Linux Serial Console Driver Lands Patch For Possible ~25% Performance Improvement

It's not an area of Linux hardware performance we normally look at, but thanks to a Red Hat engineer discovering very low serial console performance, there is an improvement queued up for introduction in Linux 5.17...

Red Hat's Wander Lairson Costa was noticing the serial console throughput on an HP Proliant DL380 Gen9 server was coming in well below expectations: expecting 10KB/s but with the current Linux kernel only hitting around 2.5KB/s. The Linux 8250 serial console driver was taking around 410 microseconds just to dispatch one single byte.

With the Linux 8250/16550 serial port console driver, Wander has managed to improve the performance in 2022. The 16550 UARTs have an on-chip FIFO buffer to which is now being used on supported systems by the 8250 serial driver's write function.

With this patch to use the 16550 UART FIFO buffer where supported, the 8250 console driver on affected systems is seeing about 25% improved performance for the serial console throughput.

That patch was sent in today by Greg Kroah-Hartman as part of the TTY/serial updates for the Linux 5.17 merge window. There are also TTY code clean-ups and other work. There is also a fix to a longtime problem over the keyboard lock LED becoming inconsistent when using Ctrl + Alt + Fn key switching between the TTY console to/from desktops.