Generalize TPU Slice Health Checks for elasticity

- Create DefaultSliceHealthChecker.
- Move slice_to_devices to __init__.
- Bring details from simple executions' docstrings to DefaultSliceHealthChecker.

PiperOrigin-RevId: 896196196

Author: lukebaumann

pathwaysutils/elastic/elastic.py

@@ -18,10 +18,13 @@
 events. It also provides a utility for waiting for slices to become active.
 """
 
+from abc import ABC, abstractmethod
 import collections
-from collections.abc import Mapping, Sequence, Set
+from collections.abc import Callable, Mapping, Sequence, Set
+import itertools
 import logging
 import time
+from typing import Any
 
 import jax
 import numpy as np
@@ -41,44 +44,87 @@
 ])
 
 
-def _plus_one(x: jax.Array) -> jax.Array:
-  """Adds one to each element in the array.
+class SliceHealthChecker(ABC):
+  """Base class for slice health checkers.
 
-  Used to test if a slice is active.
+  Implementations must provide `dispatch` and `validate` methods.
+  `dispatch` starts the health check operations, and `validate` waits for them
+  and returns the active slice indices.
+  """
 
-  Args:
-    x: The array to add one to.
+  def __init__(self, slice_to_devices: Mapping[int, Sequence[jax.Device]]):
+    self.slice_to_devices = slice_to_devices
 
-  Returns:
-      The array with one added to each element.
-  """
-  return x + 1
+  @abstractmethod
+  def dispatch(self) -> None:
+    """Dispatches the JAX operations in the background."""
 
+  @abstractmethod
+  def validate(self) -> Set[int]:
+    """Blocks on results and returns the set of active slice indices.
 
-def _simple_execution(devices: Sequence[jax.Device]) -> jax.Array:
-  """Simple execution to test if a slice is active.
+    Returns:
+      Set of active slice indices.
+    """
 
-  This function is used to test if a slice is active. It executes a simple
-  computation on the devices and returns the result. If any of the devices are
-  not active, the returned array will fail with a JaxRuntimeError used.
 
-  Simply executing this function is not enough to determine if the slice is
-  active. We also need to check the value of the returned array.
+class DefaultSliceHealthChecker(SliceHealthChecker):
+  """Default implementation that checks each slice individually.
 
-  Args:
-    devices: The devices to execute on.
+  It executes a simple computation on each slice independently to verify
+  that the devices are active and responsive.
 
-  Returns:
-    The result of the execution.
+  The computation involves creating a zero array of the same size as the
+  number of devices in the slice, adding (_SIMPLE_EXECUTION_TEST_VALUE - 1)
+  to it, and then running a pmap to add 1 to each element. This verifies
+  that the devices are active and can perform computations.
   """
-  if not devices:
-    raise ValueError("No devices")
 
-  test_input = np.zeros(len(devices), dtype=float) + (
-      _SIMPLE_EXECUTION_TEST_VALUE - 1
-  )
+  def _plus_one(self, x: jax.Array) -> jax.Array:
+    return x + 1
+
+  def _simple_execution(self, devices: Sequence[jax.Device]) -> jax.Array:
+    if not devices:
+      raise ValueError("No devices")
+
+    test_input = np.zeros(len(devices), dtype=float) + (
+        _SIMPLE_EXECUTION_TEST_VALUE - 1
+    )
+
+    return jax.pmap(self._plus_one, devices=devices)(test_input)
+
+  def dispatch(self) -> None:
+    self.results = {
+        slice_index: self._simple_execution(devices)
+        for slice_index, devices in self.slice_to_devices.items()
+    }
+
+  def validate(self) -> Set[int]:
+    active_slice_indices = set()
+    for slice_index, x in self.results.items():
+      expected = (
+          np.zeros(len(self.slice_to_devices[slice_index]), dtype=float)
+          + _SIMPLE_EXECUTION_TEST_VALUE
+      )
+      try:
+        jax.block_until_ready(x)
+        if np.allclose(x, expected):
+          active_slice_indices.add(slice_index)
+        else:
+          msg = (
+              f"Error with _simple_execution for slice_index={slice_index}. "
+              f"Expected: {expected!r}, Actual: {x!r}"
+          )
+          _logger.error(msg)
+          raise ValueError(msg)
+      except jax.errors.JaxRuntimeError as error:
+        _logger.debug(
+            "Caught JaxRuntimeError for slice_index=%s: %s", slice_index, error
+        )
+        if not is_error_due_to_slice_down(error):
+          raise
+    return active_slice_indices
 
