Try to increase open files limit and warn if it is still too low afte…

…rwards. adresses #1368

locust/main.py

@@ -6,6 +6,7 @@
 import socket
 import sys
 import time
+import resource 
 
 import gevent
 
@@ -153,6 +154,15 @@ def main():
         # list() call is needed to consume the dict_view object in Python 3
         user_classes = list(user_classes.values())
 
+    try:
+        # 24576 is the highest value I was able to set this on MacOS Mojave without getting an error
+        resource.setrlimit(resource.RLIMIT_NOFILE, [24576, resource.RLIM_INFINITY])
+    except:
+        pass # this will fail on linux, not sure about windows
+
+    if resource.getrlimit(resource.RLIMIT_NOFILE)[0] < 10000:
+        logger.warning("System open file limit setting is not high enough for load testing. See https://docs.locust.io/en/stable/installation.html#increasing-maximum-number-of-open-files-limit for more info.")
+
     # create locust Environment
     environment = create_environment(user_classes, options, events=locust.events)