Update tracing section of README

Author: talex5

README.md

@@ -220,70 +220,30 @@ Calling an operation that performs an effect (such as `yield`) can switch to a d
 When OCaml's tracing is turned on, Eio writes events about many actions,
 such as creating fibers or resolving promises.
 
-[examples/trace](./examples/trace/) shows how to consume the events manually:
+You can use [eio-trace][] to capture a trace and display it in a window.
+For example, this is a trace of the counting example above:
 
-<!-- $MDX skip -->
-```sh
-$ dune exec -- ./examples/trace/main.exe
-+tracer: starting                      
-30926487700447:ring 0: create fiber 0
-30926487702032:ring 0: running fiber 0
-30926487705057:ring 0: create switch 1
-30926487707264:ring 0: create fiber 2
-30926487707512:ring 0: running fiber 2
-30926487720213:ring 0: log "tracer: starting"
-+server: starting
-+client: connecting socket...
-+server: got connection from client
-+server: read "Hello" from socket
-30926487769298:ring 0: running fiber 0
-30926487769877:ring 0: create fiber 3
-30926487770083:ring 0: running fiber 3
-30926487771198:ring 0: create switch 4
-30926487807888:ring 0: create switch 5
-30926487808329:ring 0: create fiber 6
-30926487808555:ring 0: running fiber 6
-30926487812219:ring 0: log "server: starting"
-30926487818883:ring 0: running fiber 3
-30926487819091:ring 0: create fiber 7
-30926487819155:ring 0: running fiber 7
-30926487822428:ring 0: log "client: connecting socket..."
-30926487901604:ring 0: running fiber 3
-30926487904947:ring 0: running fiber 0
-30926487907318:ring 0: running fiber 6
-30926487917202:ring 0: log "server: got connection from client"
-30926487929993:ring 0: running fiber 6
-30926487941403:ring 0: running fiber 7
-30926487948000:ring 0: running fiber 7
-30926487971898:ring 0: resolve 7
-30926487974810:ring 0: running fiber 6
-30926487975215:ring 0: running fiber 6
-30926487977869:ring 0: running fiber 6
-30926487984514:ring 0: log "server: read \"Hello\" from socket"
-30926487990785:ring 0: resolve 6
-30926487991752:ring 0: running fiber 3
-30926488022310:ring 0: resolve 3
-30926497839725:ring 0: running fiber 2
-+tracer: stopping
 ```
+dune build ./examples
+eio-trace run -- ./_build/default/examples/both/main.exe
+```
+
+<p align='center'>
+  <img src="./doc/traces/both-posix.svg"/>
+</p>
 
+The upper horizontal bar is the initial fiber, and the brackets show `Fiber.both` creating a second fiber.
+The green segments show when each fiber is running.
 Note that the output from `traceln` appears in the trace as well as on the console.
+In the eio-trace window, scrolling with the mouse or touchpad will zoom in or out of the diagram.
 
-There are various third-party tools that can consume this data
+There are various third-party tools that can also consume this data
 (but may currently require patches to support the new system):
 
 - [Meio][] (Monitoring for Eio) provides an interactive console-based UI for exploring running fibers.
 - [Olly][] can save Perfetto traces and report statistics.
-- [mirage-trace-viewer][] renders traces visually.
-
-For example, this is how mirage-trace-viewer renders the counting example above:
-
-<p align='center'>
-  <img src="./doc/trace.svg"/>
-</p>
 
-This shows the two counting threads as two horizonal lines.
-The white regions indicate when each thread was running.
+[examples/trace](./examples/trace/) shows how to consume the events manually.
 
 ## Cancellation
 
@@ -1893,3 +1853,4 @@ Some background about the effects system can be found in:
 [Eio.Process]: https://ocaml-multicore.github.io/eio/eio/Eio/Process/index.html
 [Dev meetings]: https://docs.google.com/document/d/1ZBfbjAkvEkv9ldumpZV5VXrEc_HpPeYjHPW_TiwJe4Q
 [Olly]: https://github.com/tarides/runtime_events_tools
+[eio-trace]: https://github.com/ocaml-multicore/eio-trace

doc/trace.svg

@@ -0,0 +1,2 @@
+all:
+	eio-trace render -f both-posix.fxt both-posix.svg

doc/traces/Makefile

@@ -0,0 +1,3 @@
+(executable
+  (name main)
+  (libraries eio_main))

doc/traces/both-posix.fxt

@@ -0,0 +1,7 @@
+open Eio.Std
+
+let () =
+  Eio_main.run @@ fun _env ->
+  Fiber.both
+    (fun () -> for x = 1 to 3 do traceln "x = %d" x; Fiber.yield () done)
+    (fun () -> for y = 1 to 3 do traceln "y = %d" y; Fiber.yield () done)

doc/traces/both-posix.svg

@@ -18,9 +18,6 @@ module Rcfd = Rcfd
 module Fork_action = Fork_action
 
 let run_in_systhread ?(label="systhread") fn =
-  Eio.Private.Suspend.enter label @@ fun fiber enqueue ->
-  match Eio.Private.Fiber_context.get_error fiber with
-  | Some err -> enqueue (Error err)
-  | None ->
-    let _t : Thread.t = Thread.create (fun () -> enqueue (try Ok (fn ()) with exn -> Error exn)) () in
-    ()
+  Eio.Private.Suspend.enter label @@ fun _ctx enqueue ->
+  let _t : Thread.t = Thread.create (fun () -> enqueue (try Ok (fn ()) with exn -> Error exn)) () in
+  ()