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SubjectRe: finding out the value of HZ from userspace
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
PO Box 305, Strawberry Hills NSW 2012, Australia Fax:+61 2 9699 9174
79 Myrtle Street, Chippendale NSW 2008, Australia http://www.aurema.com

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