Make No. of Transport Threads == Available CPUs (#56488)

We never do any file IO or other blocking work on the transport threads
so no tangible benefit can be derived from using more threads than CPUs
for IO.
There are however significant downsides to using more threads than necessary
with Netty in particular. Since we use the default setting for
`io.netty.allocator.useCacheForAllThreads` which is `true` we end up
using up to `16MB` of thread local buffer cache for each transport thread.
Meaning we potentially waste CPUs * 16MB of heap for unnecessary IO threads in addition to obvious inefficiencies of artificially adding extra context switches.

Author: original-brownbear

modules/transport-netty4/src/main/java/org/elasticsearch/transport/netty4/Netty4Transport.java

@@ -78,7 +78,7 @@ public class Netty4Transport extends TcpTransport {
 
     public static final Setting<Integer> WORKER_COUNT =
         new Setting<>("transport.netty.worker_count",
-            (s) -> Integer.toString(EsExecutors.allocatedProcessors(s) * 2),
+            (s) -> Integer.toString(EsExecutors.allocatedProcessors(s)),
             (s) -> Setting.parseInt(s, 1, "transport.netty.worker_count"), Property.NodeScope);
 
     public static final Setting<ByteSizeValue> NETTY_RECEIVE_PREDICTOR_SIZE = Setting.byteSizeSetting(

plugins/transport-nio/src/main/java/org/elasticsearch/transport/nio/NioTransportPlugin.java

@@ -57,7 +57,7 @@ public class NioTransportPlugin extends Plugin implements NetworkPlugin {
 
     public static final Setting<Integer> NIO_WORKER_COUNT =
         new Setting<>("transport.nio.worker_count",
-            (s) -> Integer.toString(EsExecutors.allocatedProcessors(s) * 2),
+            (s) -> Integer.toString(EsExecutors.allocatedProcessors(s)),
             (s) -> Setting.parseInt(s, 1, "transport.nio.worker_count"), Setting.Property.NodeScope);
     public static final Setting<Integer> NIO_HTTP_WORKER_COUNT =
         intSetting("http.nio.worker_count", 0, 0, Setting.Property.NodeScope);