Re: kvm_read() vs ioctl performance

--- Julian Elischer <julian_at_elischer.org> wrote:

> Barney Cordoba wrote:
> > --- Julian Elischer <julian_at_elischer.org> wrote:
> > 
> >> Barney Cordoba wrote:
> >>> I have an app which reads stats from the kernel
> >>> periodically, and there can be a lot of
> >> iterations,
> >>> sometimes 20,000 or more. I'm thinking of
> >> converting
> >>> from an ioctl method to kvm_read(). KVM is
> >> certainly
> >>> simpler, but its not clear what overhead is
> >> involved,
> >>> since kvm_read() likely has to call the kernel
> >> also.
> >>> Does anyone have a handle on the difference in
> >>> overhead, assuming that the ioctl call is to a
> >> module
> >>> which does nothing more than copy the data and
> >> return?
> >>
> >> tried a shared memory page?
> > 
> > No, but I built a test and kvm_read is 70 times
> > faster, in
> > case anyone is interested.
> 
> cool..
> the only downside is that we are trying to get away
> from kvm direct 
> access. (which is why a shared page might give the
> same result with a 
> stable API which is not libkvm... BTW on an SMP
> machine you have
> no way to ensure that your stats are coherent if you
> use libkvm.

The app is portable, and I'd prefer not to have
different methods for LINUX and FreeBSD. When you say
"coherent", what exactly do you mean?

Barney


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