[Simh] SIMH performance

[Michael Bloom]

It is known that SIMH uses a lot of CPU time, apparently when it has no 
work to do.

I used strace to capture 10 seconds worth of system calls from SIMH on a 
(2 core) machine with 4800 bogomips per core.

The simulation was pdp11/73 running 2.11BSD

In ten seconds, there were 35610 calls into the kernel, averaging about 
356 per second.

During those 10 seconds,there were many unsuccessful calls:

23526 read calls requesting one byte and returning 0 bytes
  5990 accepts calls returning EAGAIN,
  5990 recv calls returning EAGAIN
-----
35606 unsuccessful calls

and a few successful calls:
      1 read call that returned 1024 bytes,
      1 write call returning 4096 bytes
    100 gettimeofday calls returning 0 (SUCCESS)
      2 lseek calls returning 0
  -----
    104 calls were successful,  less than 3% of all calls


There is a lot of spinning apparently going on.   What would be the 
downside of issuing select calls or FIONREAD ioctls in the thread(s) 
doing IO?  Or perhaps sleeping until the highest resolution clock 
expires and having file descriptors generate SIGIO if  IO becomes 
possible during the interim?

You could use, for example,  sigsuspend() with a signal mask that says 
to sleep until either a high resolution timer (higher than your desired 
clock resolution)  expires or until a SIGIO indicates that i/o is 
available.  In the latter case, you could use select()  to tell which fd 
or fds io is possible on.

Alternately, you could use the timeout argument of select(), and not 
have to deal with SIGIO at all. select would return either after a 
timeout, or when one of the file descriptors specified is ready.

Perhaps useful, to provide examples, might be GNU emacs, which has been 
using means such as these  (or whatever method is available on the host 
system) to perform I/O without wasted cycles, since the mid 1980's.

Note:  by it's very nature, strace slows things down.  Without it, there 
would have been more calls, both successful and unsuccessful.  
Schrodinger's Penguin, anyone?