Date: Thu, 1 Apr 2004 11:50:02 -0500 (EST)
From: "Richard B. Johnson" <root@chaos.analogic.com>
Reply-To: root@chaos.analogic.com
To: Jamie Lokier <jamie@shareable.org>
cc: Arjan van de Ven <arjanv@redhat.com>,
       Albert Cahalan <albert@users.sourceforge.net>,
       "Randy.Dunlap" <rddunlap@osdl.org>, Peter Williams <peterw@aurema.com>,
       ak@muc.de, Richard.Curnow@superh.com, aeb@cwi.nl,
       linux-kernel mailing list <linux-kernel@vger.kernel.org>
Subject: Re: finding out the value of HZ from userspace
In-Reply-To: <20040401163047.GD25502@mail.shareable.org>
Message-ID: <Pine.LNX.4.53.0404011146490.21282@chaos>
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On Thu, 1 Apr 2004, Jamie Lokier wrote:

> Arjan van de Ven wrote:
> > HZ doesn't mean nothing, esp when we go to a tickless kernel...
>
> As explained several times in this thread, HZ is meaningful because it
> affects the rounding in select/poll/epoll/setitimer.  A few userspace
> programs with low jitter soft-RT timing requirements need to
> compensate for that rounding and/or deliberately synchronise
> themselves with the tick.
>
> Such programs can determine HZ experimentally and lock onto the tick
> in the manner of a PLL, but it would be nice to simply be able to
> have the value, to reduce the number of control variables.
>
> When we go to a tickless kernel and offer high-resolution timers to
> userspace, then it will be irrelevant.  Until then, or if the kernel
> goes tickless but limits the resolution of timers for efficiency, the
> value of HZ is still relevant.
>
> Not to get irritatingly back to the subject of this thread or
> anything, but...  is the value of HZ reported to userspace anywhere?
>
> Thanks :)
> -- Jamie

I may be naive, but what's the matter with:

#include <stdio.h>
#include <sys/param.h>   // Required to be here!
int main()
{
    printf("HZ=%d\n", HZ);
    return 0;
}
It works for me.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.24 on an i686 machine (797.90 BogoMips).
            Note 96.31% of all statistics are fiction.


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