On Thu, 20 Apr 1995, John F. Woods wrote:

> >    I have USRobotics 28,800 internal modems on two NetBSD-1.0 486/DX-33 
> > machines.  I am using ppp to connect these machines.  Both systems use
> > rtscts hardware flow control and transmit to the computer at 115200 baud
> > with compression enabled.
> > Any suggestions for how to prevent the silo overflows?
> 
> I markedly reduced the incidence of silo overflows on my system by editing
> dev/isa/com.c to change the FIFO threshold from 8 to 4 characters.
Thanks to John Woods for this suggestion.  My new transfer rates are as
follows:
     500kb compressed    2757 bytes/sec (2 times without silo overflow)
                         2542 bytes/sec (1 time with 1 silo overflow)
     500kb kernel binary 4237 bytes/sec (no silo overflow)
     200kb c source text 6250 bytes/sec (no silo overflow)

In looking at com.c it appears that on init of the device it set
the FIFO threshold to 4 but on open or changing baud rates the
threshold is set to 1 if speed less than or equal to 1200 or
8 if greater.  It raises a question why on init it is set to 4 then
when used changed to 8?  It seems that for my enviornment having it 
set to 4 works better.  Again this is 1.0 code.

johnh...