package qcheck-multicoretests-util

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Various utility functions for property-based testing of multicore programs


Dune Dependency






A small library of utility functions for QCheck-based testing of multicore programs.


Multicore tests

Property-based tests of (parts of) the OCaml multicore compiler and run time.

This project contains

  • a randomized test suite of OCaml 5.x, packaged up in multicoretests.opam

  • two reusable testing libraries:

    • Lin packaged up in qcheck-lin.opam and

    • STM packaged up in qcheck-stm.opam

All of the above build on QCheck, a black-box, property-based testing library in the style of QuickCheck.

The two libraries are already quite helpful

Installation instructions, and running the tests

The multicore test suite requires OCaml 5.x:

opam update
opam switch create 5.0.0

Installing the libraries

The two testing libraries are available as packages qcheck-lin and qcheck-stm from the opam repository. The full versions require OCaml 5.x and reduced, non-Domain versions are available for OCaml 4.12.x to 4.14.x. They can be installed in the usual way:

opam install qcheck-lin
opam install qcheck-stm

Bleeding edge users can pin and install the latest main as follows:

opam pin -y

To use the Lin library in parallel mode on a Dune project, add the following dependency to your dune rule:

  (libraries qcheck-lin.domain)

Using the STM library in sequential mode requires the dependency (libraries qcheck-stm.sequential) and the parallel mode similarly requires the dependency (libraries qcheck-stm.domain).

Running the test suite

We have not released the test suite on the opam repository at this point. The test suite can be built and run from a clone of this repository with the following commands:

opam install . --deps-only --with-test
dune build
dune runtest -j1 --no-buffer --display=quiet

Individual tests can be run by invoking dune exec. For example:

$ dune exec src/atomic/stm_tests.exe -- -v
random seed: 51501376
generated error fail pass / total     time test name
[✓] 1000    0    0 1000 / 1000     0.2s sequential atomic test
[✓] 1000    0    0 1000 / 1000   180.8s parallel atomic test
success (ran 2 tests)

See src/ for an overview of the current PBTs of OCaml 5.x.

It is also possible to run the test suite in the CI, by altering .github/workflows/common.yml to target a particular compiler PR:

      OCAML_COMPILER_GIT_REF:   'refs/pull/12345/head'
      CUSTOM_COMPILER_VERSION:  '5.1.0+pr12345'
      CUSTOM_COMPILER_SRC:      ''

or a particular branch:

      CUSTOM_COMPILER_VERSION:  '5.1.0+myexperiment'
      CUSTOM_COMPILER_SRC:      ''

A Linearization Tester

The Lin module lets a user test an API for sequential consistency, i.e., it performs a sequence of random commands in parallel, records the results, and checks whether the observed results can be linearized and reconciled with some sequential execution. The library offers an embedded, combinator DSL to describe signatures succinctly. As an example, the required specification to test (a small part of) the Hashtbl module is as follows:

module HashtblSig =
  type t = (char, int) Hashtbl.t
  let init () = Hashtbl.create ~random:false 42
  let cleanup _ = ()

  open Lin
  let a,b = char_printable,nat_small
  let api =
    [ val_ "Hashtbl.add"    Hashtbl.add    (t @-> a @-> b @-> returning unit);
      val_ "Hashtbl.remove" Hashtbl.remove (t @-> a @-> returning unit);
      val_ "Hashtbl.find"   Hashtbl.find   (t @-> a @-> returning_or_exc b);
      val_ "Hashtbl.mem"    Hashtbl.mem    (t @-> a @-> returning bool);
      val_ "Hashtbl.length" Hashtbl.length (t @-> returning int); ]

module HT = Lin_domain.Make(HashtblSig)
QCheck_base_runner.run_tests_main [
  HT.lin_test `Domain ~count:1000 ~name:"Hashtbl DSL test";

The first line indicates the type of the system under test along with bindings init and cleanup for setting it up and tearing it down. The api then contains a list of type signature descriptions using combinators unit, bool, int, returning, returning_or_exc, ... in the style of Ctypes. The functor Lin_domain.Make expects a description of the tested commands and outputs a module with a QCheck test lin_test that performs the linearization test.

