On Tue, Aug 04, 2009 at 09:42:41AM +0800, Pei Lin wrote:
> 2009/8/2 Greg KH <greg@xxxxxxxxx>:
> > On Sat, Aug 01, 2009 at 03:36:21PM -0400, Robert P. J. Day wrote:
> >> On Sat, 1 Aug 2009, Greg KH wrote:
> >>
> >> > On Fri, Jul 31, 2009 at 11:23:15AM +0700, Mulyadi Santosa wrote:
> >> > > On Fri, Jul 31, 2009 at 6:52 AM, Karel Zak<kzak@xxxxxxxxxx> wrote:
> >> > > It seems that user space sees HZ as 100, while in fact my current
> >> > > running kernel is compiled with HZ=300. What do you think about it? Or
> >> > > is there something wrong with my conclusion?
> >> >
> >> > The following C program should properly show you the kernel HZ.
> >> >
> >> > thanks,
> >> >
> >> > greg k-h
> >> >
> >> > --------------
> >> >
> >> > #include <unistd.h>
> >> > #include <time.h>
> >> > #include <stdio.h>
> >> >
> >> > int main()
> >> > {
> >> > struct timespec res;
> >> > double resolution;
> >> >
> >> > printf("UserHZ %ld\n", sysconf(_SC_CLK_TCK));
> >> >
> >> > clock_getres(CLOCK_REALTIME, &res);
> >> > resolution = res.tv_sec + (((double)res.tv_nsec)/1.0e9);
> >> >
> >> > printf("SystemHZ %ld\n", (unsigned long)(1/resolution + 0.5));
> >> > return 0;
> >> > }
> >>
> >> i already knew about sysconf. so i'm going to assume there's no
> >> simpler way to get the current HZ (either command line or /proc
> >> based).
> >
> > But the point is, from userspace's point of view, the kernel HZ value
> > means NOTHING. See the above program that shows what the resolution
> > that userspace can use is. That is what userspace cares about, and what
> > the kernel provides. Internal HZ values of what the kernel might use
> > for scheduling means _nothing_.
> >
> but some applications or drivers especially for stream media i think
> they need this parameter to get a balance for performance.
> refer to the link: http://lwn.net/Articles/145973/
> i think people need it to configure their kernel.
Look at the code I provided, it gives you the value you need to care
about.
> > So if you do provide it to userspace in a proc file, what then? What is
> > userspace going to do with such a value? It should never depend on it,
> > because it doesn't matter to it.
> >
> how do we change the kernel HZ parameter except reconfiguration kernel
> and rebuild kernel?
That is the only way.
But again, it should not be an issue at all, except for some very
special server loads which wants a low HZ value.
Do you have a special workload that needs a specific HZ value to achieve
its results?
thanks,
greg k-h
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