Re: finding out the value of HZ from userspace

[Micha Feigin]

On Sat, Mar 13, 2004 at 11:41:25PM +0100, Arjan van de Ven wrote:
> On Sat, 2004-03-13 at 23:10, Micha Feigin wrote:
> > On Sat, Mar 13, 2004 at 06:24:31PM +0100, Arjan van de Ven wrote:
> > > On Thu, 2004-03-11 at 15:17, Micha Feigin wrote:
> > > > Is it possible to find out what the kernel's notion of HZ is from user
> > > > space?
> > > > It seem to change from system to system and between 2.4 (100 on i386)
> > > > to 2.6 (1000 on i386).
> > > 
> > > if you can see 1000 from userspace that is a bad kernel bug; can you say
> > > where you find something in units of 1000 ?
> > 
> > I can't see it from user space. Its in the kernel headers. The thing is
> > I am working on fixes to laptop mode. The problem is it requires
> > changing bdflush and journaled file systems journal flush times. The
> > problem is that some of these (bdflush, xfs) expect the value in jiffies
> > and not seconds or milliseconds so making the initiation script portable
> > requires knowing the value of HZ.
> 
> the kernel side is supposed to use clock_t_to_jiffies() and co for this
> to present a unified HZ to userspace. The internal kernel HZ should
> *NOT* leak out to usespace. Heck it's quite thinkable that in the future
> there will be no such HZ.
>  
> 

Kernel side doesn't do that at the moment. Even the fixed bdflush
interface in 2.6 which has dirty_writeback_centisecs as an example
converts it as
(dirty_writeback_centisecs * HZ) / 100
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