The QCheck linearization test iterates a number of test instances. Each instance consists of a "sequential prefix" of calls to the above commands, followed by a spawn of two parallel Domains that each call a sequence of operations. Lin chooses the individual operations and arguments arbitrarily and records their results. The framework then performs a search for a sequential interleaving of the same calls, and succeeds if it finds one.

Since Hashtbls are not safe for parallelism, if you run dune exec doc/example/lin_tests_dsl.exe the output can produce the following output, where each tested command is annotated with its result:

Messages for test Hashtbl DSL test:

  Results incompatible with sequential execution

                 |                                    |
     Hashtbl.add t 'a' 0  : ()            Hashtbl.add t 'a' 0  : ()
       Hashtbl.length t  : 1                Hashtbl.length t  : 1

In this case, the test tells us that there is no sequential interleaving of these calls which would return 1 from both calls to Hashtbl.length. For example, in the following sequential interleaving the last call should return 2:

 Hashtbl.add t 'a' 0;;
 let res1 = Hashtbl.length t;;
 Hashtbl.add t 'a' 0;;
 let res2 = Hashtbl.length t;;

See src/atomic/ for another example of testing the Atomic module.

A Parallel State-Machine Testing Library

STM contains a revision of qcstm extended to run parallel state-machine tests akin to Erlang QuickCheck, Haskell Hedgehog, ScalaCheck, .... To do so, the STM library also performs a sequence of random operations in parallel and records the results. In contrast to Lin, STM then checks whether the observed results are linearizable by reconciling them with a sequential execution of a model description. The model expresses the intended meaning of each tested command. As such, it requires more of the user compared to Lin. The corresponding code to describe a Hashtbl test using STM is given below:

open QCheck
open STM

(** parallel STM tests of Hashtbl *)

module HashtblModel =
  type sut = (char, int) Hashtbl.t
  type state = (char * int) list
  type cmd =
    | Add of char * int
    | Remove of char
    | Find of char
    | Mem of char
    | Length [@@deriving show { with_path = false }]

  let init_sut () = Hashtbl.create ~random:false 42
  let cleanup (_:sut) = ()

  let arb_cmd (s:state) =
    let char =
      if s=[]
      then Gen.printable
      else Gen.(oneof [oneofl ( fst s); printable]) in
    let int = Gen.nat in
    QCheck.make ~print:show_cmd
         [Gen.map2 (fun k v -> Add (k,v)) char int;
  (fun k   -> Remove k) char;
  (fun k   -> Find k) char;
  (fun k   -> Mem k) char;
          Gen.return Length; ])

  let next_state (c:cmd) (s:state) = match c with
    | Add (k,v) -> (k,v)::s
    | Remove k  -> List.remove_assoc k s
    | Find _
    | Mem _
    | Length    -> s

  let run (c:cmd) (h:sut) = match c with
    | Add (k,v) -> Res (unit, Hashtbl.add h k v)
    | Remove k  -> Res (unit, Hashtbl.remove h k)
    | Find k    -> Res (result int exn, protect (Hashtbl.find h) k)
    | Mem k     -> Res (bool, Hashtbl.mem h k)
    | Length    -> Res (int,  Hashtbl.length h)

  let init_state = []

  let precond (_:cmd) (_:state) = true
  let postcond (c:cmd) (s:state) (res:res) = match c,res with
    | Add (_,_), Res ((Unit,_),_)
    | Remove _,  Res ((Unit,_),_) -> true
    | Find k,    Res ((Result (Int,Exn),_),r) -> r = (try Ok (List.assoc k s) with Not_found -> Error Not_found)
    | Mem k,     Res ((Bool,_),r) -> r = List.mem_assoc k s
    | Length,    Res ((Int,_),r)  -> r = List.length s
    | _ -> false

module HT_seq = STM_sequential.Make(HashtblModel)
module HT_dom = STM_domain.Make(HashtblModel)
  (let count = 200 in
   [HT_seq.agree_test     ~count ~name:"Hashtbl test sequential";
    HT_dom.agree_test_par ~count ~name:"Hashtbl test parallel"; ])

Again this requires a type sut for the system under test, and bindings init_sut and cleanup for setting it up and tearing it down. The type cmd describes the tested commands.

