Eth iperf default sdkconfig: Enable lwIP IRAM optimization

examples/ethernet/iperf/README.md

@@ -149,7 +149,7 @@ I (2534456) iperf: want recv=16384
 ## Suggestions of getting higher bandwidth
 
 1. Higher MCU working frequency will get higher bandwidth.
-2. Put frequency invoked function into IRAM via macro `IRAM_ATTR` in code.
+2. Put frequently invoked functions into IRAM via macro `IRAM_ATTR` in code. Note that the lwIP IRAM optimization is already enabled by default.
 3. Priority of iperf task may also have effect.
 
 ## Troubleshooting

examples/ethernet/iperf/sdkconfig.defaults

@@ -19,4 +19,7 @@ CONFIG_ESPTOOLPY_FLASHSIZE_4MB=y
 CONFIG_ESP_INT_WDT=n
 CONFIG_ESP_TASK_WDT=n
 
+# Enable lwIP IRAM optimization
+CONFIG_LWIP_IRAM_OPTIMIZATION=y
+
 CONFIG_ESP_NETIF_TCPIP_ADAPTER_COMPATIBLE_LAYER=n