Vax 11/780 performance vs Sun 4/280 performance

[Eric Gisin]

In article <15875 at brl-adm.ARPA>, weiser.pa at xerox.com writes:
> I just tried this on my Sun-4/280 by running 30 cpueating processes.
> ("while(1);").  Sure enough, even with the load at 30, I got much better
> response than I did with only 20 of the little 50 ms sleeper programs I posted a
> day or so ago.  One way to interpret this is that when Sun's scheduler knows
> that it has 30 processes on the queue, it does a better job of sharing the
> limited resource of contexts, than if it thinks there is nothing to do, but
> every 50ms 20 jobs all suddenly leap up and call for attention...  But I don't
> know for sure.
> 

The scheduler doesn't know that it has 30 process on the queue.

With the 20 50ms sleeper jobs there will be 20*20 = 400 context switches
per second. The 16 (or whatever) sets of MMU mapping can't hold
all the active processes.

With 30 "while(1);" jobs, the scheduler reschedules a compute
bound job every N clock ticks, or a few times a second.

If you are using a screen editor, it is likely the process's
context stays in the MMU between keystrokes in the latter case,
resulting in quick interactive response.
Or it could be that the 50 ms jobs wake up with a high priority
relative to the interactive process.