How to avoid serial port buffer overruns?

From: Raphael Hertzog
Date: Wed Aug 16 2006 - 06:43:46 EST

(Please CC me when replying)


While using Linux on low-end (semi-embedded) hardware (386 SX 40Mhz, 8Mb
RAM), I discovered that Linux on that machine would suffer from serial
port buffer overruns quite easily if I use a baudrate high enough (I start
loosing bytes at >19200 bauds and I would like to make it reliable up to
115 kbauds). I check if overruns are happening with
/proc/tty/driver/serial ("oe" field).

Back when I was using the 2.4 kernel, I reduced dramatically the frequency
of overruns by using the "low latency" and "preemptible kernel" patch [1]. But
it still happened sometimes at 115 kbauds if the system was a bit loaded
(with disk I/O for example).

Now I switched to stock 2.6 and while the stock kernel improved in
responsiveness, it still isn't enough by default (even with
CONFIG_PREEMPT=y and CONFIG_HZ=1000). So I wanted to try the "rt" patch of
Ingo Molnar and Thomas Gleixner, but the patched kernel doesn't boot (see
bug report in a separate mail on this list).

Other things that I tried which didn't help (enough) are:

- tuning with hdparm
- make disk IRQ interruptible with hdparm -u 1 /dev/hda
- activating DMA is suggested but my disk is a "disk on module"
and doesn't support DMA
- using irqtune ( to reprioritize
irqtune is old and it's very difficult to know if it still works
reliably with recent kernel and as there's no way to "read" the
interrupt priorities, I have no way to know if irqtune changed anything
at all...

My questions are thus:
1/ Is there a way to patch the kernel to make it handle serial IRQ as
the highest priority? If yes, how? Where should I look at to create
such a patch?
2/ How can I identify why the serial interrupts are delayed? Or, in other
words, how can I find the code blocking for too long the treatment of
the serial IRQ?
I suppose that network IRQ and disk IRQ are responsible for that but
I'm not sure and furthermore disk interrupts are supposed to
be interruptible given the hdparm config that I use.
3/ What other suggestions do you have to avoid those serial buffer

As usual, I'll gladly try out patches/ideas and will provide any
required additional information that I didn't include in this mail.

Some infos on the hardware:
More detailed spec of the CPU are here:

bash-2.05a# cat /proc/interrupts
0: 88503366 XT-PIC timer
2: 0 XT-PIC cascade
3: 11 XT-PIC serial
4: 12 XT-PIC serial
5: 4958 XT-PIC NE2000
14: 10771 XT-PIC ide0
NMI: 0
ERR: 0


[1] I documented that in a blog post last year:
Raphaël Hertzog

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