Unix: increase the maximum number of preemptive threads

We require preemptive threads for reading DNS responses on Mac.
The default maximum number of threads is quite small (4), so increase
to 1024.

Note that these threads are created on demand.

Signed-off-by: David Scott <[email protected]>

Author: djs55

src/bin/main.ml

@@ -352,6 +352,11 @@ let hvsock_addr_of_uri ~default_serviceid uri =
       if debug || env_debug then Some Logs.Debug else Some Logs.Info in
     Logging.setup log_destination level;
 
+    if Sys.os_type = "Unix" then begin
+      Log.info (fun f -> f "Increasing preemptive thread pool size to 1024 threads");
+      Uwt_preemptive.set_bounds (0, 1024);
+    end;
+
     let host_names = List.map Dns.Name.of_string @@ Astring.String.cuts ~sep:"," host_names in
     let dns_path, resolver = match dns with
     | None -> None, Configuration.default_resolver