🚀 go-pugleaf

In article <1992Dec7.172216.21460@cbnewsc.cb.att.com> dwd@cbnewsc.cb.att.com (David W. Dykstra) writes:
>Doug, do you have any evidence that context-switch overhead is truly
>significant?

I have both xt and pty versions of layers running on a VAX-11/780 under
4.3BSD, and the pty version has noticeably reduced throughput.  When I
investigated this some time ago I concluded that it was explained by
context-switch overhead.

>... I maintain that pty-based layers are so superior to xt-driver
>based layers that they're worth the extra context switches.

The biggest problem with xt seems to be the difficulty of porting it.
Ptys have one big problem in that only a single byte of ioctl information
can be passed over the pty.  Streams seem like the best way to go, but I
don't have access to a streams implementation of layers to try out.

>I have also seen hosts that had very poor serial port drivers with
>very slow turn-around time drag down layers throughput.

I recently was moved to an Annex terminal server and found that layers
operation has significantly reduced throughput when funneling the data
through this terminal switch.  I haven't yet performed any experiments
to determine just what the bottleneck is.

A real problem with rlogin-style support on a lot of operating systems
is that every single byte of data has to be transferred to and from a
user-mode process.  What I'd really like to see is the ability to
establish a "splice" between two channels so that all data flow
occurs in the kernel.  This should be easy with streams, but none of
the BSD-based rsh/rlogin/etc. implementations I've seen follows this
general approach.  Probably the Annex software has a similar deficiency.