The type state = (char * int) list describes with a pure association list the internal state of a Hashtbl. The init_state represents the empty Hashtbl mode and the state transition function next_state describes how it changes across each cmd. For example, Add (k,v) appends the key-value pair onto the association list.

arb_cmd is a generator of cmds, taking state as a parameter. This allows for state-dependent cmd generation, which we use to increase the chance of producing a Remove 'c', Find 'c', ... following an Add 'c'. Internally arb_cmd uses QCheck combinators Gen.return,, and Gen.map2 to generate one of 5 different commands.

run executes the tested cmd over the sut and wraps the result up in a result type res offered by STM. Combinators unit, bool, int, ... allow to annotate the result with the expected type. postcond expresses a post-condition by matching the received res, for a cmd with the corresponding answer from the model. For example, this compares the Boolean result r from Hashtbl.mem with the result from List.mem_assoc. Similarly precond expresses a pre-condition.

The module is phrased as functors:

  • the functor STM_sequential.Make produces a module with a function agree_test to test whether the model agrees with the sut across a sequential run of an arbitrary command sequence and

  • the functor STM_domain.Make produces a module with a function agree_test_par which tests in parallel by spawning two domains with Domain similarly to Lin and searches for a sequential interleaving over the model.

When running the above with the command dune exec doc/example/stm_tests.exe one may obtain the following output:

Messages for test Hashtbl test parallel:

  Results incompatible with linearized model

               |                                    |
     (Add ('e', 5268)) : ()                (Add ('!', 4)) : ()
           Length : 1                           Length : 1

This illustrates how two hashtable Add commands may interfere when executed in parallel, leaving only 1 entry in the resulting Hashtbl - which is not reconcilable with the declarative model description.

The above examples are available from the doc/example directory. The doc directory also contains our 2022 OCaml Workshop paper presenting the project in a bit more detail.

Repeatability Efforts

Both Lin and STM perform randomized property-based testing with QCheck. When rerunning a test to shrink/reduce the test input, QCheck thus starts from the same Random seed to limit non-determinism. This is however not suffient for multicore programs where CPU scheduling and garbage collection may hinder reproducibility.

Lin and STM primarily uses test repetition to increase reproducibility and it is sufficient that only a single repetition triggers an issue. Currently repeating a non-deterministic QCheck property can be done in two different ways:

  • a repeat-combinator lets you test a property, e.g., 50 times rather than just 1. (Pro: a failure is found faster, Con: wasted, repetitive testing when there are no failures)

  • a QCheck PR extends Test.make with a ~retries parameter causing it to only perform repetition during shrinking. (Pro: each test is cheaper so we can run more, Con: more tests are required to trigger a race)


Float register preservation on ppc64 (new, fixed, codegen)

The sequential Float.Array STM test revealed that a float register was not properly preserved on ppc64, sometimes resulting in random float values appearing

Signal-based overflow on ppc64 crash (new, codegen)

The sequential STM tests of Array, Bytes, and Float.Array would trigger segfaults on ppc64

Frame pointer Effect crashes (new, fixed, codegen)

Negative Lin Effect tests exercising exceptions for unhandled Effects triggered a crash on a frame pointer switch

s390x Effect crashes (new, fixed, codegen)

Negative Lin Effect tests exercising exceptions for unhandled Effects also triggered a crash on the newly restored s390x backend

Sys.rename behaves differently on corner cases under MingW (new, stdlib)

Sequential STM tests targeting Sys.rename found two corner cases where MingW behaves differently

flexdll contains a race condition in its handling of errors (new, fixed, flexdll)

Parallel Lin tests of the Dynlink module found a race condition in accesses to the global variables storing the last error.

