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Re: [address@hidden: [PATCH] Make __getclktck return 100 for the Hurd]
From: |
Rich Felker |
Subject: |
Re: [address@hidden: [PATCH] Make __getclktck return 100 for the Hurd] |
Date: |
Tue, 11 Jun 2013 21:24:52 -0400 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Tue, Jun 11, 2013 at 04:13:53PM -0700, Roland McGrath wrote:
> > Basically, top doesn't report correct CPU times. This is caused by
> > sysconf() returning 1000000 while values read from /proc are true tick
> > counts, hence 100 per second at most.
>
> In fact, that is not a "true tick count". Most modern Linux kernels have
> ticks at 1024Hz, for example. The ABI for certain /proc/PID/* files is
> that certain fields are in centiseconds. That has nothing (except for an
> historical relationship) to do with the actual frequency used in the kernel.
>
> This is a bug in procfs, regardless. If it's a good idea to change what
> libc uses, then we'll change. Any caller of times or clock (i.e. any
> interface using clock_t) has to use sysconf (_SC_CLK_TCK) to know the
> translation from "clock ticks" to units of real time.
sysconf(_SC_CLK_TCK) has to match the units returned by the times()
function. That's about the only place it's relevant. My understanding
is that the kernel's times() syscall and numbers reported in /proc are
using the same units, but if not, times() is the one that's required
to match.
Rich