-  return jax.pmap(_plus_one, devices=devices)(test_input)
 
 
 def get_slice_to_devices(
@@ -94,13 +140,16 @@ def get_slice_to_devices(
 @timing.timeit
 def get_active_slice_indices(
     slice_to_devices: Mapping[int, Sequence[jax.Device]] | None = None,
+    checker: SliceHealthChecker | None = None,
 ) -> Set[int]:
   """Returns the set of active slices indices.
 
   Args:
     slice_to_devices: A mapping from slice index to devices. If None,
       `get_slice_to_devices(jax.devices())` is used to gather all available
       devices and group them by slice.
+    checker: A SliceHealthChecker instance. If None, DefaultSliceHealthChecker
+      is used.
 
   Returns:
     A set of integers representing the indices of the active slices.
@@ -109,62 +158,24 @@ def get_active_slice_indices(
     _logger.debug("slice_to_devices is None. Getting from jax.devices().")
     slice_to_devices = get_slice_to_devices(tuple(jax.devices()))
 
+  if checker is None:
+    checker = DefaultSliceHealthChecker(slice_to_devices)
+
   _logger.debug(
       "Getting active slice indices for slices: %s",
       sorted(list(slice_to_devices.keys())),
   )
 
-  active_slice_indices = set()
-
-  results = {
-      slice_index: _simple_execution(devices)
-      for slice_index, devices in slice_to_devices.items()
-  }
-
-  for slice_index, x in results.items():
-    _logger.debug("Checking slice_index=%s", slice_index)
-    expected = (
-        np.zeros(len(slice_to_devices[slice_index]), dtype=float)
-        + _SIMPLE_EXECUTION_TEST_VALUE
-    )
-    try:
-      with timing.Timer(f"Checking {slice_index=}"):
-        _logger.debug("Blocking until ready for slice_index=%s", slice_index)
-        jax.block_until_ready(x)
-        _logger.debug("Execution finished for slice_index=%s", slice_index)
-        if np.allclose(x, expected):
-          active_slice_indices.add(slice_index)
-          _logger.debug("slice_index=%s active", slice_index)
-        else:
-          _logger.error(
-              "Error with _simple_execution for slice_index=%s. "
-              "This should never happen. Expected: %r, Actual: %r",
-              slice_index,
-              expected,
-              x,
-          )
-          raise ValueError(
-              f"Error with _simple_execution for slice_index={slice_index}."
-          )
-    except jax.errors.JaxRuntimeError as error:
-      _logger.debug(
-          "Caught JaxRuntimeError for slice_index=%s: %s", slice_index, error
-      )
-      if not is_error_due_to_slice_down(error):
-        _logger.info("Re-raising error for slice_index=%s", slice_index)
-        raise
-      _logger.debug("slice_index=%s bad", slice_index)
-
-  _logger.debug("active_slice_indices=%s", active_slice_indices)
-
-  return active_slice_indices
+  checker.dispatch()
+  return checker.validate()
 
 
 def wait_for_slices(
     slice_count: int,
     poll_interval: float | int = 10,
     timeout: float | int | None = None,
     slice_to_devices: Mapping[int, Sequence[jax.Device]] | None = None,
+    checker: SliceHealthChecker | None = None,
 ) -> Set[int]:
   """Waits until after at least `slice_count` slices become active.
 
@@ -177,6 +188,7 @@ def wait_for_slices(
       timeout.
     slice_to_devices: A mapping from slice index to devices. If None,
       `get_slice_to_devices(jax.devices())` is used.
+    checker: A SliceHealthChecker instance to use for checking health.
 
   Returns:
     The active slice indices
@@ -201,7 +213,7 @@ def wait_for_slices(
     check_start_time = time.time()
 
     _logger.debug("Checking active slices...")
-    active_slice_indices = get_active_slice_indices(slice_to_devices)
+    active_slice_indices = get_active_slice_indices(slice_to_devices, checker)
     if len(active_slice_indices) >= slice_count:
       _logger.info(
           "Sufficient slices active: %s >= %s. Active indices: %s",