Replaced GAIResolver with twisted.names.client for the default resolver

changelog.d/5053.misc

@@ -0,0 +1 @@
+Replace blocking getaddrinfo hostname resolver with the non-blocking resolver from twisted.names.client.

docs/sample_config.yaml

@@ -46,6 +46,12 @@ pid_file: DATADIR/homeserver.pid
 #
 #cpu_affinity: 0xFFFFFFFF
 
+# Whether to use the default system resolver (getaddrinfo) instead of the twisted
+# asynchronous dns resolver. Using the async resolver can potentially improve
+# performance, but may also behave different from getaddrinfo.
+# Defaults to True. Uncomment to use the twisted resolver instead.
+#use_getaddrinfo_for_dns: False
+
 # 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

@@ -22,6 +22,7 @@
 import psutil
 from daemonize import Daemonize
 
+import twisted.names
 from twisted.internet import defer, error, reactor
 from twisted.protocols.tls import TLSMemoryBIOFactory
 
@@ -70,6 +71,7 @@ def start_worker_reactor(appname, config):
         daemonize=config.worker_daemonize,
         cpu_affinity=config.worker_cpu_affinity,
         print_pidfile=config.print_pidfile,
+        use_getaddrinfo_for_dns=config.use_getaddrinfo_for_dns,
         logger=logger,
     )
 
@@ -82,6 +84,7 @@ def start_reactor(
         daemonize,
         cpu_affinity,
         print_pidfile,
+        use_getaddrinfo_for_dns,
         logger,
 ):
     """ Run the reactor in the main process
@@ -96,6 +99,7 @@ def start_reactor(
         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
+        use_getaddrinfo_for_dns (bool): use getaddrinfo instead of async resolver
         print_pidfile (bool): whether to print the pid file, if daemonize is True
         logger (logging.Logger): logger instance to pass to Daemonize
     """
@@ -127,6 +131,10 @@ def run():
             change_resource_limit(soft_file_limit)
             if gc_thresholds:
                 gc.set_threshold(*gc_thresholds)
+
+            if use_getaddrinfo_for_dns:
+                reactor.installResolver(twisted.names.client)
+
             reactor.run()
 
     if daemonize:

synapse/app/homeserver.py

@@ -647,6 +647,7 @@ def start_generate_monthly_active_users():
         daemonize=hs.config.daemonize,
         cpu_affinity=hs.config.cpu_affinity,
         print_pidfile=hs.config.print_pidfile,
+        use_getaddrinfo_for_dns=hs.config.use_getaddrinfo_for_dns,
         logger=logger,
     )
 

synapse/config/server.py

@@ -56,6 +56,12 @@ def read_config(self, config):
         self.public_baseurl = config.get("public_baseurl")
         self.cpu_affinity = config.get("cpu_affinity")
 
+        # Whether to use the getaddrinfo instead of the asynchronous DNS
+        # resolver. Disabling this can potentially improve performance.
+        self.use_getaddrinfo_for_dns = config.get(
+            "use_getaddrinfo_for_dns", True
+        )
+
         # Whether to send federation traffic out in this process. This only
         # applies to some federation traffic, and so shouldn't be used to
         # "disable" federation