[feat] Allow num_tasks to be None

reframe/core/pipeline.py

@@ -523,16 +523,34 @@ def pipeline_hooks(cls):
 
     #: Number of tasks required by this test.
     #:
-    #: If the number of tasks is set to a number ``<=0``, ReFrame will try to
-    #: flexibly allocate the number of tasks, based on the command line option
-    #: |--flex-alloc-nodes|_. A negative number is used to indicate the minimum
-    #: number of tasks required for the test. In this case the minimum number
-    #: of tasks is the absolute value of the number, while Setting
-    #: :attr:`num_tasks` to ``0`` is equivalent to setting it to
+    #: If the number of tasks is set to zero or a negative value, ReFrame will
+    #: try to flexibly allocate the number of tasks based on the command line
+    #: option :option:`--flex-alloc-nodes`. A negative number is used to
+    #: indicate the minimum number of tasks required for the test. In this case
+    #: the minimum number of tasks is the absolute value of the number, while
+    #: Setting :attr:`num_tasks` to zero is equivalent to setting it to
     #: :attr:`-num_tasks_per_node
     #: <reframe.core.pipeline.RegressionTest.num_tasks_per_node>`.
     #:
-    #: :type: integral
+    #: Setting :attr:`num_tasks` to :obj:`None` has a scheduler-specific
+    #: interpretation, but in principle, passes the responsibility of producing
+    #: a correct job script to the user by setting the appropriate scheduler
+    #: options. More specifically, the different backends interpret the
+    #: :obj:`None` :attr:`num_tasks` as follows:
+    #:
+    #: - ``flux``: not applicable.
+    #: - ``local``: not applicable.
+    #: - ``lsf``: Neither the ``-nnodes`` nor the ``-n`` will be emitted.
+    #: - ``oar``: Resets it to ``1``.
+    #: - ``pbs``: Resets it to ``1``.
+    #: - ``sge``: not applicable.
+    #: - ``slurm``: Neither the ``--ntasks`` nor the ``--nodes`` option (if the
+    #:   :attr:`~config.systems.partitions.sched_options.use_nodes_option` is
+    #:   specified) will be emitted.
+    #: - ``squeue``: See ``slurm`` backend.
+    #: - ``torque``: See ``pbs`` backend.
+    #:
+    #: :type: integral or :obj:`None`
     #: :default: ``1``
     #:
     #: .. note::
@@ -542,10 +560,9 @@ def pipeline_hooks(cls):
     #:     .. versionchanged:: 2.16
     #:        Negative :attr:`num_tasks` is allowed for specifying the minimum
     #:        number of required tasks by the test.
-    #:
-    #: .. |--flex-alloc-nodes| replace:: :attr:`--flex-alloc-nodes`
-    #: .. _--flex-alloc-nodes: manpage.html#cmdoption-flex-alloc-nodes
-    num_tasks = variable(int, value=1, loggable=True)
+    #:     .. versionchanged:: 4.1
+    #:        Allow :attr:`num_tasks` to be :obj:`None`.
+    num_tasks = variable(int, type(None), value=1, loggable=True)
 
     #: Number of tasks per node required by this test.
     #:

reframe/core/schedulers/__init__.py

@@ -174,7 +174,10 @@ class Job(jsonext.JSONSerializable, metaclass=JobMeta):
     #:    based on the test information.
     #:
     #: .. versionadded:: 3.11.0
-    num_tasks = variable(int, value=1)
+    #:
+    #: .. versionchanged:: 4.1
+    #:    Allow :obj:`None` values
+    num_tasks = variable(int, type(None), value=1)
 
     #: Number of tasks per node for this job.
     #:
@@ -487,7 +490,7 @@ def nodelist(self):
         This attribute might be useful in a flexible regression test for
         determining the actual nodes that were assigned to the test.
         For more information on flexible node allocation, see the
-        |--flex-alloc-nodes|_ command-line option
+        :option:`--flex-alloc-nodes` command-line option.
 
         This attribute is *not* supported by the ``pbs`` scheduler backend.
 
@@ -514,7 +517,7 @@ def submit_time(self):
 
     def prepare(self, commands, environs=None, prepare_cmds=None, **gen_opts):
         environs = environs or []
-        if self.num_tasks <= 0:
+        if self.num_tasks is not None and self.num_tasks <= 0:
             getlogger().debug(f'[F] Flexible node allocation requested')
             num_tasks_per_node = self.num_tasks_per_node or 1
             min_num_tasks = (-self.num_tasks if self.num_tasks else

reframe/core/schedulers/lsf.py

@@ -41,7 +41,7 @@ def emit_preamble(self, job):
             self._format_option(job.stderr, '-e {0}'),
         ]
 
-        if job.num_tasks_per_node is not None:
+        if job.num_tasks is not None and job.num_tasks_per_node is not None:
             num_nodes = job.num_tasks // job.num_tasks_per_node
             preamble.append(self._format_option(num_nodes, '-nnodes {0}'))
         else:

reframe/core/schedulers/oar.py

@@ -79,8 +79,9 @@ def emit_preamble(self, job):
             tasks_opt += ',walltime=%d:%d:%d' % (h, m, s)
 
         # Get number of nodes in the reservation
+        num_tasks = job.num_tasks or 1
         num_tasks_per_node = job.num_tasks_per_node or 1
-        num_nodes = job.num_tasks // num_tasks_per_node
+        num_nodes = num_tasks // num_tasks_per_node
 
