Interesting benchmark performance (was RE: Pitiful benchmark perf ormance)

[Sean Hinde]

> > What would happen if you e.g. change INPUT_REDUCTIONS in erl_vm.h
> > from (2* CONTEXT_REDS) to CONTEXT_REDS, doubling the I/O poll
> > frequency?  (CONTEXT_REDS is set to 1000)
> > 
> > If basically all that happens in the erlang vm is that some 
> > process is waiting for I/O, ports may not be polled with any
> > predictably high frequency (then again, this is only guessing
> > wildly. Perhaps it effectively gives the ports even higher
> > priority...)
> 
> I didn't try this but did get the benchmark to spawn an 
> additional process

OK, here is my take on this:

The main emulator loop appears to be the code at the very end of sys.c. This
calls schedule() and waits for it to return before any i/o is checked.

schedule() in erl_process.c will *only* return when INPUT_REDUCTIONS have
all been used up.

As far as I can make out the only reason the echo/server benchmark doesn't
deadlock is that there is enough going on in the background in an otherwise
quiescent system to eventually use up the 2000 function calls required to
check i/o.

Maybe schedule() might be modified so that in addition to returning after
INPUT_REDUCTIONS it keeps track of activity in each pass through all
processes and returns if there has been no activity detected for a whole
pass.

As far as I can make out this would not slow things down significantly in a
busy system but would improve i/o responsiveness under normal load quite
considerably.

Please, someone who really understands these things, help me out here!

Thanks,
Sean



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