Buffer.add_string contained a race condition (new, fixed, stdlib)

Parallel STM tests of the Buffer module found a segfault, leading to the discovery of an assertion failure revealing a race condition in the add_string function

Parallel Weak Hashset usage may crash the runtime (new, fixed, runtime)

Parallel STM tests found a combination of Weak Hashset functions that may cause the run-time to abort or segfault

Sys.readdir on MingW disagrees with Linux behavior (new, fixed, stdlib)

Sequential STM tests of Sys showed how Sys.readdir of a non-existing directory on MingW Windows returns an empty array, thus disagreeing with the Linux and macOS behavior

seek on a closed in_channel may read uninitialized memory (new, fixed, runtime)

A failure of Lin In_channel tests revealed that seek on a closed in_channel may read uninitialized memory

Parallel usage of Weak could produce weird values (new, fixed, runtime)

Racing Weak.set and Weak.get can in some cases produce strange values

Bytecode interpreter can segfault on unhandled Effect (new, fixed, runtime)

In seldom cases the tests would trigger a segfault in the bytecode interpreter when treating an unhandled Effect

Ephemeron can fail assert and abort (new, fixed, runtime)

In some cases (even sequential) the Ephemeron tests can trigger an assertion failure and abort.

Parallel usage of Bytes.escaped is unsafe (new, fixed, stdlib)

The Bytes tests triggered a segfault which turned out to be caused by an unsafe Bytes.escaped definition.

Infinite loop in caml_scan_stack on ARM64 (known, fixed, runtime)

The tests triggered an apparent infinite loop on ARM64 while amd64 would complete the tests as expected.

Unsafe Buffer module (new, fixed, stdlib)

The tests found that the Buffer module implementation is unsafe under parallel usage - initially described in multicoretests#63.

MacOS segfault (new, fixed, runtime)

The tests found an issue causing a segfault on MacOS.

In_channel and Out_channel unsafety (new, fixed, runtime)

The tests found a problem with In_channel and Out_channel which could trigger segfaults under parallel usage. For details see issue ocaml-multicore/multicoretests#13 and this ocaml/ocaml#10960 comment.

Cornercase issue in Domainslib (new, fixed, domainslib)

The tests found an issue in Domainslib.parallel_for_reduce which would yield the wrong result for empty arrays. As of domainslib#100 the Domainslib tests have been moved to the Domainslib repo.

Specification of Lockfree.Ws_deque (new, fixed, lockfree)

The initial tests of ws_deque just applied the parallelism property agree_prop_par. However that is not sufficient, as only the original domain (thread) is allowed to call push, pop, ..., while a spawned domain should call only steal.

A custom, revised property test runs a cmd prefix, then spawns a "stealer domain" with steal, ... calls, while the original domain performs calls across a broder random selection (push, pop, ...). As of lockfree#43 this test has now been moved to the lockfree repo.

Here is an example output illustrating how size may return -1 when used in a "stealer domain". The first line in the Failure section lists the original domain's commands and the second lists the stealer domains commands (Steal,...). The second Messages section lists a rough dump of the corresponding return values: RSteal (Some 73) is the result of Steal, ... Here it is clear that the spawned domain successfully steals 73, and then observes both a -1 and 0 result from size depending on timing. Size should therefore not be considered threadsafe (none of the two papers make any such promises though):

$ dune exec src/ws_deque_test.exe
random seed: 55610855
generated error  fail  pass / total     time test name
[✗]   318     0     1   317 / 10000     2.4s parallel ws_deque test (w/repeat)

--- Failure --------------------------------------------------------------------

Test parallel ws_deque test (w/repeat) failed (8 shrink steps):

 Seq.prefix:  Parallel procs.:

          []  [(Push 73); Pop; Is_empty; Size]

              [Steal; Size; Size]

+++ Messages ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Messages for test parallel ws_deque test (w/repeat):

Result observations not explainable by linearized model:

  Seq.prefix:  Parallel procs.:

          []  [RPush; (RPop None); (RIs_empty true); (RSize 0)]

              [(RSteal (Some 73)); (RSize -1); (RSize 0)]

failure (1 tests failed, 0 tests errored, ran 1 tests)

Segfault in Domainslib (known, fixed, domainslib)

Testing Domainslib.Tasks with one dependency and with 2 work pools found a segfault in domainslib. As of domainslib#100 the domainslib/ test in question has been moved to the Domainslib repo.

