I have a setup in which a linux PC is supposed to communicate with my custom hardware with the serial port.

I installed "Damn Small Linux V4.10" as my distro and so far, the web server to other clients via LAN (ethernet) is running well.

The trouble I have is communication to my hardware with the serial port at high speed (56kbps).

I tested the operation of the custom hardware through the serial port with the cat command and turned off then on my hardware and the data returned was as expected.

I also tested my hardware on Slackware via a direct serial hookup and using the screen command and all data is returned as expected so I can't blame my custom hardware.

I heard about using RTLinux or RTAI but before having to try to resort to that route, I want to disable some unnecessary system daemons.

The following is what is running on my linux at time of last test:
init [5]
[keventd]
[ksoftirqd_CPU0]
[kswapd]
[bdflush]
[kupdated]
[khubd]
-su
bash
/opt/monkey-0.9.2/bin/monkey -D
[run.cgi]
ps

The run.cgi is my CGI script that is trying to retrieve valid data from the serial port and deliver the results as a web page to a remote user connected to the same network.

monkey is the web server.

When I checked top it looks like nothing is consuming CPU.

I also made my last test without using X.

Is there a way I could get performance gain (like where data can be read by serial port uninterrupted) by somehow removing processes in [ ]?

I also noticed that no "swappiness" file is available in /proc/sys/vm but there is a "bdflush" and "kswapd" files. I'm tempted to set the numbers in those files to 0, but would that cause more harm than good?

So far the only ultimate solution which is bad for my project is to lower the baud rate to my hardware to 9600bps. but I'd rather have 57600bps.

What can I do or what processes can I or shouldn't I disable for best performance and why?

You don't say what your problem is, apart from vague allusions to performance problems.

The processes in [...] are kernel processes, which means that they run kernel code exclusively, and can't be killed. In any case, if nothing consumes CPU, they are unlikely to be the source of trouble. Whether paging/swapping is taking place can be seen with tools like free or vmstat.

The problem is either the PC isn't sending data over the serial link fast enough (1 byte every 170uS) or the PC isn't receiving data fast enough.

I have no problem if I try a slow baud rate (like 9600bps)

So I guess I have no choice but to use realtime linux extension such as rtai?