Re: finding out the value of HZ from userspace

[Micha Feigin <michf@post.tau.ac.il>]

On Sat, Mar 13, 2004 at 05:49:29PM -0800, Andrew Morton wrote:
> Micha Feigin <michf@post.tau.ac.il> wrote:
> >
> >  > Wrong. Any such interface is supposed to convert automatically. Any
> >  > interface you can find that doesn't should be reported as a serious bug!
> >  > 
> > 
> >  Like I said, look at bdflush in 2.4 (this was fixed with the changed 2.6
> >  interface) and xfs proc interface in both 2.4 and 2.6.
> >  In light of your post then there is a serious bug.
> > 
> >  For example for bdflush age_buffer field (true for the other used fields
> >  also), no conversion:
> >  	bh->b_flushtime = jiffies + bdf_prm.b_un.age_buffer;
> 
> I doubt if there's any motivation to fix these things in 2.4.  If you change
> HZ in 2.4 you own both pieces.  (alpha has HZ=1024 in 2.4, so presumably
> bdflush tuning doesn't work right).
> 

There is for laptop mode which is now in the kernel so a generic startup
script can be written.

I will right a patch and post it in a new thread and see how it takes.

> In 2.6, the bdflush parameters do not exist.  They were replaced by
> /proc/sys/vm/*_centisecs, which are HZ-independent.
> 
> There are, I think, still some /proc tunables in 2.6 which do depend upon
> HZ and they should be found and fixed.  If the same tunables are present in
> 2.4 kernels then they should be converted to take centiseconds in 2.6, so
> 2.4-based tools continue to work correctly.
> 
> We have similar problems where /proc tunables are expressed in terms of
> "number of pages".  As PAGE_SIZE varies from 4096 to 65536 this is
> sometimes wrong.  Fixing this is more subtle.
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