Dead-lock in Domainslib (known, fixed, domainslib)

A reported deadlock in domainslib motivated the development of these tests:



The tests domainslib/ and domainslib/ were run with a timeout to prevent deadlocking indefinitely. domainslib/ could trigger one such deadlock. As of domainslib#100 these tests have been moved to the Domainslib repo.

The test exhibits no non-determistic behaviour when repeating the same tested property from within the QCheck test. However it fails (due to timeout) on the following test input:

$ dune exec -- src/task_one_dep.exe -v
random seed: 147821373
generated error fail pass / total     time test name
[✗]   25    0    1   24 /  100    36.2s Task.async/await

--- Failure --------------------------------------------------------------------

Test Task.async/await failed (2 shrink steps):

{ num_domains = 3; length = 6;
  dependencies = [|None; (Some 0); None; (Some 1); None; None|] }
failure (1 tests failed, 0 tests errored, ran 1 tests)

This corresponds to the following program with 3+1 domains and 6 promises. It loops infinitely with both bytecode/native:

open Domainslib

(* a simple work item, from ocaml/testsuite/tests/misc/ *)
let rec tak x y z =
  if x > y then tak (tak (x-1) y z) (tak (y-1) z x) (tak (z-1) x y)
           else z

let work () =
  for _ = 1 to 200 do
    assert (7 = tak 18 12 6);

let pool = Task.setup_pool ~num_additional_domains:3 ()

let p0 = Task.async pool work
let p1 = Task.async pool (fun () -> work (); Task.await pool p0)
let p2 = Task.async pool work
let p3 = Task.async pool (fun () -> work (); Task.await pool p1)
let p4 = Task.async pool work
let p5 = Task.async pool work

let () = List.iter (fun p -> Task.await pool p) [p0;p1;p2;p3;p4;p5]
let () = Task.teardown_pool pool

Utop segfault (known?, status?)

Utop segfaults when loading src/domain/ interactively:

$ utop
                                              │ Welcome to utop version 2.8.0 (using OCaml version 4.12.0+domains)! │
Findlib has been successfully loaded. Additional directives:
  #require "package";;      to load a package
  #list;;                   to list the available packages
  #camlp4o;;                to load camlp4 (standard syntax)
  #camlp4r;;                to load camlp4 (revised syntax)
  #predicates "p,q,...";;   to set these predicates
  Topfind.reset();;         to force that packages will be reloaded
  #thread;;                 to enable threads

Type #utop_help for help about using utop.

utop # #require "";;
utop # #require "qcheck";;
utop # #use "src/";;
type cmd = Incr | Decr | Spawn of cmd list
val pp_cmd : Format.formatter -> cmd -> unit = <fun>
val show_cmd : cmd -> string = <fun>
val count_spawns : cmd -> int = <fun>
val gen : int -> int -> cmd Gen.t = <fun>
val shrink_cmd : cmd Shrink.t = <fun>
val interp : int -> cmd -> int = <fun>
val dom_interp : int Atomic.t -> cmd -> unit = <fun>
val t : max_depth:int -> max_width:int -> Test.t = <fun>
random seed: 359528592
Segmentation fault (core dumped)

This does not happen when running a plain ocaml top-level though, so it seems utop-specific.

Dependencies (3)

  1. qcheck-core >= "0.20"
  2. ocaml >= "4.12"
  3. dune >= "3.0"

Dev Dependencies (1)

  1. odoc with-doc




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