I've read quite a few archived discussions on PPP performance in
NetBSD/mac68k.  From what I've seen, it seems very hard for the system
to keep up with any serial connections over 38400. I am using a
Quadra/Centris 660AV (25mhz '040) with 52MB ram and a SupraExpress 56
external modem. My NetBSD Mac is only able to reach about half the
throughput speed (between 2.5k/sec and 3.0k/sec usually) as my G4 with
internal modem, and even PC with a cheap Motorola winmodem (both can
reach 4.5-5k/sec and higher).

I am using my NetBSD Centris box as a router with IPNAT for 2 other
computers on an ethernet network. It is running a 1.4.3 kernel that I
compiled myself (with unnecessary drivers stripped out and -O3 and
-m68040 optimizations).  During heavy network activity, even when the
NetBSD box is idle, I still get many silo overflows:
Dec 19 14:22:50 littleone /netbsd: zstty0: 14 silo overflows, 0 ibuf
floods
Dec 19 14:23:50 littleone /netbsd: zstty0: 9 silo overflows, 0 ibuf
floods
Dec 19 14:24:51 littleone /netbsd: zstty0: 13 silo overflows, 0 ibuf
floods
Dec 19 14:25:50 littleone /netbsd: zstty0: 4 silo overflows, 0 ibuf
floods
Dec 19 14:27:44 littleone /netbsd: zstty0: 9 silo overflows, 0 ibuf
floods
Never had an ibuf flood though.
Those results are when the system is idle, I have gotten over 100 silo
overflows in a minute during heavy system activity.

I have tried various port speeds, including 57600, 115200, and even
230400, all with similar results. (although overflows seem to occur most
at 230400). I have also tried both cdtrcts and crtscts flow control
options. Is this problem simply due to a lack of support for Serial DMA
in NetBSD/mac68k? If so, can we ever expect to see this added in the
(near?) future?

I have also tried various modem init strings, including disabling modem
compression and setting the S26 register (RTS->CTS delay time), with no
luck.

I would love to be able to use my NetBSD system for this, but unless I
can find a solution to this speed problem, I have to just put it out in
the shed to collect dust...

-Halim Chtourou