Re: [9fans] Performance

[jmk@plan9.bell-labs.com]

On Wed May  9 04:44:05 EDT 2001, pschay@pobox.com wrote:
> Hi,
> 
> There are many things besides kernel compilation that
> are also really fast compared to other systems: booting,
> compiling the window system, running the window system,
> ....
> 
> Anyway, as fast as kernel compilation is, I do have some
> questions about the performance.  I tried a few ways of
> profiling the kernel during kernel compilation.  At first
> I found that a lot of the kernel time was spent in halt()
> so I changed HLT to NOP in l.s.  Now almost a third of the
> time is apparently spent in runproc().
> 
> Is this a fixable problem?  Or is it actually the desired
> behavior?  (All I can imagine is that maybe the scheduler
> sacrifices the good of the few for the good of the many and
> me + my measly kernel compilation processes are not the
> common case?  Or perhaps scheduling typically consumes this
> kind of overhead?)  I know zilch about schedulers so
> I'd greatly appreciate any insight, or pointers to
> literature.  I think I recall reading a brief posting a year
> or two ago by one of the experts who said something to the
> effect that the scheduler has lots of room for improvement.
> Why? How? Could anybody elaborate?
> 
> Thanks.
> -Pete

The system looks for processes to run by scanning the run queues
in runproc(). If it doesn't find anything it just keeps looking until
something is put in the queue. Nothing can be put in the queue
except by an external event happening, i.e. some interrupt causes
a process to change state. The HLT instruction waits until an
interrupt occurs then continues. As you observed, you can replace
HLT with a NOP (or take the call to the routine out entirely) to
no ill effect, and this is what the system used to do. However,
HLT gives the processor opportunities to conserve power and thereby
run cooler and lengthen battery life, etc.

Of course, on a multiprocessor, something can be put on the run
queue by another processor without an external event occurring on
this processor, so the HLT instruction isn't used in that case.

--jim