Document I/O threads in redis.conf.

redis.conf

@@ -938,6 +938,52 @@ lazyfree-lazy-expire no
 lazyfree-lazy-server-del no
 replica-lazy-flush no
 
+################################ THREADED I/O #################################
+
+# Redis is mostly single threaded, however there are certain threaded
+# operations such as UNLINK, slow I/O accesses and other things that are
+# performed on side threads.
+#
+# Now it is also possible to handle Redis clients socket reads and writes
+# in different I/O threads. Since especially writing is so slow, normally
+# Redis users use pipelining in order to speedup the Redis performances per
+# core, and spawn multiple instances in order to scale more. Using I/O
+# threads it is possible to easily speedup two times Redis without resorting
+# to pipelining nor sharding of the instance.
+#
+# By default threading is disabled, we suggest enabling it only in machines
+# that have at least 4 or more cores, leaving at least one spare core.
+# Using more than 8 threads is unlikely to help much. We also recommend using
+# threaded I/O only if you actually have performance problems, with Redis
+# instances being able to use a quite big percentage of CPU time, otherwise
+# there is no point in using this feature.
+#
+# So for instance if you have a four cores boxes, try to use 2 or 3 I/O
+# threads, if you have a 8 cores, try to use 6 threads. In order to
+# enable I/O threads use the following configuration directive:
+#
+# io-threads 4
+#
+# Setting io-threads to 1 will just use the main thread as usually.
+# When I/O threads are enabled, we only use threads for writes, that is
+# to thread the write(2) syscall and transfer the client buffers to the
+# socket. However it is also possible to enable threading of reads and
+# protocol parsing using the following configuration directive, by setting
+# it to yes:
+#
+# io-threads-do-reads no
+#
+# Usually threading reads doesn't help much.
+#
+# NOTE 1: This configuration directive cannot be changed at runtime via
+# CONFIG SET. Aso this feature currently does not work when SSL is
+# enabled.
+#
+# NOTE 2: If you want to test the Redis speedup using redis-benchmark, make
+# sure you also run the benchmark itself in threaded mode, using the
+# --threads option to match the number of Redis theads, otherwise you'll not
+# be able to notice the improvements.
+
 ############################## APPEND ONLY MODE ###############################
 
 # By default Redis asynchronously dumps the dataset on disk. This mode is