CP-52709: use timeslices shorter than 50ms

ocaml/libs/timeslice/dune

@@ -0,0 +1,5 @@
+(library
+	(name xapi_timeslice)
+	(package xapi-idl)
+ 	(libraries threads.posix mtime mtime.clock.os xapi-log)
+)

ocaml/libs/timeslice/recommended.ml

@@ -0,0 +1,34 @@
+module D = Debug.Make (struct let name = "timeslice_recommended" end)
+
+let yield_stop = Atomic.make false
+
+let yield_worker () =
+  while not (Atomic.get yield_stop) do
+    Thread.yield ()
+  done
+
+let yield_overhead () =
+  (* Thread.yield only has an effect if another thread exists,
+     so create one that yields back immediately *)
+  D.debug "Measuring Thread.yield overhead" ;
+  Atomic.set yield_stop false ;
+  let t = Thread.create yield_worker () in
+  let measured = Simple_measure.measure Thread.yield in
+  D.debug "Thread.yield overhead: %.6fs <= %.6fs <= %.6fs" measured.low
+    measured.median measured.high ;
+  D.debug "Waiting for worker thread to stop" ;
+  Atomic.set yield_stop true ;
+  Thread.join t ;
+  measured.median
+
+let measure ?(max_overhead_percentage = 1.0) () =
+  let overhead = yield_overhead () in
+  let interval = overhead /. (max_overhead_percentage /. 100.) in
+  D.debug "Recommended timeslice interval = %.4fs" interval ;
+  (* Avoid too high or too low intervals:
+     do not go below 1ms (our HZ is 250, and max is 1000, the kernel would round up anyway)
+     do not go above 50ms (the current default in OCaml 4.14)
+  *)
+  let interval = interval |> Float.max 0.001 |> Float.min 0.050 in
+  D.debug "Final recommeded timeslice interval = %.4fs" interval ;
+  interval

ocaml/libs/timeslice/recommended.mli

@@ -0,0 +1,8 @@
+val measure : ?max_overhead_percentage:float -> unit -> float
+(** [measure  ?max_overhead_percentage ()] returns the recommended timeslice for the current system.
+
+  The returned value should be used in a call to {!val:Timeslice.set}.
+
+  @param max_overhead_percentage default 1%
+  @returns [interval] such that [overhead / interval <= max_overhead_percentage / 100]
+ *)

ocaml/libs/timeslice/simple_measure.ml

@@ -0,0 +1,47 @@
+(** 95% confidence interval, and median value *)
+type t = {low: float; median: float; high: float}
+
+let span_to_s s = Mtime.Span.to_float_ns s *. 1e-9
+
+let ci95 measurements =
+  let n = Array.length measurements in
+  Array.sort Float.compare measurements ;
+  let median = measurements.(n / 2) in
+  (* "Performance Evaluation of Computer and Communication Systems", Table A. 1 *)
+  let n = float n in
+  let d = 0.98 *. sqrt n in
+  let lo = (n /. 2.) -. d |> Float.to_int
+  and hi = (n /. 2.) +. 1. +. d |> Float.ceil |> Float.to_int in
+  {low= measurements.(lo - 1); median; high= measurements.(hi - 1)}
+
+let measure ?(n = 1001) ?(inner = 10) f =
+  if n <= 70 then (* some of the formulas below are not valid for smaller [n] *)
+    invalid_arg (Printf.sprintf "n must be at least 70: %d" n) ;
+  (* warmup *)
+  Sys.opaque_identity (f ()) ;
+
+  let measure_inner _ =
+    let m = Mtime_clock.counter () in
+    for _ = 1 to inner do
+      (* opaque_identity prevents the call from being optimized away *)
+      Sys.opaque_identity (f ())
+    done ;
+    let elapsed = Mtime_clock.count m in
+    span_to_s elapsed /. float inner
+  in
+  let measurements = Array.init n measure_inner in
+  ci95 measurements
+
+let measure_min ?(n = 1001) f arg =
+  (* warmup *)
+  Sys.opaque_identity (f arg) ;
+  let measure_one _ =
+    let m = Mtime_clock.counter () in
+    Sys.opaque_identity (f arg) ;
+    let elapsed = Mtime_clock.count m in
+    span_to_s elapsed
+  in
+  Seq.ints 0
+  |> Seq.take n
+  |> Seq.map measure_one
+  |> Seq.fold_left Float.min Float.max_float

ocaml/libs/timeslice/simple_measure.mli

@@ -0,0 +1,33 @@
+(** Measure the speed of an operation in a very simple and robust way.
+    More detailed measurements can be dune using [Bechamel].
+*)
+
+(** 95% confidence interval, and median value *)
+type t = {low: float; median: float; high: float}
+
+val measure : ?n:int -> ?inner:int -> (unit -> unit) -> t
+(** [measure ?n ?inner f] measures [n] times the duration of [inner] iterations of [f ()].
+
+    Returns the median of the inner measurements, and a 95% confidence interval.
+    The median is used, because it makes no assumptions about the distribution of the samples,
+    i.e. it doesn't require a normal (Gaussian) distribution.
+
+    The inner measurements use a simple average, because we only know the duration of [inner] iterations,
+    not the duration of each individual call to [f ()].
+    The purpose of the [inner] iterations is to reduce measurement overhead.
+
+    @param n iteration count for the outer loop, must be more than [70].
+    @param n iteration count for the inner loop
+    @param f function to measure
+
+    @raises Invalid_argument if [n<70]
+ *)
+
+val measure_min : ?n:int -> ('a -> unit) -> 'a -> float
+(** [measure_min ?n:int f arg] is the minimum amount of time that [f arg] takes.
+
+    This should be used when we try to measure the maximum speed of some operation (e.g. cached memory accesses),
+    while ignoring latencies/hickups introduced by other processes on the system.
+
+    It shouldn't be used for measuring the overhead of an operation, because the hickups may be part of that overhead.
+ *)