         # Emit main resource reservation option
         options = [tasks_opt.format(

reframe/core/schedulers/pbs.py

@@ -78,9 +78,10 @@ def __init__(self):
         self._submit_timeout = self.get_option('job_submit_timeout')
 
     def _emit_lselect_option(self, job):
+        num_tasks = job.num_tasks or 1
         num_tasks_per_node = job.num_tasks_per_node or 1
         num_cpus_per_task = job.num_cpus_per_task or 1
-        num_nodes = job.num_tasks // num_tasks_per_node
+        num_nodes = num_tasks // num_tasks_per_node
         num_cpus_per_node = num_tasks_per_node * num_cpus_per_task
         select_opt = self.TASKS_OPT.format(
             num_nodes=num_nodes,

reframe/core/schedulers/slurm.py

@@ -191,7 +191,7 @@ def emit_preamble(self, job):
                 self._format_option(job.exclusive_access, '--exclusive')
             )
 
-        if self._use_nodes_opt:
+        if self._use_nodes_opt and job.num_tasks is not None:
             num_nodes = job.num_tasks // job.num_tasks_per_node
             preamble.append(self._format_option(num_nodes, '--nodes={0}'))
 

unittests/test_schedulers.py

@@ -91,6 +91,13 @@ def minimal_job(make_job):
     return make_job()
 
 
+@pytest.fixture
+def no_tasks_job(make_job):
+    ret = make_job()
+    ret.num_tasks = None
+    return ret
+
+
 @pytest.fixture
 def fake_job(make_job):
     ret = make_job(sched_options=['--account=spam'])
@@ -157,6 +164,23 @@ def _expected_lsf_directives(job):
     ])
 
 
+def _expected_lsf_directives_minimal(job):
+    return set([
+        f'#BSUB -J testjob',
+        f'#BSUB -o {job.stdout}',
+        f'#BSUB -e {job.stderr}',
+        f'#BSUB -n {job.num_tasks}'
+    ])
+
+
+def _expected_lsf_directives_no_tasks(job):
+    return set([
+        f'#BSUB -J testjob',
+        f'#BSUB -o {job.stdout}',
+        f'#BSUB -e {job.stderr}'
+    ])
+
+
 def _expected_flux_directives(job):
     return set()
 
@@ -165,13 +189,8 @@ def _expected_flux_directives_minimal(job):
     return set()
 
 
-def _expected_lsf_directives_minimal(job):
-    return set([
-        f'#BSUB -J testjob',
-        f'#BSUB -o {job.stdout}',
-        f'#BSUB -e {job.stderr}',
-        f'#BSUB -n {job.num_tasks}'
-    ])
+def _expected_flux_directives_no_tasks(job):
+    return set()
 
 
 def _expected_sge_directives(job):
@@ -199,6 +218,9 @@ def _expected_sge_directives_minimal(job):
     ])
 
 
+_expected_sge_directives_no_tasks = _expected_sge_directives_minimal
+
+
 def _expected_slurm_directives(job):
     return set([
         '#SBATCH --job-name="testjob"',
@@ -229,8 +251,17 @@ def _expected_slurm_directives_minimal(job):
     ])
 
 
+def _expected_slurm_directives_no_tasks(job):
+    return set([
+        '#SBATCH --job-name="testjob"',
+        '#SBATCH --output=%s' % job.stdout,
+        '#SBATCH --error=%s' % job.stderr,
+    ])
+
+
 _expected_squeue_directives = _expected_slurm_directives
 _expected_squeue_directives_minimal = _expected_slurm_directives_minimal
+_expected_squeue_directives_no_tasks = _expected_slurm_directives_no_tasks
 
 
 def _expected_pbs_directives(job):
@@ -258,6 +289,9 @@ def _expected_pbs_directives_minimal(job):
     ])
 
 
+_expected_pbs_directives_no_tasks = _expected_pbs_directives_minimal
+
+
 def _expected_torque_directives(job):
     num_nodes = job.num_tasks // job.num_tasks_per_node
     num_cpus_per_node = job.num_cpus_per_task * job.num_tasks_per_node
@@ -284,6 +318,9 @@ def _expected_torque_directives_minimal(job):
     ])
 
 
+_expected_torque_directives_no_tasks = _expected_torque_directives_minimal
+
+
 def _expected_oar_directives(job):
     num_nodes = job.num_tasks // job.num_tasks_per_node
     num_tasks_per_node = job.num_tasks_per_node
@@ -308,6 +345,9 @@ def _expected_oar_directives_minimal(job):
     ])
 
 
+_expected_oar_directives_no_tasks = _expected_oar_directives_minimal
+
+
 def _expected_local_directives(job):
     return set()
 
@@ -316,6 +356,10 @@ def _expected_local_directives_minimal(job):
     return set()
 
 
+def _expected_local_directives_no_tasks(job):
+    return set()
+
+
 def test_prepare(fake_job):
     sched_name = fake_job.scheduler.registered_name
     if sched_name == 'pbs':
@@ -347,6 +391,20 @@ def test_prepare_minimal(minimal_job):
     assert expected_directives(minimal_job) == found_directives
 
 
+def test_prepare_no_tasks(no_tasks_job):
+    prepare_job(no_tasks_job)
+    with open(no_tasks_job.script_filename) as fp:
+        found_directives = set(re.findall(r'^\#\S+ .*', fp.read(),
+                                          re.MULTILINE))
+
+    sched_name = no_tasks_job.scheduler.registered_name
+    expected_directives = globals()[
+        f'_expected_{sched_name}_directives_no_tasks'
+    ]
+    assert_job_script_sanity(no_tasks_job)
+    assert expected_directives(no_tasks_job) == found_directives
+
+
 def test_prepare_no_exclusive(make_job, slurm_only):
     job = make_job()
     job.exclusive_access = False