Fwd: [RFC] Kernel shared variables

[John Baldwin]

On Saturday, June 02, 2012 1:27:58 pm Attilio Rao wrote:
> >> The gettimeofday() implementation is a different story than what is asked 
here.
> >
> > But the goal is to have fast clocks, right ? What else is planned ?
> >
> > In fact, I think that if the whole goal is only fast clocks, then we
> > do not need any additional system mechanisms, since we can easily export
> > coefficients for rdtsc formula already. E.g. we can put it into elf auxv,
> > which is ugly but bearable.
> 
> Not sure if there is anything else besides gettimeofday() that we want
> right now, in particular on global basis.
> I just mean to say that I don't think Giovanni put a lot of effort in
> correctness/robustness of gettimeofday userland implementation, so we
> should not judge that part of the patch too tightly.

I think this is an important question actually.  Is there anything that really 
needs to be here besides gettimeofday()?  I mean, is there any real-world 
application that needs to call getpid() or getppid() a bunch of times?  Things 
that are static like that the application can easily cache (and should if it
actually needs it).  gettimeofday() is different because it is dynamic.

> >> > Interesting question is how much shared the shared page needs be.
> >> > Obvious needs are shared between all same-ABI processes, but I can also
> >> > easily see a need for the per-process private information be present in
> >> > the 'private-shared' page. For silly but typical example, useful for
> >> > moronix-style benchmarks, see getpid().
> >>
> >> Really the performance benefits of having fast getpid() is marginal if
> >> compared to heavilly used things like gettimeofday(). I cannot think
> >> of a per-process page implementing a fast syscall that can bring many
> >> perfomance advantages.
> >
> > This is completely true, but there may be other process-private data that
> > could benefit from the low access cost. I just do not know right now.
> 
> I don't know either, thus I don't think there is a big urgence for
> per-process shared pages at all.

I can't think of anything useful.

-- 
John Baldwin