On Fri, Jun 11, 2010 at 02:26:50PM -0700, J. J. Farrell wrote:
> Can anyone point me at any measurements and/or analysis of the cost
> of moving back and forth between user and kernel space - when doing
> an ioctl call into a driver, for example. Interested in current
> OpenSolaris on x86-64 in particular.
>
> I'm looking at the additional costs of sharing work between modules
> in user and kernel space, trying to get some quantitative feel for
> how much it costs to hop back and forth. It's obvious we want to
> minimise the number of transitions, but I'd like some understanding
> of the numbers to see how much effort it's worth putting into
> minimising them.
>
> I'm also interested in other context switch costs - thread sleep and
> wakeup principally. A much more complex area to analyse, but any
> pointers to useful write-ups or measurements would be welcome.
>
no offence intended, but unless you have some performance data
indicating that your application is spending too much time context
switching it seems to me like you're over-optimizing.
that said, i don't know of any generic no syscall overhead writups lying
around. but you could always measure this yourself using something
like libmicro:
http://hub.opensolaris.org/bin/view/Project+libmicro/
just try benchmarking a super simple system call like getpid().
you should be aware of the fact that different x86 machines use
different system call mechanisms. if you look in /usr/lib/libc/ you'll
see three different version of libc, all which use different syscall
mechanisms. the default mechanism for a system calls is chosen at boot
time by lofs mounting one of the copies of libc above onto
/lib/libc.so.1. so depending on what version of libc your using you'll
get different performance numbers. (and not all syscall mechanisms are
supported by all x86 processors.)
ed
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