Allow configuration of CPU affinity

Make it possible to set the CPU affinity in the config file, so that we don't
need to remember to do it manually every time.

synapse/app/_base.py

@@ -15,6 +15,7 @@
 import gc
 import logging
 
+import affinity
 from daemonize import Daemonize
 from synapse.util import PreserveLoggingContext
 from synapse.util.rlimit import change_resource_limit
@@ -40,7 +41,8 @@ def start_worker_reactor(appname, config):
         config.gc_thresholds,
         config.worker_pid_file,
         config.worker_daemonize,
-        logger
+        config.worker_cpu_affinity,
+        logger,
     )
 
 
@@ -50,6 +52,7 @@ def start_reactor(
         gc_thresholds,
         pid_file,
         daemonize,
+        cpu_affinity,
         logger,
 ):
     """ Run the reactor in the main process
@@ -63,6 +66,7 @@ def start_reactor(
         gc_thresholds:
         pid_file (str): name of pid file to write to if daemonize is True
         daemonize (bool): true to run the reactor in a background process
+        cpu_affinity (int|None): cpu affinity mask
         logger (logging.Logger): logger instance to pass to Daemonize
     """
 
@@ -73,6 +77,9 @@ def start_reactor(
         # between the sentinel and `run` logcontexts.
         with PreserveLoggingContext():
             logger.info("Running")
+            if cpu_affinity is not None:
+                logger.info("Setting CPU affinity to %s" % cpu_affinity)
+                affinity.set_process_affinity_mask(0, cpu_affinity)
             change_resource_limit(soft_file_limit)
             if gc_thresholds:
                 gc.set_threshold(*gc_thresholds)