diff --git a/ffcv/loader/loader.py b/ffcv/loader/loader.py
index 0531fc2c..9283912d 100644
--- a/ffcv/loader/loader.py
+++ b/ffcv/loader/loader.py
@@ -2,7 +2,7 @@
 FFCV loader
 """
 import enum
-from os import environ
+from os import environ, sched_getaffinity
 import ast
 from multiprocessing import cpu_count
 from re import sub
@@ -140,7 +140,7 @@ def __init__(self,
         self.recompile = recompile
 
         if self.num_workers < 1:
-            self.num_workers = cpu_count()
+            self.num_workers = len(sched_getaffinity(0))
 
         Compiler.set_num_threads(self.num_workers)
 
diff --git a/ffcv/pipeline/compiler.py b/ffcv/pipeline/compiler.py
index 987356cc..9ebae469 100644
--- a/ffcv/pipeline/compiler.py
+++ b/ffcv/pipeline/compiler.py
@@ -4,7 +4,7 @@
 from multiprocessing import cpu_count
 import torch as ch
 import warnings
-
+from os import sched_getaffinity
 
 class Compiler:
 
@@ -15,7 +15,7 @@ def set_enabled(cls, b):
     @classmethod
     def set_num_threads(cls, n):
         if n < 1 :
-            n = cpu_count()
+            n = len(sched_getaffinity(0))
         cls.num_threads = n
         set_num_threads(n)
         ch.set_num_threads(n)
diff --git a/ffcv/writer.py b/ffcv/writer.py
index 1b70f74f..3615dd0a 100644
--- a/ffcv/writer.py
+++ b/ffcv/writer.py
@@ -1,6 +1,6 @@
 from functools import partial
 from typing import Callable, List, Mapping
-from os import SEEK_END, path
+from os import SEEK_END, path, sched_getaffinity
 import numpy as np
 from time import sleep
 import ctypes
@@ -143,7 +143,7 @@ def __init__(self, fname: str, fields: Mapping[str, Field],
         self.num_workers = num_workers
         # We use all cores by default
         if self.num_workers < 1:
-            self.num_workers = cpu_count()
+            self.num_workers = len(sched_getaffinity(0))
 
         if not is_power_of_2(page_size):
             raise ValueError(f'page_size isnt a power of 2')