ocaml/libs/timeslice/timeslice.ml

@@ -0,0 +1,48 @@
+(* avoid allocating an extra option every time *)
+let invalid_holder = -1
+
+let last_lock_holder = Atomic.make invalid_holder
+
+let me () = Thread.self () |> Thread.id
+
+let lock_acquired () =
+  (* these need to be very low overhead, so just keep track of the last lock holder,
+     i.e. track only one high-priority lock at a time
+  *)
+  Atomic.set last_lock_holder (me ())
+
+let lock_released () = Atomic.set last_lock_holder invalid_holder
+
+let[@inline always] am_i_holding_locks () =
+  let last = Atomic.get last_lock_holder in
+  last <> invalid_holder && last = me ()
+
+let yield_interval = Atomic.make Mtime.Span.zero
+
+(* TODO: use bechamel.monotonic-clock instead, which has lower overhead,
+   but not in the right place in xs-opam yet
+*)
+let last_yield = Atomic.make (Mtime_clock.counter ())
+
+let periodic_hook (_ : Gc.Memprof.allocation) =
+  ( if not (am_i_holding_locks ()) then
+      let elapsed = Mtime_clock.count (Atomic.get last_yield) in
+      if Mtime.Span.compare elapsed (Atomic.get yield_interval) > 0 then (
+        let now = Mtime_clock.counter () in
+        Atomic.set last_yield now ; Thread.yield ()
+      )
+  ) ;
+  None
+
+let periodic =
+  Gc.Memprof.
+    {null_tracker with alloc_minor= periodic_hook; alloc_major= periodic_hook}
+
+let set ?(sampling_rate = 1e-4) interval =
+  Atomic.set yield_interval
+    (Mtime.Span.of_float_ns @@ (interval *. 1e9) |> Option.get) ;
+  Gc.Memprof.start ~sampling_rate ~callstack_size:0 periodic
+
+let clear () =
+  Gc.Memprof.stop () ;
+  Atomic.set yield_interval Mtime.Span.zero

ocaml/libs/timeslice/timeslice.mli

@@ -0,0 +1,30 @@
+val set : ?sampling_rate:float -> float -> unit
+(** [set ?sampling_rate interval] calls [Thread.yield ()] at most [interval] seconds.
+
+  The implementation of [Thread.yield] guarantees since OCaml 4.09 that we'll switch to a different OCaml thread,
+  if one exists that is not blocked (i.e. it doesn't rely on [sched_yield] which may run the same thread again,
+  but uses pthread mutexes and condition variables to ensure the current thread isn't immediately runnable).
+
+  The setting is global for the entire process, and currently uses [Gc.Memprof] to ensure that a hook function is called periodically,
+  although it depends on the allocation rate of the program whether it gets called at all.
+
+  Another alternative would be to use {!val:Unix.set_itimer}, but XAPI doesn't cope with [EINTR] in a lot of places,
+  and POSIX interval timers rely on signals to notify of elapsed time.
+
+  We could also have a dedicated thread that sleeps for a certain amount of time, but if it is an OCaml thread,
+  we'd have no guarantees it'd get scheduled often enough (and it couldn't interrupt other threads anyway,
+  by the time you'd be running the handler you already gave up running something else).
+
+  It may be desirable to avoid yielding if we are currently holding a lock, see {!val:lock_acquired}, and {!val:lock_released}
+  to notify this module when that happens.
+*)
+
+val clear : unit -> unit
+(** [clear ()] undoes the changes made by [set].
+  This is useful for testing multiple timeslices in the same program. *)
+
+val lock_acquired : unit -> unit
+(** [lock_acquired ()] notifies about lock acquisition. *)
+
+val lock_released : unit -> unit
+(** [lock_acquired ()] notifies about lock release. *)

ocaml/tests/common/dune

@@ -28,6 +28,7 @@
     xapi-stdext-date
     xapi-stdext-threads.scheduler
     xapi-stdext-unix
+    xapi_timeslice
   )
 )
 

ocaml/tests/common/suite_init.ml

@@ -11,4 +11,6 @@ let harness_init () =
     Filename.concat Test_common.working_area "xapi-inventory" ;
   Xcp_client.use_switch := false ;
   Pool_role.set_pool_role_for_test () ;
-  Message_forwarding.register_callback_fns ()
+  Message_forwarding.register_callback_fns () ;
+  (* for unit tests use a fixed value *)
+  Xapi_timeslice.Timeslice.set 0.004

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 = []) () =