CP-52709: use timeslices shorter than 50ms

ocaml/libs/timeslice/dune

@@ -1,5 +1,5 @@
 (library
 	(name xapi_timeslice)
-	(package xapi)
+	(package xapi-idl)
 	(libraries threads.posix mtime mtime.clock.os)
 )

ocaml/xapi-idl/lib/dune

@@ -26,6 +26,7 @@
    unix
    uri
    uuidm
+   xapi_timeslice
    xapi-backtrace
    xapi-consts
    xapi-log

ocaml/xapi-idl/lib/xcp_service.ml

@@ -163,6 +163,25 @@ let setify =
   in
   loop []
 
+(** How long to let an OCaml thread run, before
+    switching to another thread.
+    This needs to be as small as possible to reduce latency.
+
+    Too small values reduce performance due to context switching overheads
+
+    4ms = 1/HZ in Dom0 seems like a good default,
+    a better value will be written by a boot time service.
+ *)
+let timeslice = ref 0.05
+
+let apply_timeslice () =
+  let interval = !timeslice in
+  D.debug "Setting timeslice to %.3fs" interval ;
+  if interval >= 0.05 then
+    D.debug "Timeslice same as or larger than default: not setting"
+  else
+    Xapi_timeslice.Timeslice.set interval
+
 let common_options =
   [
     ( "use-switch"
@@ -236,6 +255,11 @@ let common_options =
     , (fun () -> !config_dir)
     , "Location of directory containing configuration file fragments"
     )
+  ; ( "timeslice"
+    , Arg.Set_float timeslice
+    , (fun () -> Printf.sprintf "%.3f" !timeslice)
+    , "timeslice in seconds"
+    )
   ]
 
 let loglevel () = !log_level
@@ -454,6 +478,7 @@ let configure_common ~options ~resources arg_parse_fn =
         failwith (String.concat "\n" lines)
     )
     resources ;
+  apply_timeslice () ;
   Sys.set_signal Sys.sigpipe Sys.Signal_ignore
 
 let configure ?(argv = Sys.argv) ?(options = []) ?(resources = []) () =