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Re: [rbraun at sceen dot net: [PATCH] Make __getclktck return 100 for the Hurd]
- From: Samuel Thibault <samuel dot thibault at gnu dot org>
- To: Roland McGrath <roland at hack dot frob dot com>
- Cc: Richard Braun <rbraun at sceen dot net>, bug-hurd at gnu dot org, libc-alpha at sourceware dot org
- Date: Tue, 18 Jun 2013 00:58:12 +0200
- Subject: Re: [rbraun at sceen dot net: [PATCH] Make __getclktck return 100 for the Hurd]
- References: <20130611222607 dot GA2793 at mail dot sceen dot net> <20130611224917 dot E93042C07D at topped-with-meat dot com> <20130611225504 dot GA8246 at mail dot sceen dot net> <20130611225546 dot B6CA52C09F at topped-with-meat dot com> <20130611230505 dot GA9058 at mail dot sceen dot net> <20130611231353 dot ED42E2C07D at topped-with-meat dot com>
Hello,
Roland McGrath, le Tue 11 Jun 2013 16:13:53 -0700, a écrit :
> > 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.
Well, more or less so. It's centiseconds on x86, but other bases are
used on other archs.
> This is a bug in procfs, regardless. If it's a good idea to change what
> libc uses, then we'll change.
The problem is that applications such as top do assume that /proc is
expressed in _SC_CLK_TCK, which does indeed work on Linux: depending
on the arch, _SC_CLK_TCK will return the proper base for /proc. Other
applications simply hardcode /proc as being centiseconds-based. So
setting _SC_CLK_TCK to 100 and using centiseconds in /proc saves porting
headache in both cases.
Samuel