Re: finding out the value of HZ from userspace

[Peter Williams <peterw@aurema.com>]

Andi Kleen wrote:
>>So it does and POSIX.1 (_SC_CLK_TCK) compliant as well.  Unfortunately, 
>>the presence of this functionality makes it VERY difficult to understand 
>>why ticks are being converted from HZ==1000 values to HZ=100 values when 
>>they are being exported to user space especially as this conversion 
>>throws away precision.  Can anyone enlighten me?
> 
> 
> There are two different cases here: 
> 
> Timer tick as visible to user space in the minimum delay of select()
> and other kernel functions with timeout. That is what AT_CLKTCK aims at.

Which is a good reason for USER_HZ to be the same as HZ.

> 
> And exports of values with jiffie units in sysctls in /proc. This was in fact i
> always a bug because they should have used ms or s as unit 
> (there are readily usable utility functions to do this for sysctl). Otherwise
> writing documentation becomes quite difficult. But there are already i
> configurations that set or read these values and was not a good idea to 
> subtly and silently break them. Especially since they predate any exporting 
> of HZ to user space. So the the conversion factor was added.
> 
> This is not only obscure sysctls, ps and top are also consumers of such
> jiffies values in /proc
> 

These programs could (and should) use sysconfig(_SC_CLK_TCK) to find out 
how many ticks there are in a second so this does not constitute a good 
reason for USER_HZ not being equal to HZ.

BTW, in ignorance of sysconfig(_SC_CLK_TCK) and because of statements to 
the same effect in Robert Love's book, I had been assuming that this was 
the reason for USER_HZ and HZ not being equal.  But now that I've been 
told about sysconfig(_SC_CLK_TCK) I can see no valid reason.  That 
doesn't mean that there aren't any but the reasons you've advanced 
certainly aren't them.

Peter
-- 
Dr Peter Williams, Chief Scientist                peterw@aurema.com
Aurema Pty Limited                                Tel:+61 2 9698 2322
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