Speed up Bootstrapping Computation (#409)

* Added fix for heavy recomputation of sample level metrics.

* Moved parallelization to where it is actually useful.

---------

Co-authored-by: Clémentine Fourrier <[email protected]>
Co-authored-by: Nathan Habib <[email protected]>

src/lighteval/metrics/stderr.py

@@ -26,7 +26,7 @@
 
 import math
 import random
-from typing import Callable
+from typing import Callable, Optional
 
 import numpy as np
 from scipy.stats import bootstrap
@@ -45,18 +45,27 @@ def mean_stderr(arr):
 
 
 class _bootstrap_internal:
-    def __init__(self, metric: Callable, number_draws: int):
-        self.metric = metric
+    def __init__(self, number_draws: int, metric: Optional[Callable] = None):
         self.number_draws = number_draws
+        self.metric = metric
 
     def __call__(self, cur_experiment):
         # Creates number_draws samplings (with replacement) of the population by iterating on a given seed
         population, seed = cur_experiment
         rnd = random.Random()
         rnd.seed(seed)
         samplings = []
-        for _ in range(self.number_draws):
-            samplings.append(self.metric(rnd.choices(population, k=len(population))))
+        import multiprocessing as mp
+
+        with mp.Pool(mp.cpu_count()) as pool:
+            samplings = pool.starmap(
+                self.metric,
+                tqdm(
+                    [(rnd.choices(population, k=len(population)),) for _ in range(self.number_draws)],
+                    total=self.number_draws,
+                    desc="Sampling bootstrap iterations",
+                ),
+            )
         return samplings
 
 
@@ -65,28 +74,15 @@ def bootstrap_stderr(metric: Callable, population: list, number_experiments: int
     by sampling said population for number_experiments and recomputing the metric on the
     different samplings.
     """
-    import multiprocessing as mp
-
-    pool = mp.Pool(mp.cpu_count())
-
     res = []
     number_draws = min(1000, number_experiments)
-    # We change the seed every 1000 re-samplings
-    # and do the experiment 1000 re-samplings at a time
     number_seeds = number_experiments // number_draws
 
-    hlog(f"Bootstrapping {metric.__name__}'s stderr.")
-    for cur_bootstrap in tqdm(
-        pool.imap(
-            _bootstrap_internal(metric=metric, number_draws=number_draws),
-            ((population, seed) for seed in range(number_seeds)),
-        ),
-        total=number_seeds,
-    ):
+    hlog(f"Bootstrapping {metric.__name__}'s stderr with {number_seeds} seeds.")
+    for seed in range(number_seeds):
         # sample w replacement
-        res.extend(cur_bootstrap)
+        res.extend(_bootstrap_internal(metric=metric, number_draws=number_draws)((population, seed)))
 
-    pool.close()
     return mean_stderr(res)