush(ctx, wb, 0)) {
+			return false;
+		}
+	}
+
+	wb->head += spall_build_name((char *)wb->data + wb->head, wb->length - wb->head, name, name_len, SpallEventType_NameThread);
+	return true;
+}
+
+SPALL_FN bool spall_buffer_name_process(SpallProfile *ctx, SpallBuffer *wb, const char *name, int32_t name_len) {
+	if ((wb->head + sizeof(SpallNameContainerEvent)) > wb->length) {
+		if (!spall__buffer_flush(ctx, wb, 0)) {
+			return false;
+		}
+	}
+
+	wb->head += spall_build_name((char *)wb->data + wb->head, wb->length - wb->head, name, name_len, SpallEventType_NameProcess);
+	return true;
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // SPALL_H

src/spall/trace_spall.ml

@@ -0,0 +1,53 @@
+open Trace_core
+module Collector_spall = Collector_spall
+module Writer = Writer
+
+type output =
+  [ `Stdout
+  | `Stderr
+  | `File of string
+  ]
+
+let collector ~(out : [< output ]) () : collector =
+  let oc, close_oc =
+    match out with
+    | `Stdout -> stdout, false
+    | `Stderr -> stderr, false
+    | `File path -> open_out path, true
+  in
+  let pid = Trace_util.Mock_.get_pid () in
+  let st = Collector_spall.create ~pid ~oc ~close_oc () in
+  Collector_spall.collector st
+
+open struct
+  let register_atexit =
+    let has_registered = ref false in
+    fun () ->
+      if not !has_registered then (
+        has_registered := true;
+        at_exit Trace_core.shutdown
+      )
+end
+
+let setup ?(out = `Env) () =
+  register_atexit ();
+  let make_col out = Trace_core.setup_collector (collector ~out ()) in
+  match out with
+  | `Stderr -> make_col `Stderr
+  | `Stdout -> make_col `Stdout
+  | `File path -> make_col (`File path)
+  | `Env ->
+    (match Sys.getenv_opt "TRACE" with
+    | Some ("1" | "true") -> make_col (`File "trace.spall")
+    | Some "stdout" -> make_col `Stdout
+    | Some "stderr" -> make_col `Stderr
+    | Some path -> make_col (`File path)
+    | None -> ())
+
+let with_setup ?out () f =
+  setup ?out ();
+  Fun.protect ~finally:Trace_core.shutdown f
+
+module Private_ = struct
+  let mock_all_ () = Trace_util.Mock_.mock_all ()
+end

src/spall/trace_spall.mli

@@ -0,0 +1,40 @@
+(** Spall backend for Trace.
+
+    This emits Spall binary traces (.spall), viewable at
+    https://gravitymoth.com/spall/spall.html .
+
+    Reference format: spall.h (vendored alongside this library). *)
+
+module Collector_spall = Collector_spall
+module Writer = Writer
+
+type output =
+  [ `Stdout
+  | `Stderr
+  | `File of string
+  ]
+(** Output destination for tracing. *)
+
+val collector : out:[< output ] -> unit -> Trace_core.collector
+(** Make a collector writing to the given output. *)
+
+val setup : ?out:[ output | `Env ] -> unit -> unit
+(** [setup ()] installs the Spall collector.
+
+    @param out
+      where to write events:
+      - a {!output} value, or
+      - [`Env] (default): enabled if the [TRACE] environment variable is set.
+        ["1"] or ["true"] writes to [trace.spall]; any other value is the path;
+        ["stdout"] / ["stderr"] write to those streams. *)
+
+val with_setup : ?out:[ output | `Env ] -> unit -> (unit -> 'a) -> 'a
+(** [with_setup () f] sets up the collector, calls [f()], then shuts down. *)
+
+(**/**)
+
+module Private_ : sig
+  val mock_all_ : unit -> unit
+end
+
+(**/**)

src/spall/writer.ml

@@ -0,0 +1,45 @@
+open struct
+  let[@inline] add_u8 buf n = Buffer.add_char buf (Char.chr (n land 0xff))
+  let add_u64_le = Buffer.add_int64_le
+  let[@inline] add_f64_le buf f = add_u64_le buf (Int64.bits_of_float f)
+end
+
+let write_file_header buf ~timestamp_unit =
+  add_u64_le buf 0x0BADF00DL;
+  add_u64_le buf 3L;
+  add_f64_le buf timestamp_unit;
+  add_u64_le buf 0L
+
+let write_header ~size ~tid ~pid ~first_ts : string =
+  let buf = Bytes.create 20 in
+  Bytes.set_int32_le buf 0 (Int32.of_int size);
+  Bytes.set_int32_le buf 4 (Int32.of_int tid);
+  Bytes.set_int32_le buf 8 (Int32.of_int pid);
+  Bytes.set_int64_le buf 12 first_ts;
+  Bytes.unsafe_to_string buf
+
+(* type=3 Begin: type(u8) when(u64) name_len(u8) args_len(u8) name_bytes *)
+let write_begin buf ~ts ~name =
+  let name_len = min (String.length name) 255 in
+  add_u8 buf 3;
+  add_u64_le buf ts;
+  add_u8 buf name_len;
+  add_u8 buf 0;
+  Buffer.add_substring buf name 0 name_len
+
+(* type=4 End: type(u8) when(u64) *)
+let write_end buf ~ts =
+  add_u8 buf 4;
+  add_u64_le buf ts
+
+(* type=8 NameProcess / type=9 NameThread: type(u8) name_len(u8) name_bytes *)
+let write_name buf ~kind ~name =
+  let ty =
+    match kind with
+    | `Process -> 8
+    | `Thread -> 9
+  in
+  let name_len = min (String.length name) 255 in
+  add_u8 buf ty;
+  add_u8 buf name_len;
+  Buffer.add_substring buf name 0 name_len

trace-spall.opam

@@ -0,0 +1,37 @@
+# This file is generated by dune, edit dune-project instead
+opam-version: "2.0"
+version: "0.12"
+synopsis:
+  "A fast binary backend for trace, emitting Spall profiler format (.spall)"
+maintainer: ["Simon Cruanes"]
+authors: ["Simon Cruanes"]
+license: "MIT"
+tags: ["trace" "tracing" "spall" "profiling"]
+homepage: "https://github.com/ocaml-tracing/ocaml-trace"
+bug-reports: "https://github.com/ocaml-tracing/ocaml-trace/issues"
+depends: [
+  "ocaml" {>= "4.08"}
+  "trace" {= version}
+  "mtime" {>= "2.0"}
+  "thread-local-storage" {>= "0.2"}
+  "base-unix"
+  "dune" {>= "2.9"}
+  "odoc" {with-doc}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "--promote-install-files=false"
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+  ["dune" "install" "-p" name "--create-install-files" name]
+]
+dev-repo: "git+https://github.com/ocaml-tracing/ocaml-trace.git"