Buffers vs. tty throughput

[John R. Levine]

I am running ISC 2.2 Unix on an Intel 302, a pretty much vanilla cached 25
MHz 386 with 16MB, 8MB on the motherboard, and 8MB on an expansion card.
The kernel has TCP/IP, X, and the Norton unerase, of which only the latter
two are turned on.

I have an internal Telebit with a 16550 UART, and uucp connections generally
run between 800 and 1100 cps, depending on who's on the other end.  I noticed
that my system is fairly disk bound, and that since I'm not running NFS I
don't fill up all my RAM.  It seemed sensible to increase my disk buffers, so
I built a new kernel with 4000 rather than 2000 buffers.  It works OK, except
for one thing -- uucp throughput stinks.  It was down around 400 cps.  I
rebooted the old kernel and throughput is back up.  As far as I can tell, the
only difference between the old and new kernels is NBUF.

What is going on?  The most likely thing I can think of is that somewhere in
the buffer management code there is an N^2 algorithm that runs with
interrupts masked, and with 4000 buffers it takes so long that even with a
16550 I lose interrupts.  I note that in the mtune file, 2000 is the maximum
number of buffers you're normally supposed to configure, but losing
performance when you add more buffers is pretty pitiful.  As I recall, a
16550 has a 16 character silo, so to lose characters you'd have to stay
masked for upwards of 15ms at a time.

Regards,
John Levine, johnl at iecc.cambridge.ma.us, {spdcc|ima|world}!iecc!johnl