Drop support for cpu_affinity (#5525)

This has no useful purpose on python3, and is generally a source of confusion.

changelog.d/5525.removal

+Remove support for cpu_affinity setting.

docs/sample_config.yaml

@@ -23,29 +23,6 @@ server_name: "SERVERNAME"
 #
 pid_file: DATADIR/homeserver.pid
 
-# CPU affinity mask. Setting this restricts the CPUs on which the
-# process will be scheduled. It is represented as a bitmask, with the
-# lowest order bit corresponding to the first logical CPU and the
-# highest order bit corresponding to the last logical CPU. Not all CPUs
-# may exist on a given system but a mask may specify more CPUs than are
-# present.
-#
-# For example:
-#    0x00000001  is processor #0,
-#    0x00000003  is processors #0 and #1,
-#    0xFFFFFFFF  is all processors (#0 through #31).
-#
-# Pinning a Python process to a single CPU is desirable, because Python
-# is inherently single-threaded due to the GIL, and can suffer a
-# 30-40% slowdown due to cache blow-out and thread context switching
-# if the scheduler happens to schedule the underlying threads across
-# different cores. See
-# https://www.mirantis.com/blog/improve-performance-python-programs-restricting-single-cpu/.
-#
-# This setting requires the affinity package to be installed!
-#
-#cpu_affinity: 0xFFFFFFFF
-
 # The path to the web client which will be served at /_matrix/client/
 # if 'webclient' is configured under the 'listeners' configuration.
 #

synapse/app/_base.py

@@ -19,7 +19,6 @@ import signal
 import sys
 import traceback
 
-import psutil
 from daemonize import Daemonize
 
 from twisted.internet import defer, error, reactor
@@ -68,21 +67,13 @@ def start_worker_reactor(appname, config):
         gc_thresholds=config.gc_thresholds,
         pid_file=config.worker_pid_file,
         daemonize=config.worker_daemonize,
-        cpu_affinity=config.worker_cpu_affinity,
         print_pidfile=config.print_pidfile,
         logger=logger,
     )
 
 
 def start_reactor(
-    appname,
-    soft_file_limit,
-    gc_thresholds,
-    pid_file,
-    daemonize,
-    cpu_affinity,
-    print_pidfile,
-    logger,
+    appname, soft_file_limit, gc_thresholds, pid_file, daemonize, print_pidfile, logger
 ):
     """ Run the reactor in the main process
 
@@ -95,7 +86,6 @@ 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
         print_pidfile (bool): whether to print the pid file, if daemonize is True
         logger (logging.Logger): logger instance to pass to Daemonize
     """
@@ -109,20 +99,6 @@ def start_reactor(
         # between the sentinel and `run` logcontexts.
         with PreserveLoggingContext():
             logger.info("Running")
-            if cpu_affinity is not None:
-                # Turn the bitmask into bits, reverse it so we go from 0 up
-                mask_to_bits = bin(cpu_affinity)[2:][::-1]
-
-                cpus = []
-                cpu_num = 0
-
-                for i in mask_to_bits:
-                    if i == "1":
-                        cpus.append(cpu_num)
-                    cpu_num += 1
-
-                p = psutil.Process()
-                p.cpu_affinity(cpus)
 
             change_resource_limit(soft_file_limit)
             if gc_thresholds:

synapse/app/homeserver.py

@@ -641,7 +641,6 @@ def run(hs):
         gc_thresholds=hs.config.gc_thresholds,
         pid_file=hs.config.pid_file,
         daemonize=hs.config.daemonize,
-        cpu_affinity=hs.config.cpu_affinity,
         print_pidfile=hs.config.print_pidfile,
         logger=logger,
     )

synapse/config/server.py

@@ -57,7 +57,6 @@ class ServerConfig(Config):
         self.user_agent_suffix = config.get("user_agent_suffix")
         self.use_frozen_dicts = config.get("use_frozen_dicts", False)
         self.public_baseurl = config.get("public_baseurl")
-        self.cpu_affinity = config.get("cpu_affinity")
 
         # Whether to send federation traffic out in this process. This only
         # applies to some federation traffic, and so shouldn't be used to
@@ -336,29 +335,6 @@ class ServerConfig(Config):
         #
         pid_file: %(pid_file)s
 
-        # CPU affinity mask. Setting this restricts the CPUs on which the
-        # process will be scheduled. It is represented as a bitmask, with the
-        # lowest order bit corresponding to the first logical CPU and the
-        # highest order bit corresponding to the last logical CPU. Not all CPUs
-        # may exist on a given system but a mask may specify more CPUs than are
-        # present.
-        #
-        # For example:
-        #    0x00000001  is processor #0,
-        #    0x00000003  is processors #0 and #1,
-        #    0xFFFFFFFF  is all processors (#0 through #31).
-        #
-        # Pinning a Python process to a single CPU is desirable, because Python
-        # is inherently single-threaded due to the GIL, and can suffer a
-        # 30-40%% slowdown due to cache blow-out and thread context switching
-        # if the scheduler happens to schedule the underlying threads across
-        # different cores. See
-        # https://www.mirantis.com/blog/improve-performance-python-programs-restricting-single-cpu/.
-        #
-        # This setting requires the affinity package to be installed!
-        #
-        #cpu_affinity: 0xFFFFFFFF
-
         # The path to the web client which will be served at /_matrix/client/
         # if 'webclient' is configured under the 'listeners' configuration.
         #

synapse/config/workers.py

@@ -46,7 +46,6 @@ class WorkerConfig(Config):
         self.worker_name = config.get("worker_name", self.worker_app)
 
         self.worker_main_http_uri = config.get("worker_main_http_uri", None)
-        self.worker_cpu_affinity = config.get("worker_cpu_affinity")
 
         # This option is really only here to support `--manhole` command line
         # argument.